Neighborhood effects in mixed-species flocks affect foraging efficiency of intermediate and little egrets

Abstract

Birds aggregate for various survival needs, and ardeids form both conspecific and mixed-species flocks. However, it remains unclear whether and how ardeids’ foraging while joining a flock is affected differently in conspecific versus mixed-species flocks. We studied egret flocks in southwestern Taiwan to assess the flock size and neighborhood effects on flock formation and foraging performance of egrets by examining landing tendencies and expelling behaviors of Little (Egretta garzetta) and Intermediate (E. intermedia) egrets, and assessing strike rates, strike efficiency, and intake rates of singles and egrets in conspecific and mixed-species flocks. We found no evidence of non-random landings to mixed-species flocks by either species, but egrets showed a trend of landing closer to conspecifics when conspecifics represented lower proportions in flocks. Intermediate Egrets displayed expelling behavior more frequently. Expelling by earlier arrived egrets interrupted subsequent flock formation on 45.2 % occasions, but was not affected by egrets’ foraging performance. Solitary Little Egrets had higher strike and fish intake rates, but lower strike efficiency, than solitary Intermediate Egrets, whereas their biomass intake rates were similar. Strike and fish intake rates increased in flocks compared to singles in both species, but declined for Little Egrets as flocks increased from medium (11–30 birds) to large sizes (> 30). Intermediate Egrets had higher strike, fish intake, and biomass intake rates, but Little Egrets had lowered strike efficiency and fish- and biomass-intake rates, when near each other than when near conspecifics. Overall, Intermediate Egrets in mixed-species flocks improved foraging efficiency and food intakes by neighborhood effects at costs of reduced foraging efficiency to Little Egrets. The variation in landing tendency over flock compositions suggests potential interactions of the neighborhood effect with egrets’ conspecific attraction.