A BREEDING BIRD CENSUS AND NESTING SUCCESS IN CENTRAL AMERICA

Abstract

SUMMARYIn a park‐like area of 3.75 acres adjoining primary forest in Costa Rica, at an altitude of 2,500 ft., 83 nests, made by about 49 pairs of birds and two single females, were found in one year. Over a period of 20 years, 64 species were recorded as nesting in this same area.The difficulties of learning the actual rate of success of nesting birds are discussed, and it is concluded that, in view of the impossibility of assessing the effects of visits of inspection to nests in natural habitats, statements of breeding success are at best rough approximations of what happens in the absence of an observer.In the area of the census, nest‐success (the proportion of nests in which at least one egg was laid that produced at least one living fledgling) was 38–53% in four different years. During the four years 41 % of 208 nests were successful.Of 756 nests of 23 species of altricial birds of the Central American lowlands that build open or roofed nests in clearings and second‐growth, 37% were successful. When the computation is restricted to nests found before the last egg was laid (class B nests), 35% of 434 nests were successful, and 30% of 883 eggs produced living fledglings.In the neighbouring forests, nesting success was much lower, only 23.5% of 136 open or roofed nests producing at least one fledgling. Many forest birds increase their chances of success by entering neighbouring clearings to breed, but few open‐country birds build their nests in the forest.In both forest and clearings, hole‐nesting birds in Central America are much more successful than open‐nesters, as has been found also in the North Temperate Zone.A comparison of the results of a single season's observations in each of six Central American localities shows an increase of nesting success with altitude. In lowland Panama, the nest‐success was only 21%, in the Subtropical Zone of Costa Rica 53%, and in the altitudinal Temperate Zone of Guatemala 55%. The effect of altitude is complicated by differences in the amount of forest in the localities chosen for study, as well as by other factors difficult to assess.In both the tropics and the North Temperate Zone, nest losses are substantially higher in woodland than in man‐made habitats, evidently because there are fewer predators in the latter; but, even in clearings in Central America, nesting success was considerably lower than it was found to be in numerous studies in the North Temperate Zone. The difference may, however, reflect the greater “wildness” of the localities where the writer's studies were made, rather than a true contrast between tropical and temperate zone conditions.Snakes appear to be of the greatest single cause of nest losses in tropical America, but mammals, a few predatory birds, ants, and possibly even bats, destroy many eggs and young.Since small broods and heavy predation permit only a small annual contribution to the adult population, it is evident that, in order to maintain the species, adults must enjoy fairly long lives. Recent statistical studies support this theoretical conclusion.