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).
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)
(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?
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.
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)