Adultery and Monogamy

King penguins mating (CC BY-SA 3.0)

King penguins mating (CC BY-SA 3.0)

Since early 1970s many studies, conducted on the mating behaviour of different species of birds, have brought forth very astonishing facts. Contrary to earlier belief that in more than 94 per cent of the bird species male-female remain faithful to one another after pair formation has been proved wrong. What’s even more surprising, it is the female that has a penchant for adultery, known as extra-pair copulation (EPC) in scientific parlance.

Now, with the use of advanced techniques to establish paternity, biologists are finding that, on average, 30% or more of the baby birds in any nest were sired by other than the resident males. In fact, experts are facing trouble finding bird species that are not prone to such evident philandering.

On the question why females are more adulterous? Scientists say they store up a variety of semen, perhaps so that different sperm will fertilize different eggs and thus assure genetic diversity in her offspring.

Dunnock (Author - Smalljim) (CC BY 3.0)

Dunnock (Author – Smalljim) (CC BY 3.0)

Among birds much more common is ‘social monogamy,’ where a male and female couple together raise chicks, but one or both may regularly mate with other individuals. The Dunnock (Prunella modularis), a bird also known as hedge accentor, hedge sparrow and hedge warbler, is a classic example of this. In species where unfaithfulness is common, both sexes often have high sex drives, and males tend to produce large quantities of sperm and be constantly ready for sex, at least in the breeding season.

According to scientists these facts support the theory of “sperm competition” (a competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction), a phrase that describes the physiological and behavioural responses of males and females of all living species. This “competition” arises not only out of an “unconscious urge to pass on genes”, but also to possibly “improve the genetic make-up of (the) offspring”. 


Monogamy means one male mates exclusively with one female, forming a long term bond and combining efforts to raise offspring together. Many mistaken beliefs about monogamy and infidelity started during the time of Darwin, when he and other naturalists made presumptions about mating based on field observations of coupled animals. Nearly all birds form pairs during the breeding season, and these biologists assumed that the pair bond was necessary for the survival of the young. Without the involvement of both parents, to feed and protect the young, experts thought, many offspring would not be able to make it to the fledgling stage. And that demand for stability, biologists assumed likely included monogamy as well.

Now, new discoveries about mating behavior of presumed monogamous pairs have led biologists to adopt new ways of talking about monogamy. Now they talk about three varieties: social monogamy, sexual monogamy, and genetic monogamy.

Whatever makes a pair of animals socially monogamous does not necessarily make them sexually or genetically monogamous. Social monogamy, sexual monogamy, and genetic monogamy can occur in different combinations.

Social monogamy does not always involve marriage in humans. A married couple is almost always a socially monogamous couple. But couples who choose to cohabit without getting married can also be socially monogamous.

Adultery or Extra-pair copulation (EPC)

Extra-pair copulation (EPC) or adultery is a promiscuous mating behaviour in monogamous species. It occurs when one of these individuals mates outside of this pairing.

European Magpie

European Magpie

Sightings like that of blue tits and magpies inspired biologists to apply DNA fingerprinting and other techniques used in paternity suits to help determine the parentage of chicks. They discovered that between 10% and 70% of the offspring in a nest did not belong to the male caring for them.

EPCs have been observed in more than 150 species, and extra-pair fertilizations (EPFs) have been documented in about 75 per cent of the more than 100 species in which molecular genetics techniques have been used to infer paternity. Though extra-pair offspring (EPO) proportion varies between different species, the percentage of extra-pair young (EPY) in populations may range from 0 to more than 50 per cent. In many songbird populations, the percentage of extra-pair young has been found to be about 10 – 25 per cent.

Some recent studies have provided strong support for the hypothesis that the evolution of extra-pair mating by females is favoured due to indirect benefits. Such benefits accrue much later in the offspring’s life than previously documented. According to the findings of these studies both male and female offspring produced by EPF have higher lifetime reproductive success than do offspring sired within the social pair. Moreover, adult male offspring sired via extra-pair matings (EPM) are more likely to sire extra-pair offspring (EPO) themselves, suggesting that fitness benefits to males accrue primarily through enhanced mating success. By contrast, female EPO benefited primarily through enhanced productiveness.

Counterbalancing the cheating

Biologists claim despite the adulterous adventures by both males and females there are evolutionary counterbalances that keep cheating in check. In the case of females that vigorously seek outside affairs there is always a risk of losing the devotion of their own mates. Studies have shown that in barn swallows (Hirundo rustica), a male that observes his mate copulating with other males responds by reducing his attention to her babies. It is true that males themselves are always attempting to philander whether or not they are paired to a steady mate at home. In an effort to spread their seed as widely as possible, some males go to exquisitely complicated lengths.

Early females get choicest males

Studies have shown that it is the early females that get choicest of males and the late-comers are left with less-desirable males with poor genes. But for the female that wishes to breed, even these poor males are preferable to no males at all. A 12-year study by Bart Kempenaers of the University of Antwerp has showed that blue tit (Cyanistes caeruleus) females that had paired with “unattractive” males were far more likely to seek adulterous copulations with better quality males and produce offspring. Other studies have shown young females with young mates are more adulterous than older females paired with older males.

Mate guarding

Northern Cardinal male (CC BY 2.0) (Author - Stephen Wolfe from Columbus, OH, USA)

Northern Cardinal male (CC BY 2.0) (Author – Stephen Wolfe from Columbus, OH, USA)

Interestingly, all the feverish sexual liaisons occur even though the males try their hardest to guard against being cuckolded. Like humans, in the bird society too indulging in sexual activity with someone else (extra-pair copulation) is not considered good. To avoid this many males resort to mate guarding, found in scores of bird species, including ducks, swallows, finches, magpies etc. In northern cardinals (Cardinalis cardinalis), a North American bird also known as the redbird or common cardinal, mate guarding can be very effective, almost the avian equivalent of locking on chastity belt. As a result the number of offspring belonging to males other than the resident male is quite less compared to many other songbirds. Cardinal males average more than 70 per cent of their time staying close to their mates when the females are fertile, but some are with them 100 per cent of the time. The more time they devote to mate guarding the lower the risk of cuckoldry. In one study, the only case of illegitimate young in a cardinal nest occurred where a male escorted his mate less than 20 per cent of the time. Same way male magpie (Pica pica) also shadows his female closely during her mating period so that he can prevent another male from attempting to copulate with her. 

A majority of songbird species nest in pairs and present to the world a facade of monogamy. Till 1990’s, these birds were admired by social conservatives as paragons of family values, mother and father working together faithfully to fledge their demanding chicks. Now new genetic technologies revealed that most socially monogamous birds were playing a lot of away games.

For a long time, ornithologists failed to spot this secret dishonest activity because it happened mostly just before dawn, when even the hardiest birdwatcher is not fully awake. ANU’s Professor Andrew Cockburn and his many collaborators have done some real work in this field. They rose unspeakably early for decades to study Australia’s superb fairy wren (Malurus cyaneus) and the results were remarkable. They found the bird is a real medal winner as far as extra-pair sex is concerned. Females leave their nest before dawn and head straight to males singing in the dawn chorus.

Singing in the dawn chorus gives males two reasons to rise early: first to prevent their social mate from going to another male, and secondly, to get a little bit on the side from some other male’s social mate. It has been proved that European blue tit (Cyanistes caeruleus) males that start singing early in the morning sire more chicks with other females. These are the offspring that he doesn’t have to raise because the female and her social mate do all the heavy lifting.

Common guillemot (Uria aalge)

Common Guillemot

Common Guillemot

Guillemots are among the few species where males gain by duping other males into bringing up their young. By doing this they have time to mate with more females. Males use a variety of tactics to try to mate with females paired with other males. These ‘extra-pair matings’ occur regularly, but only if the intruder is able to avoid the defenses of the male partner.

The very commonly used tactic, among the many, is to wait for the moment when the ‘husband’ mounts his ‘wife’. Whenever, such opportunity comes by, the intruder runs at the mounted male at high speed, pushes him off and takes his place. Once knocked off the cliff the original mate takes several minutes to gather his wits and return, meanwhile, the intruder completes the act of mating, with no resistance from female. The original male is now left with no choice, but to accept the situation and rear the offspring that are not his own.

Black swans (Cygnus atratus)

Like other swans, black swans (Cygnus atratus) are mostly monogamous and pair for life, but they also have divorce rate of about 6 per cent. Recent studies conducted on the paternity of these birds have shown that around a third of all broods had extra-pair paternity. In addition, an estimated one-quarter of all pairings are homosexual, mostly between males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs.

Cattle Egret (Bubulcus ibis)

Among cattle egrets as much as 30 per cent extra-pair copulations have been noted. In addition they also engage in low levels of brood parasitism. There are a few instances of cattle egret eggs being laid in the nests of snowy egrets (Egretta thula) and little blue herons (Egretta caerulea), although these eggs rarely hatch. There is also evidence of low levels of intraspecific brood parasitism, with females laying eggs in the nests of other cattle egrets.

Among these birds new mate is chosen in each season and when re-nesting following nest failure. The nest is a small untidy platform of sticks in a tree or shrub constructed by both parents. Sticks are collected by the male and arranged by the female, and stick-stealing is rife.

House Sparrows mating (GFDL)

House Sparrows mating (GFDL)

House sparrow (Passer domesticus)

House sparrows are monogamous species and mate for life, but the birds from pairs often engage in extra-pair copulations. Consequently about 15 per cent of their fledglings are unrelated to their fathers. Since house sparrow males guard their females watchfully to avoid being cuckolded, most extra-pair copulation occurs away from nest sites. Among these birds males may sometimes have multiple mates, and bigamy is mostly limited by aggression between females. Many birds that are unable to find nests and mates end up in serving as helpers around the nest for mated pairs. There is a purpose behind it, their chances of being chosen to replace a lost mate increases. Lost mates of both sexes can be replaced quickly during the breeding season. The formation of a pair and the bond between the two birds is tied to the holding of a nest site, though paired house sparrows can recognise each other away from the nest.

King penguins (Aptenodytes patagonicus)

King penguins are serially monogamous (a mating practice in which individuals may engage in sequential monogamous pairings). They have only one mate each year, and stay faithful to that mate. However, fidelity between years is only about 29 per cent. The long breeding cycle may contribute to this low rate.

These birds have an unusually lengthened breeding cycle, taking some 14–16 months from laying to the fledging of offspring. Even though birds attempt to breed annually, they are usually only successful one year in two, or two years in three in a triennial pattern on South Georgia. Their reproductive cycle begins in September to November, after they return to colonies for a prenuptial moult. Those that were unsuccessful in breeding the previous season will often arrive earlier. They then return to the sea for three weeks before coming ashore in November or December.

Zebra finches (Taeniopygia guttata)

Zebra finches although socially monogamous, are not sexually monogamous and hence do engage in extra-pair courtship and attempts at copulation. In a laboratory study, females copulated over several days, many times with one male and only once with another male. Results showed that notably more eggs were fertilised by the extra-pair male than expected proportionally from just one copulation verses many copulations with the other male. 

Eastern bluebird (Sialia sialis)

In eastern bluebirds, studies have shown that around 35 per cent of offspring is due to EPC. In socially polygynous birds, extra-pair paternity (EPP) is only half as common as in socially monogamous birds. Some ethologists consider this finding to be a support for the ‘Female Choice’ hypothesis of mating systems in birds.

Tui (Prosthemadera  novaseelandiae)

Tui (CC BY 2.0) (Author - Sid Mosdell from New Zealand)

Tui (CC BY 2.0) (Author – Sid Mosdell from New Zealand)

New Zealand’s endemic bird tui have a tendency to ‘jump the fence’ when looking to breed. They are socially monogamous, pairing with only one partner, but the chicks in their nests are often sired by their neighbouring males.

In a study by Massey University researcher Dr Sarah Wells it was found that 57 per cent of all tui chicks were extra-pair – one of the highest rates for socially monogamous birds. Study was published in the international journal Behavioral Ecology.

Using the same DNA technology used at crime scenes, Dr Wells was able to track nearly 400 tui and their chicks at Tawharanui Regional Park in Auckland. “It seems the females are looking for good genes when they are looking to breed. They look for males with bigger white plume ornaments than their nest mate – male tui of a large body size and with large plumes were also less likely to find another male’s chicks in their nest,” Dr Wells says.

“This paternity success of large males is also likely to be the reason for the extreme difference in size between male and female tui, which we found to be the highest among socially monogamous songbirds.”

Another interesting finding from this study is that both males and females tui go to other nests to “check out” the chicks, staying for several minutes staring at the young until chased off by their parents. In some cases, the intruder was the true father of the chicks.

Dark-eyed junco (Junco hyemalis)

Dark-eyed junco, a small grayish American sparrow, practices monogamy to a point. After pairing male and female both take on additional partners in the name of better genetics.

Figuring out just why juncos so frequently break their pair-bonding has been tricky. Earlier studies have only looked at the birds’ offspring during the first year or two of their life, when there’s no noticeable difference between those who were born to pair-bonded parents and those who were the product of extra-pair couplings.

But Indiana University biologist Nicole Gerlach, after 18 years of extensive research, says the real advantages kick in once the junco grows up. She explains:

“There are a lot of species that form monogamous social pairs but are decidedly promiscuous when it comes to mating and having offspring, and the question of what females gain from these extra-pair matings has puzzled scientists for a long time. What we’ve found is that, at least in juncos, these females are doing it for their kids, and for their kids’ kids. In the long run, females are likely to have twice as many grandchildren if they mate with an extra-pair male than if they remain truly monogamous.”

It’s not just the quantity of children and grandchildren – it appears to be a matter of quality as well. The products of extra-pairs are generally more fertile and show a more favorable genetic makeup than their pair-bonded counterparts.


New research on great tits by Samantha Patrick, of the Center for Biological Studies Chizé, in France suggests that males that stray from the nest for adulterous adventures may leave an opening for their mates to cheat.

While these absentee males end up with more adopted chicks from the female mate’s flings, they also leave their own offspring in other nests. On average, the “bold” males have the same number of chicks as males who stay home.

In a three year study of a wild group of great tits (Parus major), a small common European and Asian bird, the researchers found that about 13% of the chicks were offspring from extra-pair mating by the males, with about half of the nests holding offspring from “adulterous affairs.”

Black-capped chickadees (Poecile atricapillus)

In a study it has been found that infidelity is rare among Black-capped chickadees (Poecile atricapillus), a North American songbird, but it does occur ”in a very interesting way.” On occasion, a female mated to a low-ranking male will leave the nest and sneak into the territory of a higher-ranking male nearby.

It was found that in every single case of extra-pair copulations, the female wasn’t moving randomly, but very selectively. She was mating with a bird ranked above her own mate.

Scientists suggest the cheating chickadee may have the best of both worlds: a stable mate at home to help rear the young, along with the chance to bestow on at least one or two of her offspring the superior genes of a dominant male. This fits into the idea that the female is actively attempting to seek the best-quality genes.

Barn Swallow (CC BY 2.5) (Attribution - I, Malene)

Barn Swallow (CC BY 2.5) (Attribution – I, Malene)

Barn Swallow (Hirundo rustica)

Among Barn swallows it has been found that their females are quite choosy about their adulterous encounters. A study by Dr. Anders Moller, a biologist at the University of Uppsala in Sweden has revealed that barn swallow females while cheating, invariably copulate with males that have slightly longer tails compared to those of their own mates. A lengthy tail in these birds appears to be evidence that the birds are resistant to parasites; this trait would be beneficial to a female’s young. ”Females mated to very short-tailed males engage in these extra-marital affairs the most,” says Dr. Moller. ”Short-tailed males attempt to have affairs themselves, but they’re rarely successful.”

Purple martin (Progne subis) : Elders are real wily

In a study, Dr. Gene S. Morton, a research zoologist at the National Zoo in Washington, has found that older and experienced males in purple martin (Progne subis), world’s largest swallows, will happily betray their younger counterparts. They mate with their own partners early so they may devote themselves to cuckolding neophyte males later.

For doing this, such males first establish their own nests, attract a mate and then quickly reproduce. After this the male starts singing songs designed to lure a younger male to his neighborhood. That inexperienced yearling moves in and croons a song to attract his own mate, who is promptly ravished by the elder martin. A result is that a yearling male manages to fertilize less than 30 percent of his mate’s eggs, although he is the one who ends up caring for the brood.

White stork (Ciconia ciconia)

Paired white storks greet by engaging in up-down and head-shaking crouch displays, and clattering the beak while throwing back their head. Pairs copulate frequently throughout the month before eggs are laid. High-frequency pair copulation is usually associated with sperm competition (competitive process between spermatozoa of two or more different males to fertilize the same egg) and high frequency of extra-pair copulation; however, extra-pair copulation is infrequent in white storks.

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