commonmoorhen

What is this strange goose?

Goose with speckled headThe Question: We saw this bird with a flock of Canada geese on Long Island, NY. What is it?

Submitted by: Jon-David, New York, USA

The Short Answer: This is a hybrid goose. Probably a cross between a Canada goose (Branta canadensis) and a snow goose (Chen caerulescens) or possibly one of the domesticated goose species that are raised in the United States. Hybridization among goose species is fairly common in the wild, but because we rarely witness the mating that leads to hybrid young, figuring out which species have crossed is generally a guess based on coloration. Often bird hybrids show coloration that is a mix of their two parents. For example, snow geese have a white neck and head and Canada geese have a black neck and head. This bird shows mottled black and white on the neck and head. Because Canada geese have become the most common geese in North America, most goose hybrids are crosses between Canada geese and another species.

More Information: When I say that hybridization among goose species is fairly common, I’m comparing geese to other birds and also to mammals. Hybridization among all the species of the family Anatidae (geese, ducks, swans) is more common than among other birds. And hybridization among birds is more common than among mammals. Why is that?

There are two probable methods of hybridization in geese. One is when a male goose copulates with an unwilling female of another species. I wrote recently about forced copulation in Mallard ducks, and much of what I wrote there applies to geese. As with ducks, forced copulations are relatively common in geese compared to other birds. Most forced copulations are within species, but males of some species will attempt to copulate with any similar species and some of these pairings might result in fertilization and hybrid goslings.

Alternatively, we occasionally see males and females of different species pairing up and mating and raising an entire brood of hybrid goslings. But why would they choose each other over members of their own species? Many goose and duck species “imprint” on their parents shortly after hatching. When they become mature, they search for mates that match the adults they saw in their first days after hatching. Normally, the first adults the newly hatched goslings see would be members of their own species and the imprinting process therefore prevents cross-species pairs.

But there is another behavior that is common in geese that can upend this process. Female geese often lay their eggs in the nests of other geese.  This allows a female goose to lay more eggs and have more offspring than she could protect or care for by herself.  Usually they lay their extra eggs in nests of the same species, but sometimes, by mistake or possibly on purpose, females lay eggs in the nests of other species of geese. When eggs laid in the nest of another species hatch, they may imprint on the parents of the “host” species, and the gosling may grow up to search for a mate from the species of its host parents – and not one of its actual species.

The result is a cross-species pairing. The work of Dr. Christoph Randler at the University of Leipzig suggests that this process of egg sharing and the resulting incorrect imprinting is the primary method by which hybridization occurs in geese.

But even if this kind of cross-species mating happens, it seems unlikely to create viable offspring. After all, the snow goose and Canada goose are separated by millions of years of evolution. How can they interbreed? Scientists who study how species remain isolated from each other talk about two kinds of barriers to hybridization: pre-zygotic and post-zygotic. Pre-zygotic barriers are things that tend to prevent the creation of a viable zygote. (A zygote is the cell formed by the merger of an egg and a sperm.) One example of a pre-zygotic barrier is geographic separation. If two species are on different continents and never come together, they can’t interbreed. Another example is courtship behavior. For example, if a female bird will only accept a male after he performs the correct song and dance routine, males of other species that don’t perform that routine won’t get the opportunity to mate with her. Imprinting among geese is another pre-zygotic behavioral barrier. Most geese will only mate with a member of the opposite sex who looks like the parents it saw after it hatched.

Post-zygotic barriers, on the other hand, are the result of incompatibility between sperm and egg. A male of one species may mate with a female and the egg and sperm join to create a zygote, but if the zygote doesn’t develop correctly, it won’t produce a viable embryo or juvenile.

It turns out that while birds have many pre-zygotic barriers to hybridization, they have fewer post-zygotic barriers than mammals. So if a male goose gets through all the pre-zygotic barriers by forcing a female goose of another species to copulate with it, chances are pretty good the zygote created will develop into a fully functional hybrid goose. Or, if the imprinting process goes awry because a female lays extra eggs in the nest of a female of another species, chances are that when the offspring chose a mate of the host species, the result will be fully functional hybrids.

If a male mammal of some species gets past pre-zygotic barriers and mates with a female of another species, chances are slim that the resulting zygote will develop. But why is that?

It may be a combination of factors. Because a mammal mother is connected to her developing offspring by a placenta, her immune system has direct contact with the developing embryo. If the embryo is too different from her, her immune system may well attack the embryo. Birds don’t have this issue because once the mother bird seals the fertilized egg into a shell and she lays it, her immune system no longer has any contact with it. It also appears that mammals rely more heavily on regulatory genes than birds do. These genes can diverge quickly over evolutionary time and create large differences in development. Mixing the regulatory genes of two different species in a single zygote probably causes havoc that results in a non-functioning embryo.

Also, birds as a group have an unusually stable arrangement of chromosomes, which means that chromosomal incompatibility creates fewer barriers to hybridization. In meiosis, chromosomes from the father and mother pair up in the process of creating a viable zygote. If the parents’ chromosomes don’t match, this usually creates problems. The well-known infertility of mules, a cross between a horse and a donkey, for example, is at least partly because horses have 32 pairs of chromosomes and donkeys have 31. The genetic information is very similar, it’s just arranged on chromosomes differently. Most mammal chromosome numbers fall in the range of 18 to 30 pairs (humans have 23). Most birds on the other hand, fall in a smaller range of 38-40 pairs, and all geese have 40 pairs. This means that problems with chromosomal pairing are less of an issue in hybridization of goose species.

For all these reasons, you are far more likely to see a snow goose-Canada goose hybrid than you are to see a hybrid between two species of mammal.

Sources:

Randler, C. “Do forced extrapair copulations and interspecific brood amalgamation facilitate natural hybridisation in wildfowl?.” Behaviour 142(2005):477-488.

Randler, C. (2006). Behavioural and ecological correlates of natural hybridization in birds. Ibis, 148(3), 459-467.

Randler, C. (2006). Extrapair paternity and hybridization in birds. Journal of avian biology, 37(1), 1-5.

Randler, C. (2008). Hybrid wildfowl in central europe – An overview. Waterbirds, 31(1), 143-146.

Kraus, van Hooft, P, Megens, H, et al. (2012). Widespread horizontal genomic exchange does not erode species barriers among sympatric ducks. BMC evolutionary biology, 12;45.

Ellegren, H. (2010). Evolutionary stasis: The stable chromosomes of birds. Trends in ecology & evolution, 25(5), 283-291.

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9 Responses

  1. Gestur Says:

    Jon-David, thanks much for this fine write up. I really enjoyed reading it. I came to this site because I’ve been seeing what I’m pretty sure is a cross between a Snow goose and a Canada goose here in Minneapolis. His legs and feet are very large and pinkish in color out of water. What I’ve been musing about after reading your piece is that both this year and last, he’s been swimming and eating moss with his girlfriend, who looks to be a true Canada goose (black legs and feet, classic white cheek patch). Moreover, I always see them alone, by themselves, as if they were a mating pair, not mixed in with a bunch of other geese. If so, and if they were to successfully mate, mightn’t that also be a way of getting some kind of cross between, in this case, a Snow goose/Canada goose cross and a Canada goose? And how do you reckon—what are the theories for why—she decided to hook up with this cross? [She was the offspring of a pure Canada goose pairing, perhaps her mother an unwilling mate, but raised by her mother and her mate, a Snow goose/Canada goose cross?]

  2. Gestur Says:

    Tom, oops I see now that Jon-David submitted the question not the response. Thanks again for writing this up.

  3. Tom Says:

    David, Sorry for the delay. This is pure speculation, but let’s try this. What if you’ve got a hybrid male, mated with a hybrid female? And then let’s imagine that a female Canada goose who was mated with a male Canada goose, decides to lay an egg in the hybrid family’s nest. If a female hatches from that egg, she might imprint on her hybrid parents and then look for a hybrid male for her mate. As to why she might do this, the theory is that imprinting is a method of reproductive isolation that normally prevents hybridization between two species. And it probably works 99.9% of the time. But the situation we’re hypothesizing is one of those in which it wouldn’t work.

  4. Gestur Says:

    Tom, thanks for your thoughtful response to my queries. I think your speculation has all the bases covered, although as you note, with very slim odds of occurrence. I was wondering if you would perhaps also respond to the premise of my speculative question, namely that Oscar (I’ve taken to calling him Oscar, as in Oscar de la Renta, the sharp dresser) and his girlfriend might successfully mate. In an email I sent around describing this little experience with Oscar, a responder reminded me that interspecies hybrids are usually sterile. If that’s so here, why is Oscar’s girlfriend, a true Canada goose, hanging around? Specifically, let’s assume your speculation here is the case and so she is imprinted to seek out a hybrid mate like Oscar. So my question is: Do you think that repeated failures of mating with Oscar (to result in a brood of fun little goslings) might ever cause her to leave? Or do you think that in this rare case, imprinting would trump anything like infertility concerns, even assuming she could find another hybrid mate without any social media to turn to. And lastly and more generally, is there any evidence that ‘normal’ matings between pairs of Canada geese ever come to an end because of infertility?

    Gestur

  5. Tom Says:

    The idea that interspecies hybrids are usually sterile applies to mammals, but as my article pointed out, not as much to birds. And certainly not to plants. So that rule is not universal. Hybrids between goose species are relatively common in the wild. So there’s a fair chance that Oscar and his girlfriend will have goslings. However, pairs do often split if there are no offspring, whether it’s a hybrid pair or a pure-bred pair, so if for some reason they aren’t successful, they probably will split up.

  6. Gestur Says:

    Tom, thanks so much for this—once again—very informative response to my question, and also for your forbearance.

  7. liz Says:

    a question – my neighbor saw a bird, larger than a Canada goose which we have plenty of here and a duck – black in color long neck, very aggressive. What do you suppose it is? I asked her if, perhaps, it was a swan – she said definitely not. We are in the Rochester NY area and have a very large pond behind our houses that are stocked with fish and it had half of a large gold fish in its mouth. Any suggestions?

  8. Tom of AskaNaturalist.com Says:

    Well, if it wasn’t a swan, and if it was catching fish, it does not sound like a swan, then my first guess would be a great blue heron. They can look very dark sometimes, especially if the light is dim. If she can get a picture, have her email it to me at tom@askanaturalist.com

  9. Eliza Says:

    I have a female Canadian goose on a nest in our pond and I haven’t seen the gander in almost a week. Could he have left her?
    She is sitting on at least 6 eggs….is it normal for the gander to leave? I feel bad for her….won’t she get hungry without her mate…doesn’t she need a break to feed from sitting on the eggs?

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