Ecological and behavioral correlates of individual flocking propensity of a tropical songbird

Bird flocks are characteristic of environments worldwide, and there is a rich literature investigating the purported costs and benefits for attendant species. Few studies, however, have investigated the potential sources of individual variation in flocking behavior. We hypothesized that flocking propensity would vary among individuals as a function of intrinsic, environmental, and social characteristics, as well as flock-joining opportunities as a function of home range size and number of flocks present. We tracked wedge-billed woodcreepers (Glyphorynchus spirurus) using radiotelemetry from January-February, 2011 and January-March, 2012 at Tiputini Biodiversity Station in eastern Ecuador. Individual home ranges overlapped a mean of 2.6 (0.2 standard error [SE]) individuals by a mean of 44% (0.09 SE) per overlapping individual. Flocking propensity was 32% in 2011 and 20% in 2012, with considerable individual variation (2-35%). Expected flocking propensity was 45% based on average home range size and number of flocks in the study area; assuming that dominant individuals excluded subordinates in areas of home range overlap reduced the predicted value to 35%. Individual flocking propensity was positively correlated with body condition, frequency of aggressive behavior, and probability of joining a flock given presence, and negatively correlated with number of overlapping conspecifics. Our results suggest that woodcreepers compete for access to flocks within areas of home range overlap, which results in limited opportunities for some individuals to join flocks. The costs and benefits of group behavior may, therefore, vary among individuals within a species.