The animal kingdom’s most diverse array of reproductive behavior is on display in spring when avian breeding strategies flutter to life.
By Rex Graham
A female Greater Painted-snipe mates serially with up to 20 males in a season in southern Africa and Asia. Some supposedly monogamous female Adélie Penguins “divorce” one male before egg-laying with another. Even outside the breeding season in the Caribbean, a female Purple-throated Caribs will exchange sex for access to a male’s nectar-rich territory. Ornithologists call the hummingbird’s behavior “prostitution.”
Monogamy & promiscuity
As recently as 1960, the consensus among ornithologists was that over 90% of all birds were monogamous. Females chose a mate with favorable traits, which they pass on to their chicks. Right? Actually, it’s not that simple even for socially monogamous species.
Genetic analyses of the eggs in the nests of monogamous birds have revealed that roughly 20% of the broods have mixed paternity. “In extreme cases, the mismatch between the social and sexual mating systems is nothing short of spectacular,” French, Canadian and U.S. scientists wrote in a 2015 summary paper in Cold Spring Harbor Perspectives in Biology.
The stray imperative
In a classic and surprising experiment, researchers vasectomized male Red-winged Blackbirds so they would be infertile, and then measured what they expected would be nesting failures. The results were shocking: of 39 nests in the territories of 6 vasectomized males, 69% produced chicks. How? Females sneak away when the males aren’t looking.
Given the opportunity, males and females in many socially monogamous species will – well – stray. When given more opportunities to stray, more straying is the result.
The way courtship, breeding and chick-rearing influence evolution is much more complicated than Darwin had imagined. The reproductive repertoires of many birds are simply unknown.
Infidelity, promiscuity and divorce may be our hush-hush cocktail-party topics, but it’s normal for many woodpeckers, ducks and even a few ordinary-looking sparrows. One in 7 female Adélie Penguins (a monogamous species) divorce (switch mates before egg-laying starts in a given breeding season). Dunnocks use any of a wide variety of mating systems, depending on circumstances.
Guianan Cock-of-the-rock and Greater Sage-grouse males are famously promiscuous. These guys are at the mercy of what Darwin called the aesthetic preferences of females. Winning the girl requires flamboyant reds and oranges in Cock-of-the-rocks, and beautiful feather ornaments in the sage-grouse plus big inflatable air sacs that help them produce popping sounds.
Aesthetics and evolution
Even females dragonflies are attracted to males’ movements and vibrant colors. The flashy ornaments of male (and in some cases female) birds don’t seem to help them survive better. However, if the most robust females mate with the flashiest males, flash and vigor, over time, may emerge together in the same offspring. It’s as if evolution awards style points.
What’s not to like about a magnificent male Peacocks, Black Grouse, Golden Pheasants or Raggiana Birds-of-paradise? Birders congregate every mating season at their courtship leks to ooh and aah over the colors, competitive struts, jumps, puffed-up feather displays, vocalizations and other behaviors. The female birds share our fascination. Unlike monogamous males, each lekking male’s goal is to win the favor of as many receptive females as possible.
Monogamous males are visually less conspicuous, but are usually much more attentive fathers to their chicks. Similar-looking male and female Northern Gannets cement their monogamous bond by facing each other, raising their apricot-tinged necks skyward and waggling their gray bills in clattering ecstasy.
Forced copulation (rape)
Not all male birds are such romanctics. Saltmarsh Sparrow males don’t meet at leks and they don’t wait for female consent. They roam around coastal saltmarshes from the Everglades to Cape Cod in the U.S. and basically rape (ornithologists call it “forced copulation”) the females. Males take no part in nest-building, chick-rearing or territory defense. In this case, evolution has somehow favored these nasty sperm donors.
Avian sexual aggression is common. For example, after male and female Harlequin Ducks form a pair, they must mobilize to fiercely defend their matrimonial bond against unattached raiders. Male chickens are well known to force themselves on hens in the barnyard, but the females can opt shed the sperm of an unwanted rooster.
White Pelican males in rookeries frequently take advantage of unattended chicks less than 21 days old. This also is when they’re seeking adult females, and the forced copulations are probably mistakes. “If this is the case, forced copulations in pelicans can be interpreted as a non-adaptive by-product of potentially adaptive adult copulation attempts, as opposed to simply aberrant behavior,” researchers at the University of Regina (Canada) wrote in The Wilson Journal of Ornithology.
In most cases, avian promiscuity actually is a form of fertility insurance. Supposedly monogamous males don’t put all their reproductive effort in one nest, and their mates minimize their risk of picking an infertile male.
A Greater Painted-snipe lays her eggs in multiple nests from April to July in southern Asia and Africa to increase her fertility odds. She increases her reproductive output not only by mating with multiple males, but also by producing smaller-than-expected eggs so she can produce more of them faster.
About 852 birds, or 9% of all species, are cooperative, or communal, breeders, according to a 2006 review study. That number is 3-fold higher than an estimate a few years earlier. In these family units, one or more males mate with one or more females. Often the males are related to one another, but not to the females, which themselves are siblings or daughters. Non-breeding “helpers” merely assist with feeding the chicks and help out by defending the territory against intruders and predators.
What’s in it for the “helpers?”
Darwin himself was puzzled by the evolution of this kind of altruism. He couldn’t explain the evolutionary motivation of sterile honey bee helpers. Birds are much more complex. Scientists in the 1960s came up with the term “kin selection” to explain why helper birds would risk their own survival and reproduction: by sacrificing for relatives they ultimately favor their family line.
It’s easy to understand why a helper would want his own nest. Communal nests are full of intrigue, treachery and jealousy. For example, after one female lays her eggs in the communal nest, she may peck and eject her sisters’ or daughters’ eggs. Florida Scrub-jay helpers want out, and they can inherit part of the communal property as a territorial “bud” where they begin new families.
In some cases, there are too few males to go around. Some species have a plan for that: homosexual pairings that borrow nearby males as sperm donors. In the reverse situation, too many males and a shortage of females can lead to rape.
If no member of the species is available, the reproductive barrier separating species can be broken, which is how hybrids are produced.
The origin of avian reproductive biology begins with the dinosaurs. Male sauropods may have mated with females in their territories, who left eggs for the males to manage. Male-only parenting was the model.
The most primitive line of birds, which includes the Greater Painted-snipe, Jacanas, Plains Wanderer, Tinamous and Ratites (Ostriches, Emus, Rheas, Kiwis and other flightless birds), have male-only care of chicks. This model evolved to bi-parental care, female-only care, bi-parental monogamous and polygamous models, care by one female with multiple males, and multiple males and females with or without helpers.
There’s more. About 1% of all bird species are brood parasites in which neither male nor female tend eggs or chicks. Brown-headed Cowbird females mate with only 1 male, and lay their eggs in other species’ nests. Nobody knows why female cowbirds would be monogamous.
Common Moorhens add another intriguing twist. In about 25% of cases studied by a University of Cambridge (U.K.) zoologist, 2 females (mother and daughter) mate with the same male, but before they initiate egg-laying in their communal nest they first lay eggs in neighbors’ nests. The aggrieved neighbors don’t reject the parasitic eggs. “The cost of desertion was greater than the cost of acceptance,” David Wingfield Gibbons wrote in Behavioral Ecology and Sociobiology.
Many avian reproductive strategies also are found in mammals. Our curiosity in what to us are very unappealing avian behaviors is actually enabling researchers to probe more deeply how evolution operates across the animal kingdom.