About 60 years ago, biologists believed that more than 90 percent of all birds were monogamous, and that males and females shared nesting duties and were true to their mates.
Then DNA fingerprinting was developed, and blood samples of parents and youngsters could reveal if paired birds were truly faithful. The short answer was “no.”
The newly discovered behavior became known as extra-pair copulation (EPC). It occurs when males or females of monogamous pairs copulate with birds of their species other than their mates. The behavior is widespread among birds and includes virtually all songbird species studied.
In the bird world, success is determined by the percentage of genes contributed to the gene pool, and EPCs can in fact increase birds’ success rate. Males typically practice mate-guarding while their mates are fertile to prevent other males from moving in and copulating with their mate. When a male moves out of his territory to solicit EPCs from other females, he runs the risk of another male fertilizing his mate. In bird talk, when a paired female receives sperm from another male, the female’s mate is said to be cuckolded.
Consider monogamous males, like robins. Participating in EPCs is more obvious for males since they have a good chance of leaving more offspring behind. On the downside, when a male leaves his territory, he is no longer mate-guarding and runs the risk of being cuckolded. Polygynous males, like pheasants, pair with two or more females. A male pheasant would probably not be interested in EPCs because he attracts a harem, and if he runs off looking for a female that would accept EPCs, other males could move in to his cluster of females and mate, very likely reducing the number of youngsters fathered by the territory holder. This male pheasant is better off to stay in his territory with his females and advertise to invite other females to join his harem.
How about females? The earliest researchers thought that males were the drivers for EPCs. But not according to the Female Choice Hypothesis. This view holds that few monogamous females will find great males for mating. That is simply because the best males will be taken by the earliest, more experienced females. Most females, then, will pair with the best males they can find, and then fly into the territories of higher quality males hoping for EPCs. Females looking for polygynous males (again like pheasants} aren’t as rushed. Females can take their time to find the best male because he will accept all interested females. Now that she has a good male, she’s not interested in EPCs, because she would likely be settling for a lower quality male.
An early study revealing EPCs in Black-capped Chickadees was done by Susan Smith of Mount Holyoke College in 1988. She studied non-migratory populations during winter at feeding stations and during the breeding season. All birds were color-banded for individual identification, and she had assessed the rank of all the females, their mates, and other males. Over 14 years, she observed females initiating EPCs 13 times: Four when a male entered a female’s territory, and nine when females flew into the territories of other males. Cleverly, in all 13, the females copulated with males of higher rank than their own mate.
As a general rule with migratory birds, older males return earlier than first-year males and have first choice for prime habitat. And they are usually selected early by the experienced females. First-year males, then, are faced with intense competition and are usually left with lesser quality habitat and inexperienced females, both of which reduce their success. Is there a better way?
About 30 songbird species exhibit delayed plumage maturation in which first-year males have a dull plumage, sometimes female-like, that serves as a social signal to adult males, indicating that they are subordinate. While an adult male will chase another adult male from his territory, he will probably not chase a subordinate male that is not a threat. In this way, a first-year male might gain a piece of land for a territory next to an adult male.
Biologist Erick Greene studied Lazuli Buntings in Montana, where preferred dense shrub habitat was patchy and limiting. Adult males arrived first and selected territories in prime habitat. Later, first-year males arrived and, surprisingly, exhibited three plumage types: mostly brown with traces of blue (female-like), brightly colored (male-like), and intermediate.
The adult males drove away birds with bright male-like and intermediate plumages, but they allowed birds with dull, brownish plumage to establish territories in the good habitat adjacent to their own. Why? At least two reasons. First, adult males could fly into the territories of these young, dull males and copulate (EPCs) with their females, but the dull males would likely not be able to stimulate the adult females for EPCs. Second, the territories of the young, dull males surround and protect the territories of the adults from intrusion by the bright male-like and intermediate plumaged young males that had been chased away earlier by the adult males, because they likely could stimulate the adult females for EPCs.
The ability of males to fertilize eggs in females other than their mates can increase the number of offspring they father, and females can increase the quality and variety of sperm fertilizing their eggs. These traits are other examples of the amazing behaviors of birds.
This article was first published in the May/June 2021 issue of BirdWatching magazine.