Divorces in Birds

Greater flamingo   (pix SShukla)

Greater flamingo                        (pix SShukla)

For centuries, it was believed that birds that nested together pretty much stayed together and were faithful to their partners. However, DNA analyses of nestlings revealed in 1980s that the male who helps tend them is not always their genetic father.

Most researchers use the term “divorce” for paired birds that separate or fail to reunite during the next breeding season. More than 9,000 bird species pair up with a mate for certain times and purposes, says Jeffrey M. Black of the University of Cambridge in England. Persistent partnerships have evolved in fewer species, including very old and more modern ones.

Black has collected estimates of divorce rates in more than 100 species. In house martins and greater flamingos the percentage of breakage in pair bonds goes nearly up to 100, while in Australian Ravens and the waved albatross it is roughly zero. In Masked Boobies it is 40 to 50 per cent. The new scrutiny has tarnished even that ultimate icon of romance, the swan. According to André A. Dhondt, a Cornell University ornithologist, five percent of whooping swan pairs end in divorce, and as many as 1 in 10 pairs of mute swans split up. However, Bewick’s Swans almost never separate.

Birds in female-dominated populations more likely to ‘divorce’ their mates

A new research has found that birds in female-dominated populations are more likely to ditch and ‘divorce’ their mates while promiscuity increases in predominantly male environments.

Findings of a joint study by the University of Sheffield and the University of Bath, published in the journal Current Biology, gives the first conclusive proof that rates of divorce and infidelity in birds are affected by the adult sex ratio of the population they live in – a theory previously discounted by biologists. It was also found that both sexes were more polygamous when the ratio was skewed towards the opposite sex.

The study, which examined the pair bonding and mating behaviour of 197 different species of bird, found the divorce rate was higher in species with a female-biased sex ratio.

However, when the number of males outweighed females, the frequency of extra-pair mating increased in socially monogamous birds.

Professor András Liker, Marie Curie Research Fellow at the University of Sheffield during the study, said: “If there is more of one sex than another, members of the rarer sex have a higher chance of getting a new partner for breeding than members of the common sex.

“Basically, the rarer sex has more opportunity to ‘play the field’ and either cheat on the partner or leave in favour of a new mate.

According to Professor Tamás Székely, of Biodiversity Lab at the University of Bath, “If there are lots of unpaired members of the opposite sex, there is more opportunity for the rarer sex to attract several partners”.

“We found that a female bias in sex ratio destabilises the pair bond system, although it is unclear as to whether this is due to lots of unpaired females luring males away from their mate or if the male, given a greater choice of partners, decides to switch partners more frequently.

“We also found that extra-pair mating was more common when there were more males than females – this could be due to females ‘shopping around’ when they have more males to choose from, or it could be that there is more rape by males unable to secure a permanent mate.

11 hypotheses on bird divorce

Jeffrey M. Black of the University of Cambridge in England has counted at least 11 hypotheses to explain the dynamics of bird divorce. The list includes theories that emphasize the role of habitat and one nicknamed “the keeping company hypothesis.” “If you keep company all year long, you’re more likely to stay together for life,” Black says. He finds the idea appealing for the geese he studies, which strike him as “traditionalists.” Species that stay together for only a few weeks, and must then find each other again the next season, tend to be less loyal.

Breeding dispersal

In an experiment conducted from 1948 to 1964 in the Forest of Dean in Gloucestershire, 250 nest boxes were carefully recorded for their locations and then analyzed for their inhabitance. It was found that females were less likely to return to a earlier breeding site following the death of, or divorce from, their former partner. It has been observed that whenever divorce takes place, the females usually move greater distances away than the males. As a result, females that keep the same mates from year to year end up moving shorter distances for each mating period than those that divorce. Divorce has little influence on the likelihood of males moving away from their original nest site. The study found that males that keep the same mate do not move significantly smaller distances than males that divorce.

Black Swan - 5; pix SShukla; Chandigarh; January 2011 (Small File)Black swan (Cygnus atratus)

Like other swans, the black swan is largely monogamous, pairing for life (about 6% divorce rate). Recent studies have shown that around a third of all broods exhibit extra-pair paternity. 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.

Great Tits (Parus major)

André A. Dhondt, a Cornell University ornithologist and his colleagues monitored cavity nesting Great Tits at nine sites in Belgium and found divorce rates in these birds ranged from essentially zero to 51 per cent.

He says these differences are mostly due to the variation in habitat sizes. In his opinion large forest patches seem to promote higher divorce rates. Here, birds gather in big flocks during the winter and search for food. At the same time, they get a good look at the mating options. Moreover, mates wintering in these flocks are more likely to get out of synch, leading one member of a pair to start breeding with another bird before the original partner is ready. On the other hand in smaller patches of woods with good gleanings or urban habitats with luxurious bird feeders, birds are less likely to form roving flocks. These neighborhoods have lower divorce rates, says Dhondt.

Spotted Sandpiper (CC BY-SA 3.0)

Spotted Sandpiper                (CC BY-SA 3.0)

Spotted Sandpiper (Actitis macularius)

Spotted Sandpiper divorces can get much nastier. These breakups stretch the meaning of divorce, since each thriving female keeps a harem of males on her territory. According to Lewis W. Oring of the University of Nevada, when another female tries to take over the stable, the resident fights hard. He’s seen a female puncture a rival’s eye or break her leg. During these fights, males “sit and watch the females duke it out,” he says. “It’s in the males’ best interest to have the best females.”

Great Skuas (Stercorarius skua)

Great Skuas, large seabirds not given to subtlety, sometimes kill each other during disputes over a mate. Robert W. Furness of the University of Glasgow in Scotland states he has witnessed females making all-out attacks to win another bird’s territory and partner. “The unattached female will swoop down with talons out,” he says. At first, she may just brush over the resident female, but as the fight escalates, those talons get used. The male Great Skua may be out at sea during the fight and come home to find a new mate waiting for him. If the male happens to be on land during the altercation, “he certainly doesn’t come rushing to the defense of his partner,” Furness observes.

Black Billed Magpie

Black Billed Magpie

Black-billed magpie (Pica hudsonia)

Pairs of adult Black-billed magpie remain together year-round and often for life unless one dies. In such a case the remaining individual may find another mate. Despite the fairly strong bonding in these birds divorces do happen: one South Dakota study found low rates of divorce (8 per cent) but another study that took seven years in Alberta found divorce rates as high as up to 63%.

Magpies generally breed from late March to early July. They usually nest once a year, but may re-nest if their first attempt fails early. The female lays up to thirteen eggs, but the usual clutch size is six or seven. Only the female incubates, for 16–21 days. The male feeds the female throughout incubation.

Sandhill Crane (Antigone Canadensis)

Cranes form long-term pair bonds that may last their lifetime and are monogamous breeders. Initial breeding attempts often fail, and in many cases newer pair bonds will dissolve (divorce) after unsuccessful breeding attempts. Pairs that are repeatedly successful at breeding will remain together for as long as they continue to do so. In a study of sandhill cranes, a species of large crane of North America and extreme northeastern Siberia, conducted in Florida,

Trumpeter Swan brood

Trumpeter Swan brood

seven out of the 22 pairs studied remained together for an 11-year period. Of the pairs that separated 53 per cent were due to the death of one of the pair, 18 per cent due to divorce and the fate of 29 per cent of pairs were unknown. Similar results had been found by acoustic monitoring (sonography / frequency analysis of duett and guard calls) in 3 breeding areas of common cranes in Germany over 10 years.

Trumpeter swan (Cygnus buccinator)

Trumpeter swan is a species of swans that are found in North America. They pair for life, and both parents participate in raising the young, but primarily the female incubates the eggs. “Divorces” have been known between birds, in which case the mates will be serially monogamous, with mates in differing breeding seasons. Occasionally, if his mate dies, a male trumpeter swan may not pair again for the rest of his life.

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