Variation in Clutch Sizes

The number of eggs in a set laid by a female bird tends to vary among taxonomic groups. Usually petrels, albatrosses, and shearwaters lay 1 egg, auks and vultures lay 1 or 2, terns and gulls 2-3, shorebirds and cormorants 3-4, hawks and songbirds 2-5, grouse and ptarmigan 5-12, ducks 7-12, and pheasants and partridges 8-18. Clutch sizes differ not only among major taxonomic groups and among species, but also among populations and individuals of the same species (as is apparent from the ranges of clutch sizes given for many species in this guide). For instance, both the European Robin (a thrush only distantly related to our robin) and the Snow Bunting lay larger clutches in the northern than in the southern parts of their ranges. In addition, older females of some species lay more eggs than do younger females.

Few topics have fascinated students of birds more than the causes of such variations in clutch size. Why do birds near the equator lay fewer eggs than related birds near the poles do? Why do seabirds that forage close to shore lay more eggs than those that forage far from shore? Why do tropical rain-forest birds generally have smaller clutches than those that dwell on tropical savannas? Why do birds that are colonial or nest at relatively high densities often lay fewer eggs than solitary relatives do? Why, in multiple-brooded birds, does clutch size often decline as the breeding season progresses? Why do small species tend to have larger clutches than large species do?

The quick answer to all of these questions is that birds lay the number of eggs that will permit them to produce the maximum number of offspring -- but that number varies with latitude, habitat, body size, etc. That answer is provided by evolutionary theory, which says that winning the game of natural selection involves producing as many surviving young as possible. A female laying too many eggs may lose them all as a result of being unable to properly incubate them, may attract nest robbers, may be too weakened by the reproductive effort to survive the winter, or (most likely) may be unable to properly care for the young. On the other hand, by laying too few eggs, the bird will fledge fewer young than it is capable of rearing.

Consider some more detailed explanations of trends in clutch size. Ornithologist N. P. Ashmole has offered an explanation of one of these trends -- the increase in the number of eggs per set from equator to pole. Such "latitudinal variation" in clutch size is related to the amount of food produced per unit area of habitat. More specifically, clutch size is positively related to resource abundance during the breeding season relative to the density of bird populations (abundance per unit area) at that time. If, when the birds are not breeding, their population sizes are limited by food shortages, then population density would be low at egg-laying time. And if resources increase only slightly during the breeding season, then natural selection would not favor large clutches, since food for the hatchlings would be limited. But if the increase in food were large during the breeding season, then, everything else being equal, raising a large brood should be possible. Thus the largest clutches should be found in high latitudes, where there is an enormous increase in productivity in the spring and summer (as anyone who has braved northern mosquitoes knows only too well), and the smallest clutch sizes might be expected in nonseasonal tropical rain forests, where productivity is rather uniform throughout the year.

According to Ashmole's hypothesis, there should be considerable uniformity of clutch size within a locality, since the seasonality of production should affect all the local birds. Such uniformity is precisely what has been found in tests conducted by avian ecologist Robert Ricklefs. In both the Western and Eastern Hemispheres, for instance, the most common number of eggs in the wet tropics is 2 or 3, but in temperate and arctic regions it is 4 to 6. For one series of 13 localities spread from Borneo to Alaska, 48-88 percent of the passerine species in each locality fall within a range of 1 egg. For example, in an equatorial rain forest in Borneo, 86 percent of the species laid an average of 2-3 eggs, and in another rain forest in west Java 75 percent were in that range. In a thorn forest in Oaxaca, Mexico, about one-half of the bird species laid an average of 3-4 eggs, and on an Alaskan tundra all of the species averaged between 4.5 and 5.5.

Most important, average clutch size under Ashmole's hypothesis is predicted to be closely and inversely related to resource productivity during the nonbreeding season; the lower the off-season productivity, the larger should be the clutch. In order to test the hypothesis, woodpecker specialist Walter Koenig of the University of California tabulated the sizes of 411 complete clutches of Northern Flickers from a wide range of localities. He found that, as predicted, clutch size declined significantly as one moved from localities where resources are scarce in the winter to ones where they are abundant. Koenig found that, as predicted by Ashmole's hypothesis, flicker clutch size is not related to resource productivity during the breeding season; he found no correlation between the two.

On the other hand, average clutch size should be positively related to breeding season resource productivity per breeding pair of birds. Such a relationship was found in a series of localities spread from Costa Rica to Alaska. Thus it isn't the breeding season productivity per se that counts, but that productivity in relationship to the bird density at that season. When there are few winter survivors, so that breeding density is low, the breeders will not seriously compete for resources, and so will have a chance to raise large broods.

Seasonal differences in food resources seem to explain latitudinal (and, similarly, other geographic and habitat) trends in clutch size. Food is also obviously the key to the difference between onshore and offshore feeding seabirds -- the former can rear more chicks because they can visit the nest with food more often. Competition for food also probably explains why clutch sizes are smaller in dense rather than sparse populations. Declining food resources also explain the evolution of smaller clutch sizes in late breeders.

Although the broad evolutionary influences on clutch size seem reasonably well understood, a great deal remains to be done before the full array of factors determining the number of eggs per clutch are worked out in detail. Some biologists feel, for example, that clutch size in passerines will be negatively related to the chance that the nest will be robbed -- vulnerable populations should produce smaller clutches. The reasons that have been given include: smaller nests should be more difficult for predators to find, the adults will have more energy to invest in a second brood if the first clutch is lost, and (since less time is invested in the first nest) renesting can begin earlier while spring conditions still prevail. But it has not yet been possible to sort out such factors definitely, so it seems likely that the determinants of clutch size will be the subject of ornithological research for some time to come.

SEE: Average Clutch Size; Brood Patches; Natural Selection.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.