Colonial versus solitary breeding: nesting patterns of Brown-throated Martins Riparia paludicola in Morocco

This study explored reasons for variable nesting habits in a hole-nesting bird species that sometimes breeds colonially and sometimes alone. Nesting was surveyed over ten years in natural sand cliffs and similar features created by human activity along a river and nearby habitats. Two-thirds of nests were in small colonies of 2–16 birds, where nests were less than 25 m from neighbouring breeders. Many others were hundreds of metres apart. Individual breeding locations were abandoned after only a few years’ occupation, often without obvious habitat change. Birds responded very rapidly to the sudden exposure of fresh sand cliffs by starting new colonies. If floods occurred in the first half of the season, nesting in the second half shifted to less vulnerable areas. Substantial changes from year to year, in the location of breeding activity and the ratio of colony to solitary nests, were unrelated to variations in rainfall (a likely measure of feeding conditions) in the previous breeding season or the six months prior to breeding. The proportion of nests in colonies was positively correlated with the numbers of birds breeding, indicating that coloniality was not limited by available sites. Behaviour at colony and solitary nests suggested that one benefit of group nesting is more frequent and stronger response to birds of prey. Extra adults visiting nests alongside the breeding pair were more frequent at colonies, though the significance of this is unknown. There was no evidence for neighbouring breeders coordinating their visits to and from colonies. Costs of group nesting (more frequent disturbance by groups of non-breeders, and time in defence of nests) were higher at colonies, though levels of aggression were very low. Coloniality is more likely to be an incidental result of birds independently selecting good nesting sites in an unpredictable habitat, than because of social benefits of breeding close together.