What’s Up With the Geese? Canada Geese, Reproduction, and Conspecific Brood Parasitism

On Saturday April 1, goose watchers at N2B saw a female goose fly into the nest. After a lot of biting, pushing, and tussling, she laid an egg despite MG and PG’s best attempts to repel her. One of the local eggs dropped out of the nest, although MG and PG ended up incorporating the intruder’s egg into their own clutch.

At one point, it was thought that Canada geese were strictly monogamous. Close observation has yielded a more nuanced picture. Although Canada geese are generally monogamous, extra-pair copulation and conspecific brood parasitism, which is what we saw here, occur at higher rates than researchers and observers initially realized.

Canada Geese, Reproduction, and Conspecific Brood Parasitism

A study of 253 free-living Canada geese in 42 clutches over three breeding seasons found that:

  • About 60% of clutches were free of extrapair fertilization and conspecific parasitism
  • About 14% of clutches included eggs that were related to the host male but unrelated to the host male, indicating extra-pair fertilization (what some of you call ‘cheating’) on the female’s part.
  • About 26% of clutches included eggs that were unrelated or semi-related to the host female, indicating conspecific brood parasitism.

Why would a goose lay an egg in another goose’s nest, especially given the likelihood that it won’t hatch in sync with the local eggs? In most species, CBP occurs when nest loss, social interactions, nest-site availability, food availability, or environmental conditions prevent birds from laying eggs and rearing offspring in their own nests. The invading goose might have lost her nest or lacked a nest entirely: extrapair copulation could presumably initiate egg production in an unmated, un-nested goose. Whatever happened, the invading goose had an egg to lay, and she was determined to lay it in a nest!

April 1, 2024: The geese at N2B defend their nest from another group.
April 1, 2024: The geese at N2B defend their nest from another goose.

The study cited above also found a high level of relatedness between nesting mothers and nest parasitizers, although we don’t know that this is intentional. If female Canada geese return to their natal breeding areas to nest, the percentage of related females will increase over time, statistically increasing the likelihood of a parasitic female being related to her host by chance. Related or not, neither resident goose was excited about the intruder and one of their eggs was lost in the scuffle, decreasing productivity by one before hatch even started.

What’s up with Anatidae?
April 1, 2024: the resident geese struggle with the intruder.
April 1, 2024: The resident geese struggle with the intruder.

Roughly 256 species of bird engage in conspecific brood parasitism. CBP is more common in precocial birds than altricial birds, and some precocial families have very high rates. Anatidae, which includes ducks, geese, and swans, have CBP levels of around 60%. By contrast, CBP levels in Falconidae and Accipitridae – altricial, non-social breeders with small clutch sizes – are very low.  Note how highly correlated both precociality and CPB are with birds that aren’t classified as neoaves in the avian tree of life.

Why do Anatidae have such high levels of CBP? Short answer: we don’t know. However, several families with high CBP nest in groups. A tendency towards sociality could increase CBP even when pairs establish and defend their own nests. More geese might equal fewer nesting territories, more extrapair copulation, and more social interactions overall.  Waterfowl also nest in highly variable environments. Phenotypic or facultative plasticity – adopting different behaviors or strategies based on environmental conditions – would be a very helpful trait in a species whose nesting grounds might be ideal one year and completely underwater the next. Can’t find a place to lay your eggs because nesting habitat is so reduced? Lay them in someone else’s nest! While this is a risky strategy, yet another study found that 28% of eggs laid by moorhens in another nest hatched: a steep reduction, but greater than none. Conspecific brood parasitism isn’t as good as laying eggs in one’s own nest, but it’s better than laying them in no nest at all.

Food for Thought

Most intriguingly to me, researchers have studied whether birds could have inherited parasitic nesting from their dinosaur ancestors. On a recent peregrine survey, I wondered what reproductive and migratory behaviors birds might have inherited from dinosaurs after watching birds chase, court, and copulate. Eagles were migrating and talon-locking, peregrines were diving and chasing, and everyone was bursting with spring fever! Were there temperate-zone species in the late Cretaceous that threw a circannually-entrained sexfest every spring? Were some dinosaur species loudly bellowing, roaring, or hooting their prowess and availability from every hilltop? Did some species migrate or irrupt in search of food? Were they sparring with one another as they went? Hissing, biting and pecking to prevent brood parasitism? Give us the diorama, please!

In the spring, a young dinosaur's thoughts lightly turn to love. Image by Firefly AI.
The diorama! Food gifts. Vocalizing! Bright colors. Spring flowers! Firefly AI attempts to render dinosaurs behaving like spring birds, even though these dinosaurs don’t look very bird-like.

It can be disappointing to find out that birds don’t necessarily model human ideals of fidelity or cooperation, especially when the birds in question have a reputation for monogamous behavior. I think professor and ornithologist Bridget Stutchbury put it very well when she wrote “There are many similarities in how competition and conflict have shaped the evolution of behavior among animals, but we must not forget that birds do not have the same feelings, thoughts, or decision-making processes as humans.”

Things That Helped Me Learn

The effects of anthropogenic alteration of nesting habitat on rates of extra-pair fertilization and intraspecific brood parasitism in Canada Geese Branta canadensis – https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1474-919X.2011.01203.x

Conspecific Brood Parasitism: Reevaluating a Bird’s-Eye View – https://www.proquest.com/openview/e87c3c0109f5a3ef33f4049f8c162b1c/1?pq-origsite=gscholar&cbl=18750&diss=y

Conspecific Brood Parasitism Among Birds: The Effects of Phylogeny, Mode of Reproduction and Geographic Distribution – https://www.researchgate.net/publication/324905224_Conspecific_Brood_Parasitism_Among_Birds_The_Effects_of_Phylogeny_Mode_of_Reproduction_and_Geographic_Distribution#pf4

I was fascinated and somewhat dismayed to learn about the BOBJ theory proposed by Richard Dawkins, a theory that individuals with an average fitness that is less than the population average fitness persist within the population by making the best of a bad job. All individuals in a population are presumed to assess their potential fitness-enhancing opportunities in terms of their physical condition, social status, or probability of success, and subsequently make behavioral or developmental “decisions” that lead to greater success than if this choice had not been made. But ‘fitness’ and ‘success’ are defined in a very alpha vs beta king-of-the-mountain way reminiscent of the most harmful corners of internet dialogue. How do we know the invading goose had lower ‘fitness’ or what that even means to a goose? Why is helping at a relative’s nest versus having one’s own young classified as ‘unfit’? I’m not saying that birds don’t seek awesome mates and excellent nesting sites. But BOBJ seems like a very reductive view of fitness and birds.