Promiscuity in mating  behaviour  of Birds


Maneka Gandhi


(Bihar Times) Nick Davies, a British ecologist observed a pair of Dunnocks. They had been feeding together, hopping gently towards a bush. The male proceeded to one side, the female to the other. Once out of the male's field of vision, the female instantly flew into the nearby undergrowth, where she mated with a different male Dunnock who had been hidden there. Then she rejoined her mate, acting as though nothing had happened.

Loyalty, duty and the evergreen romance of the permanent couple are some of the virtues attributed to birds by poets. We assumed birds don't cheat. Well, they do. Even birds considered as paragons of fidelity will indulge in a fling if they can get away with it. 

In the 1980s, DNA analyses and parentage testing of different baby nestlings revealed that the male who tends them is not always their genetic father. "A lot of birds have boyfriends on the side ." says Jeffrey Black of Cambridge University.
Scientists now know that many females two-time; that they even go for formalised divorces - even among birds that were thought to pair for life, like swans.  Researchers use the term "divorce" for paired birds that separate or fail to reunite during the next breeding season. Researchers find that females often instigate divorce.

Blue tits, common European songbirds, mate for a season, the male defends his territory with song, and the female lays and incubates the eggs. But over 40 % of  female blue tits get around all over blue tit town, cuckolding their mates. When researchers examined the fate of chicks sired in and out of wedlock, they found illegitimate chicks.

Lewis Oring of the University of Nevada has found North American plover "home-wreckers"--individuals that break into a marriage, drive out the wife and pair with her mate. When single females go hunting for a married male, his mate will often try to stop the flirtation aggressively. Researchers at Oxford University found the female Peruvian warbling-ant bird, when threatened by competition, attempts to block out the song. The birds attempt to avoid being "jammed" by changing their tune and this helps to explain how music evolves.

House Martins and Greater Flamingos are the Elizabeth Taylors of the bird world, with 100% of pairings resulting in "divorce.". Oystercatchers are relatively faithful with a divorce rate of eight % and  Whooping Swan pairs with 5%.

In bird societies that nest communally , 40 to 60 % of offspring are a result of extra-pair mating. DNA fingerprinting,  has revealed that as many as a fifth of the eggs produced by female birds believed to be monogamous had not, after all, been sired by their regular partners. In the case of the Fairy-Wrens only about 30% of the young are fathered by the husband. In fact, the Superb Fairy Wren is the most promiscuous bird in the world where both have multiple partners.

Spotted sandpipers are wading birds with long legs and long, slender bills. The females keeps a harem of males on her territory. When another female tries to take over the stable, the resident fights hard, often puncturing a rival's eye or breaking her leg. The males sit and watch the females bash each other.
Why do female birds cheat? Their reasons are usually the same as women: boredom, lots of time, the more she flirts the more she is envied by her own kind.  Some female birds like female bee-eaters sleep with additional males to reduce the levels of harassment from the many males that hang around the communal nest sites. These males act as a "hired gun" to protect females from predators or males that would kill their young.
Some birds get divorced and remarried to move up the social ladder. Oystercatchers and black capped chickadees females dump a mate in order to move up in the world. Females living in poor nesting spots are particularly likely to initiate a split and snap up a more upwardly mobile partner with a superior nesting spot and more access to food.

Sometimes the pair is just not compatible, Divorce comes about as an initiative of both individuals to search for a more compatible mate. So if a male bird isn't pulling his weight as a food provider, or is less fertile than his neighbors, the female may well take her chances elsewhere, as found in the case of the Great Skuas.

A shore-dwelling female bird called the Kentish Plover leaves her partner as soon as her eggs hatch. The male stays on to look after the babies. The female moves on to establish another family but she, like human females , weighs the cost. First, she has to make sure that the other parent will not abandon the nest as well. Secondly, she has to be sure of finding another mate and assess whether she has the energy to make another nest. Sometimes when the male Kentish Plover sees that the female is getting ready to leave with a handsomer male, he tries the cunning psychological ploy to prevent his partner fleeing the nest – he pretends to be ill, deliberately loses weight and shows he cannot bring up the children alone. The wife gives in and stays back.

Superb Starling females cheat on their mates for a variety of reasons. Some females mate with subordinate males from within their social group when they need help to raise their chicks. The additional male then also acquires food and tends to the nestlings.

In a study of three species of monogamous shorebirds, the researchers found that "illegitimate" chicks were found overwhelmingly in the nests of partners.

Females might pair up with a guy who is a good provider, but if she can also mate with the guy with the brightest feathers, the longest tail—and get away with it—she'll go for the tall dark stranger as well ! Song performance and plumage brightness are important to female blue tits. Males with poor song performance and dull plumage are most likely to be cuckolded by their mates.

Similarly, for male Barn Swallows on the make, success is all about staying handsome. If the male’s appearance changes for the better, his female is less likely to cheat.

Females engage in extra-pair mating to guard against the possibility that their chosen husband is infertile. Because females can often store sperm for several weeks, they may only copulate with males before settling down with a mate as this may be adequate to fertilize all eggs in the event of his infertility.

What effect does divorce have on the males ?  In the case of the Great Skuas , divorced males( not widowers) have a higher probability of remaining unpaired. That could be because the divorcees were losers from the start. 

When the female deserts the male and her home, the male keeps the territory and nest and she has to find a new address. In some cases , if a female bird  is caught cheating, the partner punishes her by doing less work in raising the chicks, or in extreme cases, leaves her to raise the chicks on her own. But when the nests are close together , the single mother gets help from other females to raise her brood.
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