The costs of raising nidifugous offspring: brood rearing by giant Canada geese (Branta canadensis maxima)

Competing hypotheses that have been advanced to explain the phenomenon of posthatch brood mixing by waterfowl can be distinguished by whether they assume that adults experience costs in rearing nidifugous offspring. To test this, time budget data were collected for giant Canada geese (Branta canadensis maxima) at Cambridge, Ontario, in 1990. Breeding adults with broods devoted more time to vigilance (p = 0.001) and less time to feeding (p = 0.001) than adults that hatched clutches but were without broods, suggesting a cost to rearing nidifugous young. However, as goslings matured, parents allocated less time to vigilance (p < 0.001) and more time to locomotion (p = 0.005), and time spent feeding did not change (p = 0.336). In addition, brood size did not affect the time parents allocated to vigilance (p = 0.543) or feeding (p = 0.727), suggesting that caring for additional young has negligible effects on parents. Goslings were selective about the adult with which they associated (they were positioned closer to females than to males), but neither brood size nor brood age affected the feeding time of goslings (p = 0.94 and 0.76, respectively) or time spent vigilant (p = 0.22 and 0.69, respectively), suggesting that goslings gained no obvious advantage from greater foraging opportunities or better predator detection by congregating in larger broods.

Mate retention in Giant Canada Geese

Of 73 pairs of Giant Canada Geese (Branta canadensis maxima) in which both members of the pair were marked with individually identifiable neckbands, 68 (93%) remained together for as long as both members were alive or retained their markers. Collectively, these data represent 183 years of pair histories (sum of pairs × years of records). One pair that had raised a brood in the summer they were captured and marked separated during the next breeding season; neither re-paired with another bird, and then they rejoined the following autumn and remained together for five consecutive breeding seasons (and winters). Members of four (5.5%) pairs obtained new mates while their former mates were still alive. Of 17 surviving members of pairs in which the mate died (or disappeared) over winter, 15 (88%) re-paired. Data on reproductive success of 4 of these new pairs were not obtained, but 10 of 11 of the other new pairs successfully reared broods.