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.