The Evolution Of Low Reproductive Rates In Birds

Present theory as to the selective factors controlling reproductive rates, particularly in birds, is based on Darwinian theory as modified and developed in Lack's (1954) book "The Regulation of Animal Numbers" and antecedent papers. His conclusion, of utmost simplicity, is that natural selection will favor the individual (or pair) that contributes the greatest number of effective breeding individuals to following generations. Therefore, the reproductive rate of any species or population will tend to approximate the maximum number of young its members can produce under average prevailing conditions. In this context "average" may represent a long time indeed and species that cannot accommodate themselves to extreme conditions that recur no oftener than every several thousand years may be wiped out. It is probable that the reproductive rates of all living things have this selective basis. Conditions have varied enormously, however, and thus we find some organisms which produce great numbers of eggs, others that bring forth fewer young, but in a more advanced state of development, and still others that have only a few young but care for them carefully. To mention only large predators, at or near the top of their food chains, it is instructive to compare the number of eggs or young produced by sharks, pike (Esox), crocodiles, bears, and eagles. All birds, with unimportant exceptions, lay relatively few eggs. These they incubate, and later they care for the young. Nevertheless, there is quite a contrast between a wren (Troglodytes aedon) that raises two or three broods of four or five young each in a season and a California Condor (Gymnogyps californianus) which raises, at the most, one young every other year. Indeed the reproductive rate in this condor and in a few other birds mentioned later is so low that it is difficult to believe that it does, in fact, represent the maximum possible rate under the conditions that confront these species. This was brought out in a paper by V. C. WLnneEdwards (1945). He is of the opinion that some other selective basis must be found, and advanced a theory of population control or "numerical homeostasis" (Buechner, 1963) based upon group or social behavior. Social awareness, it is implied, will limit population density and prevent overexploitation of the food supply. These ideas were developed in Wynne-Edwards (.1962) later book "Animal Dispersion." It is the present writer's belief that the reproductive rates of all birds can be best explained in terms of natural selection as analyzed by Lack and that Wynne-Edwards' theory is without a sound genetic basis just insofar as he departs from it. Before seeking to demonstrate this it is necessary to summarize the reproductive ecology of the species of birds that are in part at least responsible for the difference of opinion. The low reproductive rates of certain large birds involve the following factors: a (1) Clutch size. Some of the largest raptors such as the Andean Condor (Vultur gryphus), the California Condor, and the Martial Eagle (Polemaetus bellicosus) lay invariably one egg per clutch; others like the Crowned Eagle (Stephanoaetus coronatus) sometimes one egg, sometimes two. All of these species lay relatively small eggs and there is no physical reason why they could not lay and incubate a clutch of several eggs. Clutches of a single egg, while unusual among birds, do occur in various other forms such as some of the tropical pigeons, the lyrebirds (Menura), the larger penguins, and all of the petrels (Procellariiformes). In a few snecies of eagles which some-