Avian mate choice and plumage

how feathers shape species

Mating among birds is usually a matter of mutual agreement: both male and female are being selective in their choice of mates (although their criteria are likely to differ). Selectivity in mate choice is central to the Darwinian process of sexual selection, determining which individuals will mate and produce offspring. The chosen individuals are more successful in mating and reproduction. Thus, their genes are passed on to the next generation while the genes of un-chosen individuals are not. As the process continues, the genes that determine both the winning traits and the choosiness become more frequent in the population.

The outcomes of all that choosing vary enormously, depending on the ecology of the species, the previous evolutionary history, and the occurrence of genetic variation upon which selection can happen. If there is no plumage variation, there is nothing to choose! Avian feathers serve various functions, one of which is visual display during courtship. Genetic variation in plumage among potential mates provided birds with choices of color and pattern as they decide with whom to mate. Those choices, together with variations in other traits, shape the appearance and behavior of the lineage.

Imagine a species (Species A) that makes open-cup nests (as do all related species…that’s an evolutionary history factor) in shrubs (instead of under logs or roots; that’s an ecology factor). Suppose that in this species it is (for whatever historical reason) the female that incubates the eggs and cares for the nestlings. In an open-cup nests (compared to a cavity nest, for instance), the incubating female would be exposed to searching predators, and her frequent feeding visits to a particular location would also be noted by lurking predators. So inconspicuous, perhaps camouflaged, plumage would lead to better survival of the female and the eggs and chicks. And a male that preferred an inconspicuous female would have better reproductive success than one that somehow preferred a female that flashes a noticeable red crest or a long, bright blue tail.

If there is some variation among the males in the colorfulness of their plumage, those drab females might prefer males that are a bit colorful, perhaps with a yellow head or vivid magenta wings, rather than males that are more like the females. If those plumage patterns are heritable, the sons and daughters of such adults would bear the same traits, and eventually the whole population would have drab females and colorful males.

However, if (in species B) the males also do some of the incubation and parental care, they and their eggs and nestlings might suffer more predation. If they were colorful, then they and any female that somehow preferred a gaudy male would probably have lowered reproductive success. And so their genes would become less frequent in the population, and males and females would look similar in color.

Now go back to species A. Imagine that the population is spread over quite a large geographic area, such that the birds in one area just don’t get to another area, or vice versa. Now it is possible for the birds in that area to become different from the rest of the population. Suppose that some males don’t have simple yellow heads, they have additional blue crests on top. Then females there might find that they like males with blue crests (instead of plain yellow heads), for example. Then these more fancy males would come to predominate in that area. That might happen in several different areas, with different outcomes in each. If some of these fancy males just happened to wander into a different area, they might not be preferred by the females there. So thus, species A has begun to diversify into several new species, each with different male ornaments (and female preferences).

The classic example of diversification driven principally by mate choice is in the neotropical manakins. There are over fifty species of manakins. In general, the females are greenish and plain, while the males sport a spectacular array of plumage patterns and colors, often with behaviors that show off those features. By being very choosy, females maintain the dramatic differences among the males, and interbreeding is rare to non-existent.

Another example might be the wood warblers of North America, in which the males of different species are generally somewhat more colorful and distinctive than the females. Again, interbreeding between species is not common, but in a few cases, hybridization occurs between two species (e.g., Townsend’s and hermit warblers). Here in Juneau, a few years ago, observers noted that a warbler nest was tended by a mixed set of parents belonging to the same species (yellow-rumped warbler) but of two different varieties. One seemed to conform to the plumage patterns of Audubon’s warbler, while the other one was either a typical myrtle warbler, or the result of a previous mixed-mating of the two forms. In either case, more hybrids were being produced by this pair.

yellow-rumped-warblers,-male-and-female-at-nest-with-very-young
Two different subspecies of warbler tend their chicks together

Evolution by mate choice is common and widespread among birds. However, two other kinds of mating behavior tend to obscure the typical patterns. The first is that, even among ostensibly monogamous pairs, both sexes may go gallivanting, and do some of their copulations outside of the pair bond, and broods of mixed parentage occur. The choices for intra-pair copulations and extra-pair copulations may or may not be the same.

A second kind of mating behavior totally subverts the normal patterns of mate choice. In many ducks and some geese, there are forced copulations, in which males attack and try to copulate with females, which struggle and resist, but commonly suffer injury (sometimes lethal). There is clearly no female preference involved. The extraordinary complexity of the female reproductive tract in these species probably evolved as a way of reducing the fertilization success of the forced copulations; nevertheless, some small percentage of the embryos can be fathered by these violent males. Injury to the violated females is likely to reduce their nesting success, but I have not found data on that. The origin and continuation of this behavior of males is not entirely clear.

barrow's-goldeneye's-mating
This female Barrow’s Goldeneye (left) has made her choice from among competing males

Live-bearing and egg-laying

variation that defies generalization

Animals produce offspring by two principal modes of reproduction. Vivipary (or viviparity) means producing ‘live young’—readily recognized as ‘living’ because newly produced offspring wriggle, squirm, squall, or squeak. The intended contrast is with ovipary (or oviparity)—producing eggs that house an embryo inside a shell; usually the eggs do not wriggle or squall. Of course, fertilized eggs are not dead, as might be supposed by the contrast with ‘living’ young! Fertilized eggs are very much alive, but early development takes place inside the shell instead of inside a parent. All the nutrition for early development inside an egg must come from the egg yolk and therefore be provided by the parent before the embryo is enclosed in the eggshell.

(There is an intermediate condition –ovovivpary/ovoviviparity—in which fertilized eggs are held within a female and hatch inside her. The embryo may be nourished by eating other eggs or embryos or perhaps by a kind of placenta, with a direct connection to the mother. This might indicate ways that, in the course of evolutionary time, vivipary evolved from ovipary. But leave that aside for present purposes.)

Vivipary and ovipary—these two modes of reproduction are scattered widely in the animal kingdom. It would be convenient if we could make lots of solid generalizations about either of these modes of reproduction, either about their taxonomic distribution or about their advantages and disadvantages. But alas, not so. There are only a few strong generalizations and there are almost always exceptions. Consider first the birds and then the mammals.

All birds lay eggs. That’s one good generalization with respect to taxonomy. But how birds treat their eggs varies. Most birds make a nest in which the eggs and then the chicks are tended—ducks, hawks, most songbirds are examples. However, brush turkeys and mallee fowl in Australia don’t incubate their eggs in the conventional way. Instead they build a huge mound of dirt and vegetation, in which the heat of decomposition incubates the eggs. An adult may guard the nest and regulate temperature in the mound by opening or covering it, but that’s the extent of parental care.

In fact, not all birds make nests; several species of songbird and duck are brood-parasites: they avoid all matters of nest-building and parental care by laying their eggs in the nests of other birds. Penguins provide another exception. Emperor penguins and king penguins make no nest; they lay single, large eggs that are incubated on a parent’s feet, with a fold of skin covering them. The incubating adult can even shuffle around with its egg carefully held in place.

All mammals nurse their young; that’s the very definition of a mammal. But although most mammals are viviparous, not all of them are. The platypus and echidnas in Australia are exceptions, laying eggs. Some mammals make nests or dens for their young, some carry their offspring around, but others do not do either of those things.

There is also variation among the other vertebrates; for example, some snakes and some fishes are viviparous while others are oviparous. Among the invertebrates, vivipary is widespread, having evolved many times and occurring in many different taxonomic groups, but ovipary seems to be more common.

One broad generalization does seem to hold true: vivipary apparently necessitates internal fertilization of the eggs by sperm that are placed inside the body of the female. No such limitation applies to ovipary; some oviparous animals have internal fertilization and others do not, releasing sperm and eggs into water at the same time.

Scientists have long discussed the relative advantages and disadvantages of each mode of reproduction, but to my knowledge, they have not come up with a comprehensive explanation for the evolution of either mode. There seem to be exceptions to almost any general statement, and it is likely that different factors and different conditions have led to the evolution of one habit or the other in different evolutionary lineages.

Among vertebrates, egg-laying commonly means eggs are placed in some kind of nest while the eggs are incubated or tended by a parent (exceptions above). That means the adult is temporarily tied to one place (the nest) until the eggs hatch and, in many species, the chicks are also fed until they can be independent. Especially for an animal that flies, a clutch of relatively large eggs is difficult for a parent bird to carry around while the embryos develop, so a central place can be useful. A nest can also help keep the young animals warm. However, there is a risk involved—predators often learn to focus on parental activity as a clue about nest location, and an entire clutch of eggs or brood of chicks may be wiped out. Similar statements apply to mammals that use nests or dens. Some mobile invertebrates, however, simply lug a batch of eggs around, carrying them on hooks or in folds or whatever.

Vivipary, on the other hand, might mean that young are born in a relatively advanced stage of development (compared to egg-layers), having been nurtured inside the mother for some time. But no, although some viviparous mammals are born fully capable of running or swimming, others are born in a totally helpless condition that requires weeks or even years of parental care. Furthermore, there are birds, such as ducks, whose young hatch from eggs in condition to run about and feed themselves.

Pregnant mothers carry the fetus wherever they go, enabling them to move around to find places with more comfortable temperatures or better food or safer refuges—all things that they could not do with eggs in a nest. That applies also to invertebrates that carry their eggs with them, wherever they go. But there are risks to the parent, too, if the developing young impair mobility or, in some cases, require the mother to have a special diet. Pregnant bears avoid the mobility problem because they den in winter and birth relatively tiny young (but run a risk from human predators that seek out their dens).

The bottom line seems to be that, although some good generalizations emerge, there is much variation that defies wide generalization. There are balances to be found, playing this advantage against that disadvantage, and they vary with circumstances. As usual, there are many questions to ponder, and some answers may emerge from studying the details of particular species.