PS 47-120 - Asymmetric biparental care during the breeding season in a large seabird

Background/Question/Methods

Although about 80% of bird species exhibit biparental care, the investment of each partner is not necessarily equal. Sex-specific foraging strategies have been reported in a wide range of sexually size-dimorphic species. Various mechanisms, such as foraging efficiency, flight constraints, or competitive abilities for food, have been suggested to explain a significant amount of the observed variation of parental investment between males and females. Yet, same results are also found in monomorphic species, as in many seabirds, highlighting that anisogamy and/or other mechanisms may underlie sex-specific foraging patterns. In this study, we examined the foraging effort of northern gannets (Morus bassanus) at Bonaventure Island (Quebec, Canada) to determine to which extent sex might influence foraging behaviour in a species with little or no sexual dimorphism throughout the breeding season. In contrast to most studies that have addressed this question using samples of males and females stemming from different nests, a pair-based approach was used here. Forty-seven pairs of gannets were fitted with miniaturized GPS at different breeding stages (incubation, early chick-rearing and late chick-rearing) to monitor their activity at sea over 5 years (2016 to 2020). Body measurements were also recorded to assess sexual dimorphism in this species.

Results/Conclusions

Females are about 15% heavier than males from incubation to chick fledging, despite the absence of a significant difference in wingspan. Although foraging effort was higher during the chick-rearing period for all individuals, females made significantly longer trips and tended to travel greater distances than males, without having a larger home range. However, they spent relatively less time fishing than males during foraging trips and more time resting. This suggests that in females, the higher initial cost of egg production may be offset by greater efficiency in prey capture, rather than an increased effort in time spent searching for food and feeding. The same conclusion can be drawn for the maintenance of a higher body mass in females throughout the breeding season, which does not seem to require an increased effort in comparison to males. An alternative hypothesis could be that females invest more in their own maintenance and less in chick growth to compensate for the energetic investment related to egg production. A more detailed insight into foraging behaviour focusing simultaneously on foraging areas, diving behaviour, and amount of food delivered to the chick would provide a better understanding of sex-specific behavioural flexibility during the breeding season.