PARENT SESSION
Oral Session #34: Animal Ecology: Behavior and Sociobiology.
Presiding: S. Richards
Tuesday, August 6. 8:00 AM to 11:30 AM. Grand Ballroom East, Radisson.
Do birds of a feather flock together? An experimental study of mixed-species flock formation.
Ferraz, Gonçalo*,1, 1 Columbia University, New York, NY
ABSTRACT- There has been much debate over the role of anti-predator protection and foraging success as benefits of participation in mixed-species flocks. I conducted a three-year field experiment to study the effect of distribution and abundance of food on mixed-species flock formation. I observed Parus atricapillus, Sitta carolinensis, Parus bicolor, Picoides pubescens, Regulus satrapa, and Certhia americana, in a mixed coniferous-deciduous forest for a total of 125 days. Over 80% of Parus atricapillus, Sitta carolinensis, and Parus bicolor individuals were individually marked with color bands. The data indicate timing and location of presence and absence of birds in two separate sites, under three experimental food-addition treatments. Food was placed in 32 feeders dispersed throughout each site. The treatments were: no food (baseline), uniformly distributed superabundant food, and randomly distributed limited food. Mixed-species flocking was measured as the proportion of inter-specific pair-wise encounters, out of all observed pair-wise encounters during a given period. Three randomization tests produced the following results: At baseline, birds, regardless of species, do form flocks more often than expected if individuals occurred independently and at random. Also at baseline, given a pattern of species abundance and individual occurrence, species mix approximately as much as expected when the species identity of observed individuals is assigned at random. The effect of food availability on mixed flock formation is mediated by the distribution and abundance of food. Uniform superabundant food led to no change or to an increase in mixed flocking. Limited food, randomly distributed, led to a decrease in the formation of mixed flocks. I contrast my observations with previously published experimental results and suggest a mechanism of mixed-flock formation that is limited by the cost of keeping up with fast moving heterospecific flock mates.
KEY WORDS: mixed-species flocking, foraging behavior, sociality, Parus atricapillus