Ruddy Shelduck x South African Shelduck

probable Ruddy Shelduck x South African Shelduck, Wretham (Norfolk, UK), 17th February 2017 - copyright Dave Appleton

(photo ID: 3044)

This bird looked a lot like a Ruddy Shelduck, but it didn't seem to be quite right. On close inspection I noticed the bird had dark vermiculations on its rear flank feathers and rear upper scapulars - features that should not occur on pure Ruddy Shelduck.  It also had a grey spot in the middle of the crown. I've seen a few Ruddy Shelducks with some grey on the head and wondered about their genetic purity, but this one was more extreme than most.  South African Shelduck (often known as Cape Shelduck) has a grey head of course, and also shows dark vermiculations where this bird had.  It is perhaps hard to be completely sure that it isn't a second or subsequent generation backcrossed hybrid with a different Shelduck species, but as South African and Ruddy often come together in collections I suspect this is the more likely and simplest solution.

probable Ruddy Shelduck x South African Shelduck (same bird as in photo ID 3044 above), Wretham (Norfolk, UK), 17th February 2017 - copyright Dave Appleton

(photo IDs: 3045-3053)

The poor photos of the next bird don't offer many clues but in the field though it struck me as being distinctly dark and chestnutty compared to Ruddy Shelduck, especially towards the rear where there seemed to be blackish shading.  This reminded me of South African Shelduck.  On the head a grey patch was present just behind/above the eye and a smaller less distinct grey patch in front of the eye.  Not visible from the side but more obvious from the front when its head was down was a small darker grey patch on the centre of the crown between the eyes.  It was a male, but lacked any hint of a dark neck collar - not sure if pure Ruddy Shelduck always shows this in January or not?

This bird looked extremely similar to the one above and although separated by just over 2 years, the sites are only 17.4 miles apart and I think it's quite likely that both involved the very same bird.  This bird was seen with Egyptian Geese, the one above was paired to an Egyptian Goose... maybe this pair is responsible for the various records of what look like Egyptian Goose x Ruddy Shelduck hybrids around the county in recent months?

probable Ruddy Shelduck x South African Shelduck (with Egyptian Geese), Bittering, 25th January 2015 - copyright Dave Appleton

(photo IDs: 1785-1786)

Interestingly another apparent Ruddy Shelduck x South African Shelduck was recently photographed elsewhere in the county.  It was identified as Ruddy Shelduck but this bird, a female this time, showed dark inner webs to the tertials which should be a South African feature.

All this makes me wonder about some Ruddy Shelducks I've seen in the past.  At a collection containing several South African Shelducks and several Ruddy Shelducks (plus an Australian Shelduck and a hybrid Ruddy x Australian) I noticed some of the Ruddies showed a little grey in the head.  Although I was not aware that pure Ruddy Shelduck could show this I noted that there were no other features suggesting South African influence so felt there was insufficient evidence to point to them being hybrids.  Subsequently a similar escaped bird has been at my local patch, again not showing sufficeint anomalies to make me feel it should be a hybrid.  Now I'm wondering though - are these birds with grey in the head really pure Ruddy Shelducks, or could they have South African Shelduck influence (not necessarily F1 hybrids)?

Here is the escaped bird - this one shows a grey patch on the centre of the crown, a bit like the Bittering bird above, though less clear.  In these photos it also seems to show a little greyish patch on the ear-coverts and slightly greyish out webs of the tertials.  Subsequent photos of the same bird (probably taken after a moult) don't show these features.

escaped Ruddy Shelduck or possibly Ruddy Shelduck x South African Shelduck hybrid, Beetley (Norfolk, UK), 27th May 2013 - copyright Dave Appleton

(photo IDs: 1787-1788)

By the time the next photos were taken the patch on the crown remained but there is no sign of any grey elsewhere on the head or on the tertials.

escaped Ruddy Shelduck or possibly Ruddy Shelduck x South African Shelduck hybrid (same bird as in photo IDs 1787-1788 above), Beetley (Norfolk, UK), 2nd March 2014 - copyright Dave Appleton

(photo IDs: 1789-1791)

The next birds are even more subtle and again I am not sure if the slight hint of grey behind the eye is within range of variation for pure Ruddy Shelduck or not.  With so many South African and Ruddy Shelducks mixing at this collection there is plenty of potential for hybrids, but there were no apparent F1 Ruddy Shelduck x South African Shelduck hybrids present.  Are these birds backcrossed hybrids, or just Ruddy Shelducks?

captive Ruddy Shelduck or possibly Ruddy Shelduck x South African Shelduck hybrid, Grange-over-Sands (Cumbria, UK), 27th December 2012 - copyright Dave Appleton

(photo ID: 1792)

captive Ruddy Shelduck or possibly Ruddy Shelduck x South African Shelduck hybrid, Grange-over-Sands (Cumbria, UK), 28th December 2011 - copyright Dave Appleton

(photo IDs: 1793-1794)

Ruddy Shelduck Tadorna ferruginea
South African Shelduck Tadorna cana

House Sparrow x Spanish Sparrow

possible House Sparrow x Spanish Sparrow hybrid or House Sparrow x Eurasian Tree Sparrow hybrid or Italian Sparrow, Northrepps (Norfolk, UK), 18th August 2013 - copyright Dave Appleton

(photo ID: 1776)

When this curious bird was found the possibility that it was an Italian Sparrow was mooted.  Indeed it has been submitted as such but the British Birds Rarities Committee have effectively pended the record until taxonomy or identification criteria are resolved (British Birds 108: 630).  A 'classic' Italian Sparrow should not have any grey in the crown but it seems that birds in the north of the range (presumably where introgression with House Sparrow still occurs?) may have some grey in the crown. 

In the photo above it looks as though it has a hint of a dark spot at the lower back of the cheeks, which would strongly suggest Eurasian Tree Sparrow involvement.  I didn't think this was a real plumage feature though - just the way the feathers were lying.  Indeed the absence of a dark spot in the cheeks was one reason I doubted what some people eventually concluded, which is that it was a House Sparrow x Eurasian Tree Sparrow hybrid.  Most House Sparrow x Eurasian Tree Sparrows I've seen photos of have had at least a small dark spot in the cheeks (see also our House Sparrow x Eurasian Tree Sparrow page).

Another reason to doubt that was the almost complete absence of grey in the crown.  There were a few spots of grey in the forecrown - at the time we thought at least some of these were effects of moult - feather bases being visible where feathers were missing or loose - but as they appear in other photos taken later on including in 2014 when it reappeared briefly, they must have been a real plumage feature after all.  Most House Sparrow x Eurasian Tree Sparrow hybrids have more grey in the crown than this.

The bill was large, possibly suggesting Spanish/Italian Sparrow involvement and there were none of the buffy tones to the body that are often displayed by House Sparrow x Eurasian Tree Sparrow hybrids (actually some other poeple's photos do seem to show such tones, but not in all frames and covering a different area in different frames - I think those images may be showing the bird after it had been bathing in dust - I have seen House Sparrows with a similar effect from bathing in dust).  I wondered if it might instead by a House Sparrow x Spanish Sparrow hybrid.  It looks a lot like an Italian Sparrow and Italian Sparrows are meant to be the product of hybridisation between House Sparrows and Spanish Sparrows, so this does seem a realistic possibility.  A few Spanish Sparrows have turned up in the UK and spent time - even whole summers - with House Sparrows.  None have been known to produce hybrids but how many others have been overlooked or not even seen - especially if they were females?  It doesn't seem a terribly implausible explanation.

To progress this further I suspect we will need to get better knowledge of the extent of variation in House Sparrow x Eurasian Tree Sparrow hybrids and also a better understanding of what first-generation House Sparrow x Spanish Sparrow hybrids look like, and of course the range of variation in Italian Sparrow.

possible House Sparrow x Spanish Sparrow hybrid or House Sparrow x Eurasian Tree Sparrow hybrid or Italian Sparrow (same bird as in photo ID 1776 above), Northrepps (Norfolk, UK), 18th August 2013 - copyright Dave Appleton

(photo IDs: 1777-1779)

My original analysis of this bird, along with extra photos, can be found here.  More images of this bird from other photographers and on other dates appear here and here.


Note that Italian Sparrow taxon is now considered to be a result of hybrid speciation.  A population of House Sparrow x Spanish Sparrow hybrids have eventually become stabilised and more-or-less reproductively isolated from their ancestor taxa.



House Sparrow Passer domesticus
Italian Sparrow Passer italiae
Spanish Sparrow Passer hispaniolensis
Eurasian Tree Sparrow Passer montanus

White-browed Coucal x Burchell's Coucal intergrades

White-browed Coucal intergrade (White-browed Coucal x Burchell's Coucal), Liwonde (Malawi), 23rd September 2008 - copyright Dave Appleton

(photo ID: 1771)

Burchell's Coucal is quite a different-looking beast from White-browed Coucal and have often been treated as different species.  However there is significant intergradation between the two taxa and many authorities now treat them as the same species.

In Malawi Burchell's Coucals (race burchellii) occur in the south and one of the forms of White-browed Coucal (race loandae) occur in the far north.  In between birds intergrade.  On my visits I mainly saw birds in the south, just inside the range of burchellii, however one of these birds showed a few pale streaks in the black of the head and neck and also on the scapulars.  Such streaking is not present on pure burchellii so I assume this was an intergrade, albeit much closer to Burchell's in appearance than the northern forms.  I saw a more obvious intergrade further north but didn't manage to get any photos of that bird.


White-browed Coucal Centropus superciliosus
White-browed Coucal Centropus (superciliosus) superciliosus/sokotrae/loandae
Burchell's Coucal Centropus (superciliosus) burchellii/fasciipygialis

Hybrid speciation

Hybrid speciation is the process whereby a population of hybrids becomes sufficiently well reproductively isolated from its parent species that it represents a third new species.  Until recently it seems that it wasn't considered to play a significant factor in the evolution of bird species.

Where hybrids of two species are fertile and the two species ranges overlap a zone of hybridisation may occur.  Within this zone the two species come into contact with one another, hybridise, and the hybrids mate with either species or other hybrids.  Eventually a high proportion of the population within the overlap zone - perhaps even the entire population within the middle of the zone - are hybrids.  In some cases the hybrids in the middle of the zone have become stable and look very similar to one another.

If this zone is broad, observers may see three different-looking types of birds in three different geographic regions.  Sometimes this has led to all three types of bird being classified as species.  However, at the edges of the overlap zone the hybrids are not usually reproductively isolated from the species they are close to and so if you look across the whole range it can be seen that there is intergradation between the two species with hybrids in the middle.

In his "Handbook of Avian Hybrids of the World" (Oxford University Press 2006), Eugene M McCarthy devotes a section to "Hybrids treated as taxa" and he describes this sort of situation.  The clear implication is that whilst these hybrids have sometimes been classified as species, this classification was wrong as they are in fact hybrids.  He does not mention hybrid speciation at all (unless I've missed it somewhere else in the book) and it would appear that this is the end of the story.  'Hybrid species' are wrongly classified - they are hybrids but not species.

From the book, this seems to be the end of the story.  Since it was written mainstream understanding about hybrid speciation seems to taken a leap forward.  It now seems that while the situation described by McCarthy may still be true for many hybrids incorrectly classified as species, there are some cases where the situation is a little different.  In some cases hybrids in the middle of the overlap zone have not only become stable in their appearance but they have somehow managed to become reproductively isolated from the species at either side of the overlap zone.  They have actually become a new third species.

The best known example is Italian Sparrow.  Over the years taxonomists have classified the Italian Sparrow variously as a race of House Sparrow, a race of Spanish Sparrow, a third species or a hybrid between House Sparrow and Spanish Sparrow.  It is now understood that it did indeed originate as a hybrid between House Sparrow and Spanish Sparrow, but is now sufficiently well reproductively isolated from both House Sparrow and Spanish Sparrow that it has become a new species.

Italian Sparrow, Etang d'Urbino (Corsica, France), 16th April 2008 - copyright Dave Appleton

(photo ID: 1773)

Of course speciation takes a long time, and for a newly formed species there may be anomalies.  And with Italian Sparrows you can see that in the south of their range the birds' appearance is not as stable as it seems throughout most of Italy.  In the south many look more like Spanish Sparrow, but they are variable.  Nevertheless I understand that further north there is a sympatric population of Spanish Sparrows on the east coast that remain distinct.  In the north of the range where they come into contact with House Sparrow there is a narrow zone of ongoing hybridisation.

Italian Sparrow or Italian Sparrow x Spanish Sparrow intergrade, Syracusa (Sicilly, Italy), 23rd April 2012 - copyright Dave Appleton

(photo ID: 1774)

 Italian Sparrow or Italian Sparrow x Spanish Sparrow intergrade, Capo Murro di Porco (Sicilly, Italy), 2nd May 2012 - copyright Dave Appleton

(photo ID: 1775)

The language becomes quite technical (too much so for me to follow in full) but there is a summary of the situation in The Molecular Ecologist here.


I understand that Pomarine Skua (Jaeger) is now thought likely to have originated as a hybrid between Great Skua and one of the smaller northern skuas (jaegers in US) (I assume Arctic Skua - or Parasitic Jaeger if you're American - but I've not read anything that confirms that).

Pomarine Skua, MV Sapphire pelagic trip (Scilly, UK), 12th August 2012 - copyright Dave Appleton

(photo ID: 1772)

McCarthy's Handbook of Avian Hybrids of the World may not deal with hybrid speciation but his website does.  Indeed if I have understood correcftly it forms the basis of what McCarthy presents as "stabilization theory, an alternative theory of evolution", and he concludes that stabilisation processes are sufficient explanation for the origin of the vast majority of living forms, contrary to neo-Darwinian theory.  I think it's fair to say that some of McCarthy's theories (which include the origin of humans being pig x chimp hybrids) are some way from being accepted by the majority of the scientific community.

If McCarthy is right then we should not be surprised to find many other examples of avian hybrid speciation - and probably so even if he is not right.  McCarthy does mention a few other examples, though as yet I am not sure how many of these are undisputed.

Let us know if you are aware of any other bird hybrids that have become full species..


Species mentioned:
Great Skua Stercorarius skua (formerly Catharacta skua)
Pomarine Jaeger (Pomarine Skua) Stercorarius pomarinus
Parsitic Jaeger (Arctic Skua) Stercorarius parasiticus
House Sparrow Passer domesticus
Italian Sparrow Passer italiae
Spanish Sparrow Passer hispaniolensis

Fertility and viability of hybrids

I occasionally hear people claim that different species can't produce hybrids.  That's clearly absurd but it probably comes from a misrepresentation of another false claim that is much more widely held to be fact.  That is that different species can't produce fertile hybrids, and if they do then that indicates they aren't really different species at all and have been classified incorrectly.  That's also wrong, but I've heard it so often that I suspect it's sometimes taught in schools and textbooks.


Fertility in bird hybrids

It is demonstrably the case that some hybrids can be fertile.  There are hundreds of examples I could give you - let's say Western Gull x Glaucous-winged Gull, two species with an extensive overlap in their ranges and where in the centre of the overlap zone most birds are hybrids and freely and successfully breeding with one another.

a fertile type of hybrid - Western Gull x Glaucous-winged Gull hybrid, Mukilteo, Snohomish County (Washington, USA), 31st January 2011 - copyright Steve Mlodinow
(photo ID: 1453)

It is less easy to prove that some hybrids are necessarily infertile.  However if a hybrid is very common but backcrossed hybrids are unknown, then it seems reasonable to conclude that they are.  One example of a hybrid that is apparently always infertile is Muscovy Duck x Mallard.  Domestic examples of these have been bred together, deliberately and accidentally, and such a hybrid has never (I believe - correct me if I'm wrong) produced any offspring of its own.

an apparently infertile type of hybrid - Muscovy Duck x Mallard hybrid, Rheinauenpark, Bonn (Germany), summer 2004 or 2005 - copyright Joern Lehmhus
(photo ID: 0848)

Another good example is Greylag Goose x Canada Goose.  Such hybrids are very common but I have never seen or heard of any evidence to suggest that they can produce offspring.

another apparently infertile type of hybrid - Greylag Goose x Canada Goose hybrid, Swanton Morley (Norfolk, UK), 8th March 2008 - copyright Dave Appleton
(photo ID: 0052)

The fact that most Muscovy Duck x Mallard hybrids or Greylag Goose x Canada Goose hybrids don't produce offspring doesn't prove that they are necessarily infertile.  It doesn't mean that one day we might find one that does produce offspring.  But with common hybrids like this there is sufficient evidence at least to say that these hybrids are either much less likely to be fertile, or much less fertile.

And here's a point.  It may not be a black and white question of fertile or infertile, but a shades of grey question of how fertile.  Western Gull x Glaucous-winged Gull hybrids are very fertile and Muscovy Duck x Mallard hybrid are very infertile. Are there other hybrids that are a bit fertile, but not very fertile?  Do you know of any examples of hybrids that have shown they can be fertile but appear to have reduced fertility compared to their parent species

In an earlier draft of this article I speculated that Mallard x Northern Pintail hybrids might provide such an example.  They turn up fairly often, especially in NW USA/SW Canada it seems, and they're pretty consistent in appearance most of the time.

Mallard x Northern Pintail hybrid, Sweden, 10th February 2012 - copyright Carl Gunnar Gustavsson
(photo ID: 0925)

If Mallard x Northern Pintail hybrids were as fertile as their parents, I figured we should expect to see a few backcrossed birds now and then, looking much more like one or other parent than the typical F1 (first-generation) hybrid?  I was aware of one or two records that we've wondered about and that do seem quite likely to have been backcrossed hybrids, but not often and not many.  I concluded tentatively that perhaps Mallard x Northern Pintail hybrids are fertile, but not very fertile, or perhaps not all of them are fertile.  Well, Joern Lehmhus has brought to our attention studies of captive birds that have shown that Mallard x Northern Pintail hybrids are in fact very fertile.  This seems to be true of many other ducks in the genus Anas (and also in Aythya).

Most hybrids of very closely related taxa - like where the two taxa are treated as conspecific by some authorities and distinct species by others - seem to be very fertile.  Several records of vagrant Black Brants in Norfolk (UK)'s Dark-bellied Brent Goose flocks have remained with their carrier species to breed and now Dark-bellied Brent Goose x Black Brant hybrids are rather frequent.  Many of these have bred with Dark-bellied Brent Geese and produced young, although once the family ties are broken they are nigh-on impossible to identify.  They are evidently very fertile.

Dark-bellied Brent Goose x Black Brant hybrid with 3 presumed backcrossed young (Dark-bellied Brent Goose x Black Brant) x Dark-bellied Brent Goose hybrids, Burnham Overy (Norfolk, UK), 1st January 2015 - copyright Dave Appleton
(photo ID: 1738)

So it seems a rule might be:

• The more closely related two taxa are, the more likely their hybrid offspring are to be fertile (and the more distantly related they are the more likely their hybrid offspring are to be infertile)

and/or

• The more closely realted two are, the more fertile their hybrid offspring are likely to be (and the more distantly related they are the less fertile their hybrid offspring are likely to be)

A rule of thumb seems to be that hybrids between species that belong to different genera are infertile while hybrids between different species that belong to the same genus may be fertile.  I'm sure that cannot be more than a guide, as a hybrid's fertility cannot depend on how a taxonomist classifies its parents, but can anyone think of any exceptions?  Let us know.

Of course in nature things aren't always simple, so it's likely that hybrids of some closely related species won't be fertile and it might be that some hybrids of more distantly related species are fertile.  It would be good to hear from you if you know of cases that fall outside of expectations.


Finding evidence of hybrid fertility

In captivity it might be possible to do more, but with observations of birds in the wild we can only rely on collecting data from hybrid pairings where the hybrids are clearly different from either parent species.  If you see a lot of hybrids that look completely different from their parent species but consistent with one another then they're probably all first-generation hybrids.  If you see most hybrids are consistent but a minority look much closer to one or other parent species, it may be reasonable to conclude these are second-generation backcrossed hybrids.

For more variable hybrids, or hybrids that are not very dissimilar from one or both parent species it can be much harder to detect backcrosses and thereby prove that the hybrid is fertile.  Sometimes, as with the Dark-bellied Brent Goose x Black Brant hybrid's family shown above, their behaviour might give it away even when the birds' appearance does not.

Joern Lehmhus has also pointed out another factor that may influence our perception of fertillity.  He points out that the display of most Anas species has a series of several elements which are varied between the species.  Several species may even lack some elements.  These behavioral traits are inherited (this is shown in a study of Mallard x Northern Pintail hybrids in Behaviour 27: 259-272 (Sharpe & Johnsgard, 1966).  Therefore a hybrid does often not show the same combination as either of the parent species.  If  a male F1 hybrid is displaying to a female of one of the parents (or a male of one parent species displays to an F1 hybrid female) some elements will be 'wrong' and may reduce the success of reproduction in a hybrid, even if the bird is as healthy and viable as the parent species.


Viability in hybrids

Viability in hybrids (the ability for a hybrid to maintain life) is much harder to gather information about.  At least we can say that those hybrids that have been recorded are, or can be, viable.  But we cannot say that those hybrids we do not observe are not viable - it may simply be that they are rare.

There are some examples that might give us clues:

• In his "Handbook of Avian Hybrids of the World" (Oxford University Press 2006), Eugene M McCarthy's small section on viability/inviability includes a reference to a brood of European Greenfinch x Yellowhammer eggs.  Three eggs were sterile, one died in its shell and the fifth survived to maturity.  McCarthy admits that such data is of limited value for predicting viability but its inclusion presumably infers some relevance.  I would take this as anecdotal evidence that supports the idea that Greenfinch x Yellowhammer hybrids may have low viability, but of course much more data is required before conclusions can be drawn.  There's also a report of a European Goldfinch x Yellowhammer pairing - hybrids reportedly started developing in the eggs but died before hatching.  Both of these examples were published in Cage Birds journal in the early 50s - we're not quite sure how reliable they are.
• Also in McCarthy's book there is a comment about Mandarin hybrids.  Many breeders and researchers have tried and failed to breed Mandarins with other species without any success.  It has been suggested that because Mandarins have a different chromosome count to other ducks they cannot hybridise (the implication being that those few Mandarin hybrids that have been reported are erroneously identified) but McCarthy points to frequent hybrids between different species of Muntjac deer which have different chromosome counts and suggests that a better explanation might be that Mandarin does hybridise but only rarely.  I wonder if it might even be that they do hybridise but have poor viability.  McCarthy cites a case of two captive Mandarin x Laysan Duck hybrids - both lacked eyes on hatching with one surviving at first but dying in juvenile plumage.
• When a Swoose (swan sp. x goose sp. hybrid) was discovered by the Radipole Ringing Group their research found that such hybrids do not normally survive past fledging (Radipole Ringing Group Report 2007).  They didn't cite any references so I'm not sure of the source of that information, but if it was based on good evidence then that would suggest that swan x goose hybrids are not normally very viable.  Exceptions exist though, as the bird they were reporting on was discovered as a cygnet/gosling in 2002 and was still surviving when I saw it in 2010 and thereafter until at least December 2011.

8-year old Mute Swan x domestic goose hybrid, Wool (Dorset, UK), 24th October 2010 - copyright Dave Appleton
 (photo ID 0001)

I would be willing to speculate that hybrids of distantly related species are less likely to be viable than hybrids of closely related species.

Let us know if you have any information or experience that might help us build a clearer picture.


Species mentioned:
Mute Swan Cygnus olor
domestic goose - in this case Anser cygnoides x Anser anser
Greylag Goose Anser anser
Canada Goose Branta canadensis
Dark-bellied Brent Goose Branta (bernicla) bernicla
Black Brant Branta (bernicla) orientalis (formerly nigricans) or Branta (nigricans) orientalis
Muscovy Duck Cairina moschata
Mandarin Aix galericulata
Mallard Anas platyrhynchos
Laysan Duck Anas laysanensis
Northern Pintail Anas acuta
Western Gull Larus occidentalis
Glaucous-winged Gull Larus glaucescens
Yellowhammer Emberiza citrinella
European Greenfinch Chloris chloris (formerly Carduelis chloris)
European Goldfinch Carduelis carduelis

Vagrancy of bird hybrids

I've a hunch.  It really isn't any more than that, but let's see if it there might be anything to it. My hunch is that some hybrids may be more prone to vagrancy than non-hybrids.  In other words if a bird is a hybrid it may be more likely to turn up in an unexpected place than if it was not a hybrid.

Recognising that I don't have much evidence for my hunch, this article is to get discussion going and to encourage people to look out for evidence.  Let us know if you find any!


Why might it be true?

Hybrids are genetically a bit mixed up.  Apart from the obvious with their appearance being different from either parents, the fact that they're hybrids may also impact them negatively in a number of other ways.  Here are some examples we know about:

• Hybrids are often infertile - especially where the parent species are not very closely related.
• Hybrids may not be viable at all - difficult to get data on this as unviable hybrids don't survive to be studied, but there is evidence that suggests some hybrids may be unlikely to mature.
• Hybrids may be prone to physical conditions such as being intersex - we need more data to prove this but in this article on intersex birds we suggested that hybrids may be more prone to being intersex than non-hybrids.  (Let us know if you know of any other conditions that there is some evidence for thinking they might occur more commonly in hybrids).

So it seems that being a hybrid can result in being a bit messed up inside as well as being messed up outside (excuse the simplistic way of expressing that!).  And so if that's the case, then it doesn't seem too much of a stretch to imagine that they might display odd behaviours or that this might effect their ability to migrate normally.


What's the evidence?

Well I described it as a hunch because I recognise that I don't have much evidence.  I share it because I'd be interested if other people can help gather some evidence - if it exists.  So far I've got just one story that might support my hunch, but in all honestly I'm not sure it really does.

In November 2002 a bird closely resembling Hermit Warbler turned up in New York State (USA).  A number of apparent anomalies were eventually discovered to be within range for Hermit Warbler, but one feature remained problematic.  Both Hermit Warbler and Townsend's Warbler occur in western USA and are extremely rare vagrants to the east (especially Hermit Warbler with just a handful of records).  But where their ranges overlap hybrids do occur and it seems most likely that the November 2002 record was a hybrid.  The final conclusion of the New York State Avian Records Committee in its annual report was, "In conclusion, although the Jones Beach warbler could possibly have been an unusual variant Hermit Warbler, the Committee concluded that a hybrid origin was more likely and certainly could not be ruled out."  If hybrids are as equally prone to vagrancy as pure birds then the odds of a hybrid turning up in a place where pure birds are extremely rare must be very low.  Of course - and this is why this isn't a perfect example - they will be higher if the two species hybridise relatively often.  Townsend's Warbler x Hermit Warbler hybrids do occur where their ranges overlap.  Here's one for example:

Townsend's Warbler x Hermit Warbler hybrid, Sequim (Washington, USA), 28th October 2011 - copyright Steve Mlodinow

(photo ID: 1378)

Moreover the hybrids are fertile meaning backcrosses also occur (and meaning that impure birds form an even higher proportion of the population).  The New York bird, assuming it was indeed a hybrid, resembled Hermit Warbler much more closely than Townsend's Warbler, so could possibly have been a backcrossed hybrid.  I don't have any data as to what proportion of the Townsend's Warbler/Hermit Warbler populations consist of hybrids and we'd need that to know if this was a good example to support my hunch or not.  My impression is that it's reasonably low and if so then this story probably does support my hunch a little, but if it's in fact a moderately high proportion then it's completely irrelevant!


Is there any counter-evidence?

There's plenty of evidence that some hybrids do migrate normally with whichever parent species they grew up with.  But is that counter-evidence?  I don't think so.  There is plenty of evidence that some hybrids are fertile but it doesn't mean they all are.  I'm not sure what other counter-evidence to look for?  I'm open to suggestions but I think it will only come in the form of a lack of evidence for the hunch - if lots of people interested in hybrids fail to come up with any supporting evidence over a period of time, then it will start to look less likely.


Why does it matter?

Well I'm not sure it has to matter to be of interest, but I think it might help to understand when assessing records sometimes.  I can think of a couple of scenarios:

• Suppose you are considering a record of a rare vagrant.  Say a species where the escape likelihood is minimal so it's only the identification that needs to be established.  Let's say it's a Crested Auklet that's appeared in Europe or the eastern seaboard of USA.  It looks good, and it's clearly not any other species of Auklet, but it's not a classic.  One or two features seem quite unusual for Crested Auklet.  They're features that recall Parakeet Auklet a little, though it clearly isn't that.  No-one is quite certain whether the features are within the range of variation for Crested Auklet or not, and the possibility of it being a hybrid is raised.  Some people point out that Crested x Parakeet Auklet hybrids have never been recorded and what are the chances of an auklet that must have originated in the north Pacific being a hybrid that no-one's ever recorded before even in the north Pacific.  Surely, they say, it must be an odd Crested Auklet.  That seems more likely, after all there have been lots of examples where a vagrant has shown aberrant features, like the orangeless Varied Thrush that turned up in Cornwall (England) in 1982.  But if a hybrid Auklet is more prone to vagrancy than a pure one, then that would make the hybrid theory a whole lot more believable.  Knowing whether hybrids might be more prone to vagrancy could make a big difference to the way such a record is assessed.  (Have you got a real example rather than this imaginary one?  Let us know if you have.)

• A hybrid goose spent several winters with wild Pink-footed Geese in Norfolk (England) a few years ago.  Most people assumed it was a hybrid between Ross's Goose and Pink-footed Goose as there had been one or two Ross's Geese in the same flocks.  It turns out, we think, that it was in fact an Emperor Goose x Ross's Goose hybrid - or at least it was identical to some captive Emperor Goose x Ross's Goose hybrids of proven parentage.  Most people jump to the conclusion that it was therefore an escaped bird - both Emperor Geese and Ross's Geese are frequent in captivity, so a captive hybrid has escaped (or been released by a keeper who doesn't want hybrids) and joined up with the Pink-feet.  Well that might be so, but I wonder.  You see there are loads of feral and escaped geese in Norfolk and although some often appear in the same fields as the wild geese they very rarely stick with the wild geese for long.  In fact in all the years I've been studying them (and it's quite a few) I've never seen another goose that's certainly feral or escaped stay with the wild Pink-footed Geese for more than a couple of days, staying with them even when the Pink-feet fly away from their roosting areas (where they often come into contact with feral geese) to feed inland.  I gather it has happened (I'm told a couple of ringed Ross's Geese migrated with Pink-footed Geese to Iceland many years ago) but it's rare.  This bird remained with the Pink-footed Geese as they migrated between Iceland and the UK for several years.

probable Emperor Goose x Ross's Goose hybrid, north of Docking (Norfolk, UK), 26th November 2007 - copyright Dave Appleton
(photo ID: 0143)

My alternative (and very tentative) theory that this might be a genuine wild vagrant seems very far-fetched, but it isn't if my hunch about hybrids being more prone to vagrancy is right.  The breeding ranges of wild Emperor Geese and Ross's Geese don't overlap, but Emperor Geese have been recorded well outside their normal ranges and inside the (wintering) range of Ross's Goose.  So far as I know hybridisation between the two species has not been recorded in the wild, but it doesn't seem very unlikely that a wild Emperor Goose could find itself among breeding Ross's Geese at some point.  And if it did, especially if it was a male, it doesn't seem very unlikely that it would breed with one of those Ross's Geese.  We know from captivity that hybrids are viable.  Ross's Geese very rarely make it to Europe - we don't really know how rarely as the picture is clouded by escaped birds - but they're at least rare.  If out of all the hundreds of thousands of Ross's Geese only a tiny number at most have ever made it to the UK, what are the odds of a rare hybrid making it here under its own steam?  Well, if hybrids are more prone to vagrancy, then maybe they aren't as low as you might first assume?

Conclusions

None yet - we need more data.  But please let us know if you have any insight, or any examples that might throw some light on this subject.


Species mentioned: 
Pink-footed Goose Anser brachyrhynchus 
Emperor Goose Anser cangicus or Chen canagica Ross's Goose Anser rossii or Chen rossii
Parakeet Auklet Aethia psittacula
Crested Auklet Aethia cristatella
Varied Thrush Ixoreus naevius
Townsend's Warbler Setophaga townsendi (formerly Dendroica townsendi) 
Hermit Warbler Setophaga occidentalis (formerly Dendroica occidentalis)

Conservation issues arising from hybridisation

Mallard x Hawaiian Duck hybrid, Ko'Olina, Oahu (Hawaii, USA), 6th August 2012 - copyright Steve Mlodinow
(photo ID: 1257)

There are a number of examples from around the world where species are threatened by hybridisation.  Typically this arises where a non-native species is introduced to an area where a closely-related species already exists.  The two species interbreed and the hybrid young are fertile.  If there are sufficient numbers of the introduced species, especially if these are more dominant, genetically pure examples of the native species can in due course be wiped out completely.  Hybridisation may be one of a number of factors, with others like habitat loss playing a contribution.

Many examples involve the introduction of domesticated Mallards.  An example is the Hawaiian Duck which is listed as Endangered because it is "inferred to have a very small and fragmented range on a few islands, where wetlands are being lost and degraded, and where hybridisation is slowly reducing the number of pure individuals" (source: BirdLife International). 

Another example involving Mallards is the American Black Duck, a declining species in North America at least partly linked to hybridisation with Mallard.  In some areas there are no genetically pure Black Ducks remaining.

American Black Duck x Mallard hybrid, Everett (Washington, USA), 18th December 2008 - copyright Steve Mlodinow
(photo ID: 1415)

Domestic Mallards may also be responsible for threatening the Pacific Black Duck in Australasia - see "Feral Mallards: A Risk for Hybridisation with Wild Pacific Black Ducks in Australia?" by P-J Guay and J P Tracey (in The Victorian Naturalist 126. 3:87-91, 2009) (abstract here).  The same issue exists in New Zealand (where the name Grey Duck is often used for this species) - see the PhD thesis by Wiebke Muller: "Hybridisation, and the Conservation of the Grey Duck in New Zealand" (available here).

Hybridisation with domestic Mallards may also be one of a number of threats to the African Black Duck in southern Africa (mentioned in the BirdLife International factsheet).


Famously in the UK a cull of accidentally-introduced Ruddy Ducks has recently taken place.  This cull proved highly controversial though supported by the RSPB (they explain their decision here).  Birds originating in the UK had spread across western Europe reaching Spain where they began hybridising with the already threatened White-headed Duck.  It was hoped that by eradicating them from the UK (and I think other countries have deployed similar programmes) the threat of hybridisation in Spain could be reduced.

Ruddy Duck, Whitlingham Broad (Norfolk, UK), 17th January 2010 - copyright Dave Appleton

(photo ID: 1770)

 
It is not necessarily the introduction of non-native species that cause a threat to endangered species through hybridisation.  In some cases two species may co-exist with no difficulty when both species have strong populations but when one declines for other reasons it becomes suscpetible to hybridisation (with insufficient same-species mates related species are chosen instead), and that then compounds its problems.  This is the situation with Black Stilts in New Zealand.  Their decline was initially caused by predation from introduced mammals and habitat loss, but hybridisation with Pied Stilts is now a significant additional factor.  It is known that hybridisation used to occur when population levels were higher but the incidence was low compared to the total population so it did not generate significant issues for the species.  See the Black Stilt Recovery Plan (Reed & Murray, 1993) for more information.

Pied Stilt x Black Stilt hybrid, Ashley River Mouth (Canterbury, New Zealand), on or before 13th January 2008 - copyright Ian McHenry 

(photo ID: 2086)

Species mentioned:
African Black Duck Anas sparsa
Mallard Anas platyrhynchos Hawaiian Duck Anas wyvilliana
American Black Duck Anas rubipres
Pacific Black Duck Anas superciliosa
Ruddy Duck Oxyura jamaicensis
White-headed Duck Oxyura leucocephala
Pied Stilt Himantopus leucocephalus
Black Stilt Himantopus novaezelandiae

Hybrids or intergrades

Dark-eyed Junco intergrade ('Pink-sided Junco' x 'Grey-headed Junco'), Red Rocks Trading Post, Jefferson County (Colorado, USA), 7th January 2012 - copyright Steve Mlodinow
(photo ID: 1334)

 
When is it appropriate to use the term intergrade rather than hybrid?  This is a question that seems to cause some confusion at times so I thought it was worth exploring and throwing out my thoughts for others to build on.

In practice intergrade seems most often to be used in relation to crosses between different subspecies while hybrid is reserved for crosses between different species.  However this doesn't seem very satisfactory as a definition, as the taxonomic status of different taxa may move between species and subspecies or vice versa - should we suddenly change whether we call something a species or a hybrid just because taxonomists have changed their mind over whether the two taxa are different species or not?  And what about different taxa where one authority treats them as different species while another as subspecies of the same species?  Should we call them intergrades by one authority and hybrids by another? 

Having considered a number of different scenarios I reached the conclusion that I was asking the wrong question.  It is not a case of either hybrid or intergrade and working out where one starts and the other ends.  The words are not mutually exclusive.  Maybe this was obvious to some people already, but I suspect I'm not alone in having failed to appreciated it up to now.  In fact I'm sure I'm not as Eugene M McCarthy devotes a section to the issue in his Handbook of Avian Hybrids of the World (Oxford University Press 2006) (he also seems to come at it with the premise that most biologists choose the words according to whether the parent taxa are different species or not).

I realised that the term intergrade is easy to define if I stop being concerned about how it relates to the term hybrid.  It's the product of intergradation!  So if the ranges of two populations of different taxa overlap and there is intergradation between the two populations, we have intergrades.  This implies that crosses between the two taxa are fertile and backcross with either or both parent taxa or with each other.  In other words if there is extensive gene flow between the two taxa then where this genetic exchange takes place the crosses are intergrades.  This works independently of the taxonomic status of the two taxa - it doesn't matter if they are different species (like Western Gull and Glaucous-winged Gull) or different subspecies of a single species (like Red-shafted Flicker and Yellow-shafter Flicker).

In this article I defined a bird hybrid as a bird that is the offspring of parents that do not both belong to the same species.  Given the changing nature of taxonomic classification it doesn't seem to much of a stretch to assume the broadest concept of species, so allowing crosses between phylogenetically distinct taxa that may be treated as subspecies by some authorities to be classed as hybrids.

If I've got this right then it allows some crosses, such as Glaucous-winged Gull x Western Gull, to be accurately described as both hybrids and intergrades.  Where no intergradation occurs the crosses may only be described as hybrids.

Western Gull x Glaucous-winged Gull hybrid (and intergrade), Elwha River Mouth, Clallam County (Washington, USA), 28th October 2011 - copyright Steve Mlodinow

(photo ID: 1354)

Species mentioned:
Western Gull Larus occidentalis
Glaucous-winged Gull Larus glaucescens
Yellow-shafted Flicker Colaptes auratus luteus
Red-shafted Flicker Colaptes auratus cafer
Dark-eyed Junco Junco hyemalis  'Pink-sided Junco' Junco hyemalis mearnsi-group  'Grey-headed Junco' Junco hyemalis caniceps-group

Hybrids showing traits that do not appear in either parent species

Gadwall x Northern Shoveler hybrid, South Platte River, Adams County (Colorado, USA), 26th February 2012 - copyright Steve Mlodinow
(photo ID: 1318)

I think many of us tend to expect hybids to show either a combination of traits that are normally present in one parent and those that are normally present in the other parent, or to show traits that are intermediate between those shown by the two parent species - or perhaps a mixture of both combined traits and intermediate traits.  It might be interesting to look into which of these types occurs most often, and if that varies between different types of hybrid (perhaps the subject of another page if someone wants to look into that...?) but this page is about those hybrids that throw up traits that are not associated with either parent species.

An assumption that hybrids inherit their traits from their parents might be intuitive, but it's not always the case.  In this page we will give a few examples of hybrids that show traits that are not normally present in either parent species and which are not intermediate between traits shown be the parent species.

One such example is the pale cheek shown by a number of duck hybrids, especially ones involving Gadwall, like the Gadwall x Northern Shoveler shown at the top of this page.  Other examples of this are some Gadwall x Mallard hybrids and some Gadwall x Northern Pintail hybrids, like these:

Gadwall x Northern Pintail hybrid, Jumbo Reservoir (Colorado, USA), 25th March 2012 - copyright Steve Mlodinow
(photo ID: 1309)

Gadwall x Mallard hybrid, St Vrain State Park, Weld County (Colorado, USA), 10th April 2011 - copyright Steve Mlodinow
(photo ID: 1431)

It's worth remembering that any trait expressed in a bird (such as a pale buff-brown cheek) is not caused by a single gene but by many genes (I am no geneticist, so correct me if I go wrong).  In the Gadwall x Mallard hybrid you could maybe (I think it would be a stretch) argue that Gadwall has a pale cheek covered in dark speckling (making it look darker) and that Mallard has a dark cheek with no speckling, so the hybrid has inherited the pale cheek from the Gadwall and it has inherited the absence of dark speckling to make it look darker from the Mallard.  If that's not already too much of a stretch, it certainly would be for the Gadwall x Shoveler above which has a far paler cheek than a Gadwall, with or without speckling, but maybe the principle could still be part of the explanation?  Traits as we observe them could be made up of overlaying characteristics.  One set of genes might give rise to a whitish-brown element to the cheek, another set might give rise to a darker grey-brown element while another gives rise to the speckling.  If the hybrid only inherits the first then it could appear pale-cheeked.  I'm sure that's a gross over-simplification and may not be the explanation for this particular hybrid at all, but the principle that a trait as we observe it is made up of a number of underlying traits, only some of which may be inherited by a hybrid, is the point I am really trying to make.  If someone can explain it better, please do... or indeed if anyone can confirm (or refute) that this is indeed one cause for the phenonemon of hybrid showing traits not shared by either parent, then please shout!


Another possible cause (and again I invite you to support or refute this) is that the set of genes required for a bird to exhibit a trait may, if present, be either expressed (in which case we see the trait) or dormant (in which case we don't see the trait).  An example where I suspect this may be the case is the green headband of a Eurasian Wigeon.  Actually in this case it is sometimes expressed in pure Eurasian Wigeon (I wrote about that here) but it isn't most of the time, at least not much.  The fact that it is sometimes shown makes me suspect the set of genes responsible for it are always present but only sometimes expressed.  If that is the case, we might perhaps find that the set of genes are inherited by a Eurasian Wigeon hybrid.  Althouh it's not terribly distinct, that appears to be the case with this bird:

Gadwall x Eurasian Wigeon hybrid, Whitlingham Country Park (Norfolk, UK), 6th March 2011 - copyright Dave Appleton

(photo ID: 0311)

Perhaps related to this theory is the phenonemon whereby some adult hybrid geese show characters that are not normally shown by either adult parent, but which may be shown by juveniles of the parental species.  Many examples of adult hybrids between geese in the genus Anser and geese in the genus Branta show dusky marked undertail-coverts, yet no adult species with the genera (only Emperor Geese and some blue phase Snow Geese) show such markings.  On the other hand juveniles of some species may show darker markings on the undertail-coverts.

Greylag Goose x Canada Goose hybrid, Swanton Morley (Norfolk, UK), 6th April 2009 - copyright Dave Appleton
(photo ID: 0060)

Greylag Goose x Canada Goose hybrid, Swanton Morley (Norfolk, UK), 16th April 2011 - copyright Dave Appleton (photo ID: 0099)

Greylag Goose x Canada Goose hybrid, Skövde (Sweden), 19th September 2008 - copyright Carl Gunnar Gustavsson (photo ID: 1048)

Greylag Goose x Barnacle Goose hybrids, Malmoe (Sweden), 28th June 2008 - copyright Carl Gunnar Gustavsson
(photo IDs: 1060-1061)

probable Lesser White-fronted Goose x Barnacle Goose hybrid, Swanton Morley (Norfolk, UK), 27th October 2007 - copyright Dave Appleton
(photo ID: 0127)

I suspect a different explanation altogether may be responsible for this phenonemon in some other instances.  I'm not able to show a photo that illustrates this yet, but it appears that most hybrids between Ring-necked Duck and Tufted Duck ordinarily show a green head sheen.  Both parent species normally show a purple head sheen (though it may appear green in some lights) so this is not to be expected.  In this case the colour of the sheen is dependent on the structure of the feathers, not the pigment of the feathers.  I wonder if the fact that both parents share the same head colour means that they necessarily share the same feather structure?  Does anyone know that?  Or could it be that the structure is different, with both structures coincidentally resulting in a purple sheen, whereas the intermediate structure results in a green sheen?  That's all speculation, but I throw it out there in case anyone can shed any more light on it.



Specific identification dangers

In November 1988 a duck was discovered in Norfolk (UK) that was moulting out of eclipse plumage.  It showed a complex face pattern consisting of two large buff patches on the lower cheeks which were separated by a black vertical line, and a greenish tint behind the eye.  This, together with several other features, led to its identification as a Baikal Teal, an extremely rare vagrant to western Europe. Several hundred birders came to see this bird and went away happy that they had seen a Baikal Teal.  In January 1989 the bird was rediscovered at the same location.  By now it had moulted out of eclipse plumage and was clearly not a Baikal Teal!  Apparently it showed clear features of both Eurasian Teal and Northern Pintail so was considered to be a hybrid between these two species (source: Hybrid Duck resembling Baikal Teal by Steve Gantlett, Birding World 1.12: 426-427).

We now know that a number of dabbling duck hybrids show a bimaculated face pattern which may, more so in some than others, resemble that of Baikal Teal despite neither parent species showing such a bimaculated face pattern.



Oversize or undersize hybrids

Most hybrids are either the same size as one or both parent species or intermediate in size.  The example of Red-crested Cardinal x Yellow Cardinal hybrids referred to below is one where the hybrid can be larger than either parent.  I believe I have read of a gull hybrid that was larger than either parent, though cannot find the reference now.  If you have any  examples please let us know.



Heterosis and negative heterosis

In the "Handbook of Avian Hybrids of the World" (Oxford University Press 2006) Eugene M McCarthy describes the expression of parental traits in hybrids as being intermediate or combined, or sometimes heterotic.  In the example cited for heterotic traits, hybrids between Red-crested Cardinal and Yellow Cardinal can be larger and stronger than either parent.  He describes this as an example of positive heterosis, and mentions that negative heterosis may also occur where the hybrid's trait falls below the range of potential variation for either parent species.

I hesitate to copy McCarthy in using the term heterosis (and its associated adjective heterotic) for hybrids traits that are not shared by either parent species.  The reason for my caution is that heterosis seems to be widely understood (see various online dictionaries/Wikipedia for example) to describe traits that are beneficial or give rise to increased vigour (or decreased in the case of negative heterosis).  That might be assumed to be the case for a larger stronger hybrid cardinal, but it seems to me that many of the traits we see expressed in hybrids that are different from either parents are neither beneficial nor harmful but neutral.  Indeed for those hybrids that are infertile the benefit of specific traits is entirely academic anyway.  If traits arising from hybridisation are neutral, is the term heterosis still applicable?

Having said that, heterosity (is that a word?  I mean the extent to which a hybrid may be more or less vigorous than its parent species) is certainly of interest to the student of hybrids when it comes to those hybrids that are fertile.  The relative vigour of fertile hybrids when compared to either or both parent species may be of some academic interest but also of conservation significance.  Perhaps that should be the topic of another page...




Species mentioned:
Lesser White-fronted Goose Anser erythropus
Greylag Goose Anser anser
Barnacle Goose Branta leucopsis
Canada Goose Branta canadensis
Gadwall Anas strepera
Eurasian Wigeon Anas penelope
Mallard Anas platyrhynchos
Northern Shoveler Anas clypeata
Northern Pintail Anas acuta
Baikal Teal Anas formosa
Eurasian Teal Anas crecca
Ring-necked Duck Aythya collaris
Tufted Duck Aythya fuligula
Red-crested Cardinal Paroaria coronata
Yellow Cardinal Gubernatrix cristata