The survival value of flocking in neotropical birds: Reality or fiction?

Greater foraging efficiency and/or better predator avoidance have long been assumed and used as explanations for the evolution of flocking behavior in birds. Even if the debate between the validity of these two hypotheses rem ains open, one prediction is that living in flocks can favor increased surv ival rates. We gathered published and unpublished data from various tropica l forests where bird species forage (1) exclusively alone or in pairs (2) i n heterospecific flocks some of the time (facultative flock members), or (3 ) exclusively in heterospecific year-long associations (obligate flock memb ers). We controlled statistically for effects of body size, nest type, clut ch size, and phylogeny, and tested whether survival rates differed among th ese three groups. The survival rates of the obligate flock members (mean su rvival rate 68.7%, range 48.0-87.0%) were significantly higher than estimat es for the species feeding alone or in pairs (mean survival rate 58.3%, ran ge 33.0-79.0%). However, survival rates of the facultative hock members (me an survival rate 60.4%, range 40.0-79.0%) did not differ from those of the nonflocking species. Nevertheless, causes of such differences in survival c an be explained by an alternative hypothesis. Life history theory predicts that higher survival for the obligate flocking species may be a response to low fecundity and productivity. Yet, the pattern "high survivorship, strik ingly low fecundity" documented in obligate flocking species has never been observed among their solitary or facultative flocking counterparts. This r esult suggests that permanent obligate flocking can be an alternative ecolo gical factor that may drive the evolution of life histories in tropical bir ds.