Rise of the Planet of the Geese

"Never believe what you read on the internet- and yup, that includes my blog."

My tongue-in-cheek response to a birder at Motney. We'd been watching one of the Canada x Greylag hybrids and they started going on about 'hybrid swarms' and growing threats to wild geese species. It turned out they'd read about it in a chat group just this morning. This isn't the post I'd planned for today, but a good opportunity to check my understanding. Because I'm no scientist, but...

Haldane's Rule - if in a species hybrid only one sex is inviable (or sterile) that sex is more likely to be the heterogametic sex.

Whaa? Well, ducks and geese have a long history of interbreeding. Why aren't we overrun with hybrids already?

Studies have already proven that cross-breeding is much more common than we suspect in birds, but there is a weakness in female hybrid offspring that halts proceedings. It isn't that we're overlooking female hybrids, there just simply anywhere near as many out there. This lack of hybrid fitness halts hybrid speciation. Haldane's rule says that (for several science-y type reasons) in certain hybrid types one sex will be always much less successful than the other- the heterogametic sex, the one which has sex chromosomes that differ in morphology which result in two different kinds of gamete- in mammals, that's the male and in birds, that's the female.

A hybrid swarm is slightly different from a new hybrid species- it is a viable mixed population that survives through generations- and geese really don't without any addition of new stock. We can see so many hybrids because they survive through a generation or two, so individuals can be about for quite a period of our own birding careers. I reminded him of longevity in geese- they do reach their mid-twenties. And our feral populations will keep adding a few more hybrids. That mixed-up goose we'd been admiring might be about here for years to come. But such birds will be nearly all male, and not about to take over any time soon.

They then said they'd also read there were concerns over our hybrids going north and infecting populations there.

My reply included why are there so few subspecies in ducks and geese- in subspecies, birds from one (say, Nearctic) population do picked up by another population (say Palearctic) on the wintering grounds and do travel and interbreed- but genetics then deal with them over time and they, for the main, remain global, monotypic species.

Now those successful pick-ups would be happening from other migratory populations. I pointed out most migratory tendencies are genetic- so any mucky Medway hybrid was still more likely to think really not worth going all that way north just for a few weeks of nesting fun with an exotic bird, much easier to stay at home with their cousin...

I left them still wholly unconvinced, still fearing some sort of imminent rising up of the Heinz 57 variety. Thank goodness we hadn't been down the Strand having this conversation- the local Sheldwot really would have given them the heebie-jeebies...