First, a recap: At 6:49PM PST, an unknown goose settled on Wilma’s eggs. Wilma spent the next 27 minutes trying to evict the invader by nibbling and chewing on her side, wings, and neck. The intruder eventually left without responding to Wilma’s defense. Wilma immediately settled back down over her eggs, incorporating the down she’d plucked from the invader into her nest. Fred didn’t attempt to drive the invader or Wilma from his nest, and there was no interaction with or harassment from other geese. So what was going on? And why didn’t Fred help?
Canada Geese and Reproduction
At one point, it was thought that Canada geese were strictly monogamous. Close observation has yielded a more nuanced picture. Most Canada geese do not breed until their fourth year, but younger geese may form unstable pair bonds and simulate or engage in breeding behaviors, including copulation. Although sexually mature Canada geese tend toward monogamy, extra-pair copulation and intra-specific brood parasitism (egg dumping) occur at higher rates than once thought. A study of 253 free-living Canada geese in 42 clutches over 3 breeding seasons found that:
- About 60% of clutches were free of parasitism or extra-pair fertilization
- About 14% of clutches included eggs that were unrelated to the host male, which indicates extra-pair fertilization on the female's part
- About 26% of clutches included eggs that were unrelated or semi-related to the host female, which indicates parasitism or egg-dumping on the part of another female
The same study revealed that egg dumpers tended to be closely related to egg dumpees. Is this a deliberate collaboration between mothers, sisters, and other closely related females, or simply a consequence of families living closely together? It’s an intriguing question and one that deserves more study, especially since family size, aggression, and social dominance all impact nest success and survivability of young. Eaglecrest’s facilitators told me that they believed the invader was a related bird – possibly a daughter – and similar behavior was observed last year as well. So was this a related adult goose looking to dump an egg in Wilma’s nest? If so, she failed. Seven eggs were counted in the nest prior to and immediately after the intrusion.
Could the invader have been an immature goose? Younger geese have been witnessed engaging in adult behaviors, including copulation and pair-bonding. Bonded youngsters have been observed maintaining territory and building nests, although they don’t lay eggs. At least one observer believed this was an immature bird. If she was, it might explain why she attempted (but failed) to drop an egg. Perhaps hormonal activity (and maybe immature pair-bonding?) compelled her to try to lay or incubate eggs she didn’t have.
Sheer Speculation #2: The invader was attempting to take over the nest site and pair with Fred
While Canada geese aren’t as monogamous as we once thought, pair-bonding and family are extremely important in their lives. According to one study, single immature Canada geese were 1.44 times more likely to die or disappear than immature Canada geese that belonged to a family. Social status is tied to family, and family is a matter of life and death. Perhaps our invader didn't have a family or had recently lost a mate. She may have been trying to pair with Fred. Alternatively, she could have been under a compulsion. If her previous mate and nest had been largely destroyed shortly after the onset of incubation, she might have been hormonally compelled to incubate eggs, which would explain her persistence. Even more strangely, she might have fixated on Fred or the nest itself. Collias and Jahn wrote that “Subordination to dominating geese, inability to establish a foothold against the resistance of territorial incumbents, and unsuccessful fixations on certain nest sites or on certain individuals as potential mates, all serve to delay…effective breeding by many individuals.” This seems like the less likely scenario, since unpaired geese don’t usually disturb nesting pairs. But I can't rule it out, either. Maybe this kind of behavior is more common than was once believed, or has become more common as populations have expanded and increased in density. Only the geese know for sure.
So why didn't Fred help Wilma? And why didn't the invading female respond to Wilma?
Male geese protect their mates from a variety of things, including other male geese. Male geese have been known to kill one another over mates, harass and disrupt nesting, and engage in extra-pair fertilizations. By defending his mate from other males, Fred assures that the goslings hatch successfully and are all his. A non-aggressive, presumably unmated female was no threat to Fred's paternity or eggs. However, she was clearly a threat to Wilma, who responded accordingly. We've also been asked why the invading female didn't respond to Wilma's pecking and down pulling with hissing, flapping, or honking. Perhaps she was trying to prevent triggering an aggressive response from the larger and more powerful Fred. Similar 'quiet' strategies have been noted in Canada geese who are accompanying newly hatched goslings on their first trip from the nest. By remaining quiet, parents avoid triggering an aggressive response from other nesting geese that could damage or kill their young.
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.” I look forward to seeing what happens once the nests start hatching.
Resources
Social Behavior and Breeding Success in Canada Geese (Branta canadensis) Confined under Semi-Natural Conditions
Nicholas E. Collias and Laurence R. Jahn
The Auk
Vol. 76, No. 4 (Oct., 1959), pp. 478-509
Published by: University of California Press
Article Stable URL: http://www.jstor.org/stable/4082315
Reproductive Success and Survival in Relation to Experience during the First Two Years in Canada Geese
Dennis G. Raveling, James S. Sedinger and Devin S. Johnson
The Condor , Vol. 102, No. 4 (Nov., 2000) , pp. 941-945
Published by: University of California Press on behalf of the Cooper Ornithological Society
Article DOI: 10.2307/1370326
Article Stable URL: http://www.jstor.org/stable/1370326
Some Associations of Behavior to Reproductive Development in Canada Geese
Jack S. Wood
The Journal of Wildlife Management , Vol. 29, No. 2 (Apr., 1965) , pp. 237-244
Published by: Wiley on behalf of the Wildlife Society
Article DOI: 10.2307/3798427
Article Stable URL: http://www.jstor.org/stable/3798427
The History and Breeding Biology of the Canada Geese of Marshy Point, Manitoba
James A. Cooper
Wildlife Monographs , No. 61, The History and Breeding Biology of the Canada Geese of Marshy Point, Manitoba (Jul., 1978) , pp. 3-87
Published by: Wiley on behalf of the Wildlife Society
Article Stable URL: http://www.jstor.org/stable/3830608
http://www.science20.com/anthrophysis/urban_goose_reproductive_decisions_are_not_impacted_their_population_densities-86374