Although all birds lay eggs, it is curious that they do so, because the time of highest mortality in most birds usually occurs during the egg-laying period. Apparently, birds lack some adaptation that would permit them to become viviparous.

Most birds build a nest and incubate their eggs, but the incubator birds and such brood parasites as cuckoos are among the exceptions to this rule. Many females that lay a fixed number of eggs are referred to as determinant layers. The pigeons and doves are outstanding examples of this behaviour; for some as yet unknown reason, they never lay more than one or two eggs. Other species are often referred to as indeterminate layers because, in the absence of a suitable stimulus, they continue to produce eggs. More often than not, this stimulus is the presence in the nest of a certain number of eggs. Such behaviour is clearly adaptive—if eggs are lost for some reason and if other environmental stimuli are present, the missing eggs are replaced. The distinction between determinate and indeterminate layers is often blurred, for many indeterminate layers will not replace more than one or two missing eggs.

The duration of egg incubation varies from as little as nine days in some tropical perching birds to as long as 80 days in some albatrosses. In most species that form pairs, both individuals incubate and feed the young, but the female usually has the greater share of the burden. Among the exceptions to this behaviour pattern are the tinamou (partridge-like game birds), ostriches, some gallinaceous species (e.g., pheasant, grouse, turkeys), and phalaropes. In the phalaropes, the role of the sexes is largely reversed: the females are more brightly coloured than the males and, not surprisingly, are the aggressive ones in courtship and in territorial defense; incubation is carried out solely by the male, but the female aids in feeding the young.

Because many birds begin incubation with the laying of the first egg in the clutch, the eggs hatch at different times. This strategy is often employed by species whose food supply for the young may vary in abundance over a fairly short period. Hence, should food suddenly become scarce, only the smallest chick or chicks will starve rather than the entire clutch. Species in which the young hatch in a relatively well developed, almost independent state tend to have very large clutches, as in many gallinaceous birds. In this case, it might be said that the ultimate size of the clutch is regulated by the abundance and quality of the food available to the female as she produces eggs. The same explanation also accounts for clutch size in parasitic birds—i.e., those that lay eggs in the nests of other species. The breeding densities of birds vary from one pair in many square miles, as in some birds of prey, to such species as the fulmar, which forms colonies numbering as many as 250,000. Some colonies of the African weaverbird (Quelea) have been estimated to exceed 1,000,000 individuals.

One interesting aspect of reproductive behaviour in birds, possibly peculiar to them and to some mammals, is that many courtship displays are learned, or at least perfected through practice, from the parents. An example is the learning of birdsongs. It has been shown in some cases that when chicks are switched from the nest of one species to that of another, they learn some and perhaps all of the songs of the foster parents and do not develop their own species’ vocalizations. When mature, such birds often prefer to choose as mates individuals of the same species as their foster parents’ rather than those of their own species.

Courtship stimuli in birds are mostly visual and auditory, but it is possible that odour may be important in some petrels and shearwaters. As previously mentioned, most birds form pairs. In these and in many that do not, the males engage in communal, or lek-type, displays on a common courtship ground, such as the familiar strutting grounds of turkeys and many grouse. In addition, there are the incredibly bizarre communal dances of the birds of paradise; the jungle-floor dancing of the cock of the rock; the pasture display grounds of the shorebird, the ruff; and the forest arenas cleared for displaying purposes by the tiny manakins. Many of these display areas are used for many years; in some manakins, for example, certain cleared arenas have existed continuously for at least 30 years. In most lek species, the males are usually brightly coloured, and the females are rather dull in appearance. An exception occurs in some hummingbirds, the so-called hermits, in which both sexes are rather dull in coloration and in which the males group together in singing assemblies.

Mammals

Most mammals give birth to live young. The outstanding exceptions are the egg-laying monotremes of Australia, the platypus (Ornithorhynchus) and the echidnas (spiny anteaters). In the duckbill platypus, a brief courtship involving a chase in the water precedes copulation. The two eggs that are produced are placed in a burrow and hatch in eight to 10 days. In the reproductive behaviour of the spiny anteater (Tachyglossus), the female apparently lays her single egg directly into her pouch.

As already mentioned, another general aspect of reproductive behaviour in mammals is the estrous cycle, knowledge of which is essential to an understanding of the mechanisms involved in the reproduction of any mammalian species. In most cases, females are responsive to males only during that portion of the estrous cycle when they are in heat; that is to say, when one or more eggs have broken out of the ovary and are in the process of descending to the uterus. The factors causing this event vary significantly, but in some such as rabbits and cats, copulation itself is the main stimulus. In general, however, those mammals, particularly the large ones, that live in temperate areas—bears, dogs, wolves, foxes, seals, and some deer and antelopes, for example—have one estrous cycle per year. Mammals that live in warmer zones, such as some areas of the tropics, tend to have more than one estrous cycle per year. The sexual cycle in males, the height of which in some forms is referred to as the rut, is, not surprisingly, usually correlated with that of the females. The males of many species of domestic mammals, however, seem to be capable of copulating at almost any time of the year.

Another general aspect of mammalian reproductive behaviour is that they do not normally form pairs. Exceptions occur in certain carnivores and in some primates, in which parental care is divided between the sexes. As in many insects, the courtship behaviour of most mammals does not appear to be elaborate; but, just as in the former group, most mammals (humans are an exception) have an acute sense of smell. It is possible, therefore, that many of the chemical attractants wafted into the air by receptive females are actually courtship displays that are more complex than has been realized. This is not to say, of course, that visual, auditory, and tactile displays do not occur. Many deer and antelopes, for example, have rather complex ritualized visual displays employing such movements as strutting and arching of the heads, as well as conspicuous colour patterns. Males in many species discharge urine on females as a preliminary to copulation. Tactile and auditory displays have been shown to be important in aquatic mammals, such as porpoises and whales.

In addition to a number of mammalian pheromones, other odour effects occur in mammals that, aside from their simple advertising value, have an important influence on reproductive behaviour. It has been shown that, when a recently impregnated female mouse is exposed to the odour of a male other than the one with which she has mated, implantation of the egg in the uterus often fails; as a result, there is a rapid return to estrus. The odour of a strange male may signify to a female rodent an unfavourable situation in which to raise young, inasmuch as a number of male rodents attempt to attack offspring not their own. Although it is not yet certain, there might be an adaptive explanation for this behaviour. The population fluctuations of rodents have attracted much attention, and, perhaps correctly, studies have focussed on the ecological parameters of these fluctuations; for example, it has been demonstrated in the laboratory that certain behavioral mechanisms involving odours exercise profound control over the reproduction and population levels of rodents. It has also been shown that the odour of mice can stimulate the production of hormones that cause a decrease in the reproductive capacity of other mice. In another study, estrus was suppressed and many pseudopregnancies developed when four or more female mice were grouped together in the absence of a male. These results offer a partial explanation for the reduction of population growth in rodent colonies with high population densities.