THE COMPOSITION AND SOCIAL-ORGANIZATION OF MIXED-SPECIES FLOCKS IN A TROPICAL DECIDUOUS FOREST IN WESTERN MEXICO

I recorded the flocking propensity of birds within a tropical deciduous forest in western Mexico during the nonbreeding season, and determined the species composition of 57 mixed-species, canopy insectivore flocks. Each of 27 canopy insectivore species present on the study area was observed foraging in mixed-species flocks on at least half of the occasions that it was detected on bird surveys. The proportion of flocks within which a given species was detected could be predicted on the basis of its index of abundance, as determined from independently derived point count data. Therefore, flocks are not comprised of a special subset of canopy insectivores; rather, the composition of flocks appears to be a product of whichever species co-occur within the foraging range of one or more nuclear species. No two canopy insectivore species were negatively associated among flocks, but 40 of 210 possible pairs (19%) were significantly positively associated. Two long-distance migratory species (Nashville Warbler [ Vermivora ruficapilla] and Blue-gray Gnatcatcher [Polioptila caerulea]) shared features that characterize nuclear species, thereby constituting one of the first recorded instances where nonresident species play such a role. Five of the more common flocking species were equally likely to have a foraging neighbor nearby (< 3 m away), but the identity of that neighbor differed significantly among the five species. Specifically, the identity of close neighbors of the two nuclear species was a random subset of the species available, while the close associates of three attendant species were a nonrandom subset of (mostly) other attendant species. The independent associations among flocks, which characterize most species, and the observation that nuclear species were not close neighbors more than expected for any of three attendant species suggests that foraging enhancement is not the principal benefit that attendant species derive from flocking.