Trade-offs across life history stages and social association types shape winter communal roosting in a long-lived raptor.

Social interactions among conspecifics can have significant fitness implications, but how social behaviours develop in wild animals remains poorly understood. Here, we examine the intrinsic drivers of a social behaviour, communal winter roosting, in red kites Milvus milvus. In this species, communal roosting is a facultative behaviour, and the mechanisms underlying its emergence at the individual and population levels are unclear. Through longitudinal GPS tracking of 635 bird-winters from 216 red kites, we derived multi-year roosting histories to investigate (i) which individual characteristics associate with the decision to join communal roosts, (ii) how these patterns change across life stages and (iii) whether roost composition reflects assortative associations among breeding pairs or kin. Based on 33,930 nights across six consecutive winters on the breeding grounds, we identified red kites from our tagged sample joining communal roosts on average 38% of the time. Males occurred more at communal roosts than females, but in both sexes this probability drastically decreased with age and additionally decreased once they started breeding. These ontogenetic changes in communal roosting behaviour were driven by behavioural plasticity at the individual level rather than selective mortality. Red kites displayed assortative behaviour both in communal and non-communal roosting contexts. Breeding pairs showed the strongest affiliation, roosting more often together than expected by chance in non-communal roosting sites, when in proximity to their breeding territory. In contrast, sibling and parent-offspring dyads were rare, and roosting less frequently together than expected by chance within communal roosts. Overall, our results show that the structure of communal roosts in the red kite is shaped by the age, sex and social relationships of individuals. The influence of these factors may stem from trade-offs across various life history stages, driven by changes in the net benefits associated with foraging, territory and mate prospecting, as well as territory maintenance throughout an individual's life.