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

Willow ptarmigan

gallant males and choosy females

One nice day in September, I walked with a few friends in a subalpine area with small shrub thickets scattered throughout a meadow of no-longer-blooming wildflowers. A subdued clucking sound in the brush caught our ears and we stopped to look.

A little family of about six well-grown willow ptarmigan chicks wandered out of the brush and into the meadow, heads down, busily searching for bugs, seeds, berries, buds, and catkins. The brown female was nearby, clucking gently and keeping watch. As they all searched and sampled whatever looked good, we noticed another bird, standing on a small rock not far away.

willow-ptarmigan-male-in-late-summer-david-bergeson
Photo by David Bergeson

This bird was resplendent in a plumage of rich brownish red, with white legs and belly. He was overseeing the foraging efforts of his mate and chicks, alert for predators. He stood there like a proud papa wearing a cut-away coat with tails and white breeches. Very handsome! And the family was still intact, chicks shepherded by both parents.

Willow ptarmigan are unusual members of the taxonomic group of grouse: males participate in parental care until the chicks become independent in the fall. Males of all the other kinds of grouse are intent upon courting females and showing off to each other and have no role in chick-rearing.

In early spring, male willow ptarmigan begin to establish territories, which can be over ten acres in extent. Territory borders are defended vigorously. Neighboring males may march, rather peacefully, side by side along a shared border; and territories are advertised by vocalizations (the ‘rattling’ call) and flight displays. More intense competition involves charging at each other, knockdown-dragout fights, and long-distance chases, often well outside the area of contention. Females arrive a bit later than males, and they may be aggressive against other females.

Females are choosy when selecting mates. They like males with large red ‘combs’ over the eyes, large territories, and vigorous displays. Although sexually mature at age one, the yearlings are less likely than more experienced males to attract a female. A male courts a female by fanning tail and wings, strutting, stomping and bowing, and ‘waltzing’ around her with his fans facing her. If she likes what she sees, they form a pair. Most pairings are truly monogamous, with little extracurricular activity (unlike many other birds). If both members survive, the pair may stay together several years (annual survival in northern B. C. was reported to be roughly thirty to sixty percent (slightly higher for males than females).

Although the female builds the nest and does the incubation, the male guards her and the nest, sounding an alarm if a predator approaches. He tries to protect the family by distracting the predator: feigning injury, leading the predator away. An intensive defense includes loud vocalizations.

Females generally lay seven to nine eggs, sometimes as many as fourteen. During the egg-laying period, before incubation begins, females often leave the nest and go foraging. They commonly cover the eggs with grass or leaves while they are away. During three weeks of incubation, she covers the nest and eggs herself at least ninety percent of the time, seldom leaving to find food. Both male and female defend the eggs and, later, the chicks. A study in northern B. C. found that, on average, about fifty-four percent of females successfully raise at least some chicks; predation on eggs can be more serious than predation on chicks, but even so, about half the chicks that hatch don’t survive more than a few months.

Chicks can feed themselves soon after hatching and can regulate their own body temperatures quite well after a week or so. But the female broods even two-and three-week old chicks if the weather is cold and wet. Males sometimes brood, too, and if the female dies, he can rear chicks by himself. Chicks can fly at age ten to twelve days, and they move around together with the parents, often ranging beyond the territory borders. The family stays together until fall. Occasionally, parents may adopt chicks from another family.

willow-ptarmigan-eating-alder-buds-by-bob-armstrong
Photo by Bob Armstrong

As winter approaches, ptarmigan molt into the white winter plumage that camouflages them on snowy backgrounds. Not quite all-white—there are some black tail feathers. Come spring and the breeding season, male plumage turns reddish brown on head and neck, leaving the body white, while female plumage gradually turns brown (with white in the wings). As summer progresses, the reddish brown of the male spreads over the chest and back, leaving the belly (and most of the wings) white. That is why these birds are called red grouse in Scotland, and this was the plumage sported by the elegant fellow we saw.

All the birds will be white when winter comes. Winter plumage is denser than summer plumage and that, together with a seasonally greater metabolic tolerance of low temperatures and the habit of burrowing into the snow blanket, helps keep them warm.

Parallels between plant and animal reproduction

…little Peyton Places of the plant kingdom

When I was growing up, I thought of plants simply as green things that we sometimes ate, or mowed down every week or so, or from which we picked cherries and apples. At some point, I realized that they also provided perches for birds and nesting places for gray squirrels. But, essentially, they were static entities, useful no doubt, but incapable to doing anything interesting. Was I ever ignorant!

Gradually I came to understand that plants can do about as many things of interest as animals; it’s just that they often do them in different ways. To illustrate this, let’s consider the entire process of reproduction, from the getting together of male and female to the raising of offspring. And let’s divide this process into three phases: what happens before mating, what happens between mating and fertilization of eggs, and what happens after fertilization. Events at any point in the process may affect reproductive success, which is the ultimate measure of fitness: individuals that make more surviving offspring contribute more genes to future generations, and their genes come to dominate the population.

 

Pre-mating activity

In the animal kingdom, it is usually males that compete for females (with some notable exceptions) and females commonly choose their mates more selectively than males. Males may cruise around looking for receptive females (in bears, for example) or set up display areas and wait for interested females to visit (as in grouse, for example). Sometimes they set up territories that exclude other males, and advertise their real estate and potential nest sites (as in blackbirds). Males fight each other physically or establish superiority by means of visual and acoustic displays. Basically, males show off their looks, strength, and endurance by engaging in strenuous displays, or they defend a territory with suitable feeding and breeding sites. Females cruise around looking for males or territories that they like and settling down for breeding with a chosen male. Both males and females often engage in extracurricular sexual activity, usually as a way of increasing the number of offspring (for males) or the diversity of offspring.

In the plant kingdom, things are seemingly quite different, because plants don’t move around; they can’t dance and sing. Nevertheless, there is often intense competition for mates. This is especially clear in plants that use animals to pollinate flowers (think of wild iris, or fireweed, or blueberries, or skunk cabbage…). Flowers are the plant’s way of attracting an animal that will bring pollen to female flower parts and carry pollen from male flower parts to a receptive female. Flowers provide nectar to reward flower visitors, some of which (such as bees) also collect pollen for food. There may be competition to have pollen delivered to female parts (female competition), or competition to deliver that pollen (male competition). In either case, flowers that offer more nectar or pollen or flowers that attract more visitors because of aroma or the size or configuration of petals would tend to have higher reproductive success. It is important to keep in mind that reproductive success is measured not only in terms of seed production (maternal function) but also in terms of seeds fathered.

 

Post-mating and pre-fertilization events

In the animal kingdom, males compete with each other indirectly in what is called sperm competition. Sometimes sheer numbers of sperm produced by one male can overwhelm the sperm of other males that mated with the same female; in fact, among primates, sperm production is greater in species in which multiple mating is common than in species that tend to mate with only one or perhaps two females. The timing of copulation can be part of this process; in certain ducks, if a male observes that his mate has been sexually accosted by an outsider, the paired male will immediately copulate with his mate, in an attempt to flood out the rival’s sperm. Some dragonflies go this one better: after a female has mated, a subsequent male can remove the first male’s sperm from the female’s reproductive tract. In a few animals, there is evidence that females themselves can eliminate sperm from an unwanted male. Many bats have delayed fertilization, in which the sperm do not penetrate the eggs immediately, but rather float around in the female for some time. One can speculate that this might give the females time to sort out the sperm of different males.

In the plant kingdom, the analogous process is pollen competition. Pollen donors (males) may increase their chances of fertilizing eggs (to make future seeds) by delivering lots of pollen grains that cover the receptive surface of female flower parts, occupying the surface and reducing the space for the pollen of other males. Males could also increase the number of eggs they fertilize by rapid germination of pollen grains and rapid growth of the sperm-delivery tube through the style (see the illustration) to the eggs in the ovary.

Female plants have some opportunity to select among the pollen grains because both germination and growth of pollen are based not only on characteristics of each male’s pollen but also on interaction with female tissue. In some species, it is a long way (possibly several inches in ornamental hibiscus, for example) from the receptive surfaces where pollen is deposited down the style to the eggs in the ovary, so there is ample opportunity for interactions between pollen and female tissue. Some plants have very delayed fertilization, for several weeks or more, and researchers have suggested that prolonged delays enhance the opportunity for females to accomplish more sorting of pollen from different pollen donors.

 

Post-fertilization and rearing of offspring

In the animal kingdom, where multiple paternity of litters is quite common (e.g., mink, bears, many birds), half-siblings within a litter compete with each other for parental care and nutrition. When food is scarce, some offspring receive less food and eventually die. From the fathers’ perspectives, having offspring with good competitive abilities is an advantage. From the mothers’ perspectives, there is the opportunity to selectively tend to certain young, thus effectively choosing among potential fathers.

In the plant kingdom, exactly the same process may occur. If the growing seeds in the same or even in different ovaries on a plant have different fathers, some may be better than others in capturing nutrients from the maternal plant. And the maternal plant may have the chance to be selective of which immature seeds will reach maturity.

I don’t mean to imply that all organisms use all of these tactics all of the time. Indeed, it is interesting to speculate about why some species use one tactic and not another. It should be clear that the reproductive process in plants and animals can involve both competition and choice of partners in the parenting process. I hope it is also clear by now that plants are anything but inert sticks of green. They can be little ‘peyton places’ of their own.