When birds divorce: who splits, who benefits, and who gets the nest

Science News, March 7, 1998 by Susan Milius

Nobody expects a penguin to slump onto the ice and squawk brokenly to his buddies, "I shoulda seen it coming. She was always after me about money, see...."

Not many nests are wrecked by credit card excesses, so what does drive birds to divorce? Biologists are scrutinizing bird families, from courtship to breakup, with new interest these days.

For years, scientists assumed that birds which nested together pretty much stayed together without slipping off to visit alluring neighbors. In the 1980s, however, DNA analyses of nestlings revealed that the male who helps tend them is not always their genetic father. "A lot of birds are having a bit on the side," says Jeffrey M. Black of the University of Cambridge in England, "so many theories about evolution and social behavior have been turned on their heads."

From this upset, studies of feathered divorce have begun to emerge. "I think you're going to see a lot more," Black predicts.

Many researchers use the term "divorce" for paired birds that separate or fail to reunite during the next breeding opportunity. When the word first showed up in ornithology papers, "there was an uproar," Black remembers. However, ornithologists didn't seem to take to such proposed alternatives as "severance," "breakage," "dissolution," or that masterpiece of neutrality, "nonretainment." In Partnerships in Birds (Oxford University Press, 1996), which Black edited, he observes that "no sane reader should misinterpret divorce in birds as implying legal dissolution of a marriage, with alimony and lawyers' fees."

Only a minority of animals divorce--because only a few form pair bonds in the first place. These pair-bonding oddities include only about 5 percent of mammals, estimates T.H. Clutton-Brock of Cambridge. Gibbons, jackals, marmosets, and certain mice appear to mate for life.

A few invertebrates, such as burying beetles, also make a lasting commitment. The male starfish-eating shrimp guards its chosen female from the attentions of other males. A desert spider (Agelenopsis aperta) takes a lifelong mate, although they live only about 49 days.

More than 9,000 bird species pair up with a mate for certain times and purposes, says Black. 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 of birds. The percentage of pair bonds that break ranges from nearly 100, in house martins and greater flamingos, to roughly zero in Australian ravens and the waved albatross. Humans, who divorce in 40 to 50 percent of new marriages in the United States, fall into the same range as the masked booby.

The new scrutiny has tarnished even that ultimate icon of romance, the swan. Andre A. Dhondt, a Cornell University ornithologist who contributed to Partnerships in Birds, says "the biggest disappointment of the book for a nonornithologist is that swans are not the most faithful of birds." 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.

Divorce rates differ not only among species but among different populations of the same species, much as humans in Nevada untie the knot at a higher rate than those in Maine. Dhondt and his colleagues monitored great tits, little birds that nest in cavities, at nine sites in Belgium. Divorce rates ranged from essentially zero to 51 percent.

Dhondt attributes these differences mostly to habitat. Large forest patches seem to promote higher divorce rates. There, 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, Dhondt suggests. 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.

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, Dhondt says.

The decisive event in some avian breakups may last only 5 minutes, so the chances that an ornithologist will see it are slim, laments Lewis W. Oring of the University of Nevada, Reno.

Oring himself has managed to witness several divorces caused by home wreckers, who chase one of the mates away. In killdeer, these battles tend to be "subtle," he says. "There's a lot of bluffing and a lot of chasing," with occasional quick jabs to yank out feathers. He's never seen a serious injury.

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. When another female tries to take over the stable, the resident fights hard, Oring says. 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 interests to have the best females."