Protective association and breeding advantages of choughs nesting in lesser kestrel colonies

Some bird species may breed close to aggressive predators to reduce predation risk by more dangerous, generalist predators. We tested this protective nesting association hypothesis by studying solitary choughs, Pyrrhocorax pyrrhocorax, breeding within or outside lesser kestrel, Falco naumanni, colonies in northern Spain (1993-1994). We found 27 potential predators of choughs and kestrels and confirmed predation on adults and nests of both species by at least eight common predators. We experimentally assessed the defence investment of choughs and kestrels towards a stuffed eagle owl, Bubo bubo, in 19 buildings shared by both species. Lesser kestrels were more efficient at detecting the predator, and defended more vigorously than choughs. Choughs clearly selected for breeding buildings where lesser kestrel colonies were installed. Breeding success of these choughs was much higher than that of choughs breeding alone, because of a lower percentage of nest failure. Benefits to choughs probably accrued from both the 'parasitism' of the kestrels' nest defence and the dilution of predation risk in the colonies. Since lesser kestrels do not prey on choughs, this could be a good example of protective nesting association in birds. (C) 1997 The Association for the Study of Animal Behaviour.