DO TROPICAL BIRDS REAR AS MANY YOUNG AS THEY CAN NOURISH ?
Authors: Skutch. Alexander F. 
Published: April 2008

The thesis that birds lay clutches of eggs which will produce as many nestlings as they can, on the average, adequately nourish, is considered in relation to the Central American avifauna.

Birds whose usual rate of bringing food to the nest is slow can greatly augment this rate if, after an exceptionally long period of neglect, they find their nestlings unusually hungry (examples are given for two species of antbirds), or if an additional nestling is placed in the nest (an experiment with a tanager is described).

In four out of five cases, birds of four kinds in which the clutch consists almost invariably of two eggs succeeded in rearing three nestlings.

In numerous species of hummingbirds, manakins, cotingas, American flycatchers, Icteridae, etc., the male does not help to feed the nestlings; yet these birds nearly all rear broods of two, which in the same habitats is the prevalent size among species in which both parents attend the nest. If the latter were rearing as many young as they could properly provide for, we should expect the unmated females to lay clutches only half as big. Among flycatchers, when the female alone feeds the nestlings, their period in the nest is not significantly longer than at comparable nests of species in which both parents feed.

Also in nidifugous species, of which the chicks pick up their own food under parental guidance, clutches are far smaller in the Tropics than in the North.

On the basis of the time spent away from the nest by 18 incubating finches of 9 species, it is estimated that on the average 4 hours per day suffice each parent to find all the food it needs, preen, bathe, and perform other necessary activities. This would leave each parent, of the tropical species, about 8‐5 hours to devote to the feeding of the nestlings; both together would have 17 hours per day. Even allowing 5 bird‐hours for each nestling, the two parents together could attend 3; yet 7 of the 8 tropical species considered regularly lay sets of only 2 eggs.

Even when the climate appears to be favourable through much or all of the year, the breeding‐season of many tropical birds is short and the number of broods small. The fact that some individuals breed successfully at seasons when most members of the same species are resting from reproduction, points strongly to the conclusion that these birds do not attempt as many broods as they might successfully rear. Neither in the size of their broods, nor in the number per year, do tropical birds in general appear to rear as many offspring as their own powers and the environment would permit. Their rate of reproduction seems to be adjusted to their average annual mortality rather than pushed to the limits of their strength.

Two species of Tyrannidse, Myiozetetes similis and M. granadensis, are similar in appearance and habits and often build their nests in the same trees, but differ in their rate of reproduction. In the midst of their ranges, it does not appear that the more prolific M. similis has any advantage over its congener.

In a hypothetical species which with a clutch‐size of two keeps its range occupied at a high or optimum density, the fate of a three‐egg mutant is considered. It would be difficult for the more fertile genotype to displace the well‐established two‐egg strain, unless some catastrophe caused a severe reduction in the density of the population. Such catastrophes are rare in the humid Tropics.

The situation is radically different in a species expanding into a new area, or in one increasing rapidly in numbers after a great reduction in density of population. In these cases intraspecific competition is at a minimum, and the more prolific strains will tend to predominate. At high latitudes, recurrent drastic reductions in density as a result of great cold, famine, or disasters during migration, are followed by periods of free expansion; and under these circumstances it is likely that the rate of reproduction will be held to near the maximum which combined internal and external conditions allow. Lack's views appear to fit the facts in relation to the northern birds which he investigated, but seem to be in accord neither with observations nor theory when applied to the birds of humid tropical areas, where ecological catastrophes are at most rare and local.