ADAPTIVE LIMITATION OF THE REPRODUCTIVE RATE OF BIRDS
Authors: Skutch Alexander F. 
Published: April 2008

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.