For birds and other organisms, winning at the game of life is defined as leaving behind more offspring than other members of your species. Biologists call this contributing to the gene pool.
To be successful, it is critical that birds design and perfect mating strategies that produce a maximum number of surviving youngsters. Some birds are monogamous. Others engage in behavior that we might consider morally challenged.
Because raising a brood is hard work, males and females usually pair and share the workload. Monogamy occurs in more than 90 percent of avian species. The male’s role differs from species to species and can include helping with incubation or feeding the female as she incubates, provisioning nestlings, and caring for fledglings, all while defending the nest and youngsters.
Apparently, the number of offspring produced by most monogamous pairs isn’t enough. When biologists took blood samples from the parents and youngsters of many “families” of songbirds, they discovered a sordid truth — the chicks were often of mixed parentage.
Virtually all species of songbirds, and many others, participate in extra-pair copulations (EPCs). Paired males commonly copulate with other females. The behavior increases the males’ chances of fathering youngsters.
Often, the copulations are forced on females, but not always. A female can initiate an EPC by flying into the territory of a male and inviting copulation. She leaves with his sperm, which can create an evolutionary bonus, as it will certainly increase gene variability when combined with the sperm from her mate. And she may attract sperm of better quality if she mates with a male of higher dominance than her mate.
In cases where birds can monopolize desirable habitat or a group of birds of the opposite sex, polygamy is possible. In polygamy, a male or female pairs with two or more members of the opposite sex.
The strategy whereby a male mates with two or more females is called polygyny. Ring-necked Pheasants are polygynous. Desirable habitat will attract a group of hens. The male will protect and fertilize all of the females in his harem but will play no part in brood rearing. Competition between males is intense, and, over time, young and inexperienced males will have a chance to compete for harems.
How they mate
Sage-grouse and prairie-chickens
Of North America’s 278 breeding songbirds, 14 are polygynous, and 11 nest in marshes or grasslands. Red-winged Blackbirds are strongly polygynous. Females like the one pictured above visit the territories of both paired and unpaired males, evaluating the habitat. Usually, males with desirable habitat are paired, while males in poorer habitat are inexperienced and unpaired.
The female must choose: Will she be better off as a second or third female in good habitat or as the only female in poor habitat? Because the pairings and subsequent hatchings are staggered, the male can help feed hatchlings in all nests. Females generally select the polygynous male with good habitat.
In a few cases, females pair with more than one male, a form of polygamy called polyandry. Phalaropes and Spotted Sandpipers are polyandrous. Among Spotted Sandpipers, females are both larger and more aggressive than males, having higher levels of testosterone than monogamous females.
The polyandrous female seeks superior territory that usually attracts several males, and defends it against other females. She pairs with a male, lays a clutch of usually four eggs, and leaves incubation and brood rearing to him. Then she pairs with other males and repeats the egg-laying process. The female plays a con game: because she copulates with multiple males, the incubating males often raise youngsters that belong to their competitors, an evolutionary no-no.
When habitat quality is very good, a female can carry out all nesting activities herself and needs a male only to donate sperm. This mating strategy is known as promiscuity. Competition between males is strong, and in many species, males display in close proximity, sometimes on the same display area, or lek, where females can evaluate many males quickly.
Among prairie-chickens, leks are large. Males gather in small territories within the lek, making calls and dancing. Females evaluate the displays and then stop by males of choice, assuming a receptive posture for mating.
After copulating, the females build nests and carry out all nesting duties. More experienced males hold territories near the center of the lek, and are responsible for most of the matings. Younger males from the periphery continually challenge the older males for the choice territory in the middle.
Different mating systems allow birds to take advantage of reproductive strengths and habitat conditions. The variety demonstrates, once again, the amazing behaviors of birds.
Eldon Greij’s column “Amazing Birds” appears in every issue of BirdWatching magazine. This article appeared in the June 2014 issue. Eldon is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founding editor of Birder’s World magazine.
Read additional columns by Eldon Greij
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The muscles and bones that enable flight.
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THE MYSTERY OF ANTING
The behavior is common and fascinating but not often seen.
Why birds-of-paradise have gaudy plumage and extravagant displays.
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