Protective nesting associations in birds

Exploiting the nest defence behaviour of another relatively aggressive species is one of the most unusual amongst a great diversity of strategies used by animals to evade predation. Here we review 62 studies that have looked at protective nesting associations in which at least one species is a bird. Most ‘protected’ associates are found in the Anseriformes, the Charadriiformes and the Passeriformes, while most ‘protective’ associates come from the Charadriiformes and the Falconiformes. Protected associates primarily benefit from a reduced predation rate when nesting near protective species although a variety of other, often unusual benefits have been described, including early warning of predators, lower parasitism, lower brood‐parasitism, nest stability and higher mating success. Protected associates sometimes also pay costs when they or their young are killed by their aggressive associate, or when they are forced to abandon their nest. Two studies demonstrate that protected species manipulate the trade‐off between the costs and benefits of nesting near an aggressive associate, while only a handful of studies have found specific adaptations to help avoid costs. In contrast, there is little evidence to suggest that the protective associates pay costs or gain benefits. This is probably because few researchers set out to test this hypothesis specifically, but we tentatively conclude that the majority of nesting associations are commensal in nature, and that only a few are parasitic or mutualistic. Many studies show that nesting associations occur by active choice and not because the associate species choose similar habitat, but only one study does so experimentally. In two cases the nesting association is obligatory for one of the species but there was also evidence for significant local adaptation to nesting near protective species, sometimes across a broad geographical range. Finally, we identify potential pathways that may have led to the evolution of nesting associations, discuss some possible implications for the populations involved and argue that nesting associations provide many novel research opportunities.