Evolution of Hole-Nesting in Birds

Abstract

Many birds build their nests in sheltered sites like tree-holes, tunnels, rock crevices, caves or buildings, etc., where the nest is more or less out of predator''s reach. The safety provided by such sheltered nest-sites is counterbalanced by the limited supply of such sites and the relatively great competitive effort required for the acquisition and defense of them. Alternative tactics to reduce nest-predation are active nest-defense, effective mostly in large birds, and nest concealment/camouflage. The habit of sheltered nesting probably is associated with 2 basic premises: temporal priority to sheltered nest-sites in resident birds and extremely strong selection pressures towards safe nest-sites in birds with exposed feeding niches. The latter selection pressure is due to the great risk that birds foraging in an exposed manner will, by the feeding visits to their young, disclose their nest to predators. In temperate regions, many notorious nest predators, such as corvids, probably hunt by observing and following parent birds. This conjecture is supported by the findings that sheltered nesting is predominant (63-78%) among residents and migrants with an exposed foraging technique or habitat, i.e. aerial and sit-and-wait hunters and gleaners in the open. Sheltered nesting is rare among migrant gleaners that forage in cover, employed by only 2 (3%) out of 67 spp. Predications emanating from the conjecture include a relatively early spring arrival of tropical migrants with sheltered nests in comparison with birds having open nests (P < 0.02); a relatively high nest predation rate during the nestling stage compared with the incubation period among exposed foragers (P < 0.01); species living in closed and open, e.g., urban, habitats make more frequent use of sheltered nest-sites in the latter environment; strong effects of interspecific competition for sheltered nest-sites, sometimes resulting in breeding range contraction and habitat exclusion in subdominant species. The correlation between exposed foraging and sheltered nesting probably is obscured in the tropics, where a great proportion of the predators, mainly snakes and mammals, localize prey by their olfactory/auditory senses.