The ecological significance of extremely large flocks of birds

Population size is generally limited by resource availability during and outside the breeding season. Therefore, maximum size of flocks may provide important information on population regulation and the influence of diet and trophic level on maximal degree of sociality. We hypothesized that (a) flock size should increase with nutrient availability; (b) flock size should decrease with latitude because productivity is higher at lower latitude; (c) aquatic habitats should have larger flocks than terrestrial habitats because the former are less accessible; (d) smaller species should have larger flocks because they require overall less food; (e) human-impacted species that live in perturbed habitats should have smaller flocks than other species; (f) flock size should decrease with increasing trophic level because there is a reduction in biomass due to conversion at each trophic level; and (g) flocks of species depending on ancestral landscapes should have decreased in size in recent years due to human impact (e.g., land-use). We obtained 1564 observations of flocks that exceeded 100,000 individuals in order to test the predictions listed above. Most effect sizes were small to medium accounting for 1%-9% of the variance, while large effects accounting for 25% or more were only found for total nitrogen used per km² and area used for agriculture. Changes in large bird flocks were caused by habitat degradation and persecution, and temporal decline in size of large flocks revealed changes in nutrient use, reductions in nutrient cycling, and changes in flock size linked to trophic level.