The shifting roles of intrinsic traits in determining seasonal feeding flock composition in seabirds

In order to enhance foraging efficiency, seabirds usually use local enhancement to locate their patchy prey, triggering the formation of multispecies feeding flocks (MSFFs). Although MSFFs have been widely documented, few studies have attempted to describe the temporal stability of species composition within MSFFs and which intrinsic factors (i.e., taxonomic identity, foraging guild, migratory timing, migratory origin, and body size) are involved. Using an 8-year (2006-2014) database of monthly seabird counts at an upwelling zone within the Humboldt Current in central Chile (33S), we showed that MSFF compositional stability varies seasonally, with more than 1 intrinsic factor involved. On the basis of the network analysis and null models, we showed that during austral winter taxonomic affinities are most likely to be determinant, with a high assortativity within Procellariiformes and among Charadriiformes species. During austral spring, foraging affinities are more important with a high assortativity within foraging guilds (surface feeders and pursuit divers). Timing and origin of migration scarcely explain species assortativity, whereas body size demonstrates to be an important trait, probably related with competitive skills. On the basis of assemblage stability, we suggest that during migration (austral autumn and spring) and reproductive seasons (austral summer), foraging seabirds associate randomly (within phenotypically or behaviorally similar species). During winter, species exhibited strong preferences to associate within taxonomic groups and presented the highest reassociation probability. This study demonstrates that MSFF, rather than being constituted by birds randomly aggregated at a food source, are most likely formed and structured by intrinsic traits whose relative importance shifts seasonally.