Birds have relationships that are as complex as those found in humans.
In seasonal environments such as ours, birds usually restrict their nesting behavior to spring and summer. Great horned owls and rock doves are exceptions, partly because their breeding pairs are maintained throughout the year. Most species form new pairs each year.
The details of pair-formation -- almost always an emotionally charged business in humans -- also are complex in birds.
Ninety percent of the birds we see in eastern Kansas form into male/female pairs during nesting season. A brood of young ones usually requires care from at least two adults. The costs of holding territory, of doing advertising display, of building a nest, of generating and then incubating eggs, of feeding nestlings and doing postfledging care are major investments of time and energy. Sharing the costs by the members of a pair is a straightforward solution to what is clearly a serious problem for bird parents.
When a female does most of the incubating of eggs, the male cares for her needs -- he brings food to her and may draw the attention of potential nest predators.
A few monogamous species have a perennial pair bond, and the participants are together for two or more mating seasons, or even life (rock doves). A good number of species have two or more broods per nesting season. A few species are communal, with one to four females contributing eggs to one nest (anis). A few species have helpers, whereby young from earlier nestings assist their parents by providing food or aiding in nest defense (Florida scrub jays).
Simple monogamy therefore may not be all that simple. Additionally, it can become slightly more complicated even in the simplest cases by surreptitious matings with other individuals. Either or both sexes may engage in cuckoldry (a term drawn from another ornithological reproductive specialty).
Polygamy I: one male and more than one female
A few species are polygynous, a mating system in which males attract two or more females by the quality of the resources they are able to defend (red-winged blackbirds).
Sometimes females gather together for defense against predators, and a male may be able to defend such a group against other males, becoming the mate of all the females (rheas).
Sometimes males gather together in a display area and fight among themselves for a piece of display ground. Females choose males on the basis of their display performances. Some males manage many more matings than others (prairie chickens).
Polygamy II: one female and more than one male
Some species are polyandrous, with a female mating with more than one male per season. The breeding unit may be several males and one female, but the female leaves the males after laying eggs, which then are incubated by the males (jacana). Young are precocial and able to feed themselves. Males instruct the young in feeding techniques and defend them against predators.
An alternative mode is for the female to mate with one male and desert him after eggs are laid, whereupon he becomes the effective parent and the female finds another male and lays another clutch (spotted sandpiper).
Another form of polyandry is one in which a number of males and a female compose a stable reproductive unit (Galapagos hawk). The males mate with the female, share incubation and territorial defense, and provide food to the female and the eventual young. Occasionally more than one female is in the group.
Polygamy III: many males, many females
A truly unusual mating system is one in which two or more breeding males and one or more breeding females are present. The group may be polygynous, or polyandrous, depending on the sexes in the core group. All members of the group incubate and brood, and nonbreeding helpers are part of the social unit (acorn woodpecker).
Helpers at the nest
If nonbreeding helpers are present, they may be juveniles of the year or, more likely, older birds. All are related to the parents. For the most part, helpers are male offspring, 1 or 2 years old, which have not dispersed from the parental nesting territory. They may, in later years, move off and take on the role of the fully reproductive male.
Before color-banding of wild birds was used regularly by field workers, the sex and age and relationships of helpers was unknown, and the behavior was considered to be a simple form of altruism. But when individuals could be accurately identified by unique color combinations of leg bands, helpers were seen to be assisting in the early care of their sisters and brothers.
Adults with helpers prove to have greater lifetime productivity of young than adults lacking helpers. It is probable that this highly effective form of nestling care evolved in some species of birds a very long time ago. The behavior is seen in 300 bird species worldwide. It probably is older than the similar behavior of some 25 species of mammals, including humans, but not as old as that in a few species of fishes.
An entirely different form of bird reproduction is called brood parasitism, characterized by no maternal or paternal care. This is undertaken by only a few species, but is so remarkable that it deserves a full discussion, which will be presented later.
-- Richard Johnston is professor emeritus in the Kansas University Natural History Museum and Biodiversity Research Center and a member of the Kansas Ornithological Society.