Some Canada Geese (Branta canadensis) raise their broods by themselves (two-parent families), while others raise them in gang broods, defined as two or more broods amalgamated into a single cohesive unit and shepherded by four or more parents. From 1984 to 2005, I individually marked Canada Geese in New Haven County, Connecticut, so that I could compare the characteristics of adults that raise their goslings in gang broods to those of adults that raised their goslings in two-parent families. I wanted to determine if a parent's decision to form a gang brood was influenced by its age or body mass, its own parents (indicating either that the behavior has a genetic component or that the behavior is learned while a gosling), its prior experiences raising broods, or the loss of its mate. Parents tended to use the same brood-rearing approach from one year to the next: 61% of parents of gang broods (i.e., gang-brooders) during one year also were gang-brooders the next year they had goslings; likewise, 65% of parents in two-parent families during one year raised their next brood in a two-parent family. Geese that changed mates from the previous year were more likely to switch brood-rearing approaches than those that stayed with the same mate. As geese gained more years of experience raising goslings, their propensity to form a gang brood increased; only 29% of geese raising broods for the first time formed a gang brood versus 80% for geese with 5 or more years of experience. Geese raised in gang broods themselves were no more likely than geese raised in two-parent families to form gang broods once they became adults and had their own broods. These results indicate that gang brooding is a behavior learned as an adult. I tested the hypothesis that adult geese attending the same gang brood are members of the same extended family but found that geese were as likely to form a gang brood with unrelated individuals as with siblings or parents.
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Vol. 111 • No. 2