ADAPTIVE LIMITATION OF THE REPRODUCTIVE RATE OF BIRDS

Some biologists have held that the rate of reproduction in birds tends to be adjusted to the average annual mortality; others, that it is limited only by the parents' ability to raise sturdy young. The latter theory, that of maximum reproduction, is likely to be true only if three related propositions are true:(1) that an increase of clutch size is more likely to occur than some other mutation affecting the rate of reproduction; (2) that a genotype with a wastefully high rate of reproduction can supplant a genotype with a more conservative but adequate rate; and (3) that an excessive rate of reproduction is not harmful to the species. None of these assumptions has been proved. In a population not obliged to employ its full reproductive potential to maintain itself at a favourable level, mutations which limit this potential may arise and persist. They may effect this limitation by means of:(i) reduction in clutch size; (ii) reduction in the number of broods; (iii) failure of the male to attend the nest, often followed by failure to form pairs; (iv) deferment of reproductive maturity; (v) developments in territorialism that limit the number of nesting birds or the number of progeny they can rear; (vi) restriction of nesting to traditional sites; and (vii) the time-consuming construction of elaborate nests. These limitations of the rate of reproduction must be regarded as adaptive because, like other adaptations, they adjust the birds more perfectly to the conditions in which they live and reduce the stress to which they are subjected. In both tropical and temperate regions, species in which only the female feeds the nestlings have broods as large as species in which both parents feed them. It follows that the two parents are not rearing as many young as they could nourish. The view that hole-nesting birds can rear larger broods than open-nesters because their young develop more slowly, and require less food per capita per day, is untenable. Nestlings raised in holes and burrows gain weight about as rapidly as those in more exposed nests, but for safety they remain longer in their protected abodes. The larger broods of hole-nesters evidently compensate for the difficulty of obtaining nest sites, which delays the breeding of some pairs and prevents that of others. Clutch size is by no means closely adjusted to the number of young the parents can raise. If given additional nestlings, some birds attend them adequately. In other species, young are rarely fledged from all the eggs. In many cases, asynchronous hatching is not, as has been claimed, an arrangement which permits the parents to adjust to a varying food supply the number of young that they rear. In many raptors, fratricide and cannibalism reduce the size of the brood, sometimes to a single nestling, regardless of the abundance of food. The more stable the environment, the more closely the reproductive rate tends to be adjusted to the mortality; the more a population is subject to catastrophic reductions, the more the rate will approach the maximum. Primarily, the reproductive rate is controlled by heritable characters, which can adjust the rate to a stable environment but rarely respond to short-term fluctuations in external conditions or population density. The last fine adjustment of a population to its habitat is effected by processes that are density-dependent:either density-dependent regulation of the reproductive effort, or density-dependent mortality of adults or young, or a combination of the two. The general evolutionary trend in the Metazoa is toward producing fewer offspring and taking better care of them. This would hardly be possible if the more prolific genotype always prevails over those which raise smaller families and in consequence can attend their young somewhat better. The regulation of the rate of reproduction is a unique evolutionary problem, because a mutation conferring greater fertility, although often detrimental to the species, tends to diffuse through it as no other harmful mutation can. Yet it is counteracted by many factors, chiefly ecological, which operate subtly and are more difficult to appreciate than the force of numbers.     

A BREEDING BIRD CENSUS AND NESTING SUCCESS IN CENTRAL AMERICA

In 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.