Transition from incubation to brood-rearing activities influences dyadic associations differently between the sexes in a central-place foraging seabird.

1. Changes in activities following seasonal cycles can affect social interactions, which in turn influence social network structures. Because such activities are often sex-related, their effect on the social environment can affect males and females differently, particularly during periods of high activity such as the breeding season. While the effects of sex-related activities on the social network structure of species with distinct roles have been studied, their effects on species with similar roles remain unexplored. 2. We interrogated the role played by sex differences in breeding activities across the reproductive season on the expression of social network structure in a colonial seabird. 3. We studied associations between Atlantic puffins (Fratercula arctica) by colour-banding 120 sexed individuals and conducting focal observations during the breeding season. We tested how different networks, built on consecutive time series, changed from each other. 4. We found that the social network structure changed during the breeding season differently between males and females. The mean hatching date marked the turning point between two phases with distinctive social network structures: one during incubation and one during chick rearing. During incubation, social network traits changed similarly for males and females. During chick rearing, sociality increased, with males gaining higher values of degrees and betweenness than females. Network strength increased during the hatching period until the peak hatching date when it suddenly dropped back to its baseline value. 5. Our study demonstrates that Atlantic puffins have higher sociality during the chick-rearing period than during incubation and are associated more often with fewer conspecifics at hatching. Because sex-related activities in the Atlantic puffin are unclear, we highlight the importance of having an accurate baseline of sex-related roles to interpret social structures.