Week 51 – 02 July 2017:

The Leading Cause of Divorce Among Birds Is…

Mammals are a pretty scandalous lot. In most species, males and females come together only long enough for insemination, and then go their separate ways. In contrast, males and females of most species of birds form a long-lasting pair bond. Even when the current breeding attempt comes to an end, the pair may choose to mate with the same individual in subsequent attempts. It seems that familiarity with one’s mate is likely to increase the chances of reproducing successfully. Simple. And yet, divorce is surprisingly common in socially-monogamous birds. If it is advantageous to stay together, then what causes a pair to divorce?

Nataly Hidalgo Aranzamendi and her colleagues recently published a paper in which they investigated the possible causes of divorce in socially monogamous birds. In order to reduce the number of potentially confounding variables, they chose to study a species in which individuals remain paired and maintain their territories throughout the year, Australia’s Purple-crowned Fairy-wren. The breeding biology of these birds is pretty special; a mated pair may be aided by a number of subordinate males and females who help to raise the young. Most of these helpers are offspring from previous breeding attempts.

Hidalgo Aranzamendi et al. recognized three possible causes of divorce in fairy-wrens. First, an individual may be looking at better options – seeking either a better territory or a better partner. Second, if the mated pair is too closely related, an individual might seek to avoid the difficulties of inbreeding. Finally, it is possible that a divorce is not always voluntary.

The research was carried out over a period of ten years at the Mornington Wildlife Sanctuary in the Kimberly region of Western Australia. The site is recognized an an Important Bird Area by BirdLife International, and administered by the Australian Wildlife Conservancy. Three-hundred and seventeen breeding pairs were studied. These birds occupy territories along creeks and rivers. Coloured leg bands allowed the researchers to distinguish among individuals. Genetic testing screened for paternity, and incestuous matings were investigated with pedigrees.

Hidalgo Aranzamendi and her crew demonstrated that divorce is anything but simple in Purple-crowned Fairy-wrens. There was a high divorce rate among individuals that had paired with close relatives, and so divorce was a way of avoiding further incest. Females that left their mates often obtained mating opportunities on higher-quality territories. Infidelity, as demonstrated by offspring that were not the product of their social fathers, was also a predictor of divorce. In a few cases, territorial females were evicted by other females, showing that divorce is sometimes forced. What causes divorce in birds? Lots of things do.

How complex can the breeding behaviour of fairy-wrens be? The research team found that one female, socially-mated to her son, had produced offspring by copulating with a male on an adjacent territory. She divorced her son, and entered the territory of the male with whom she had previously copulated. That male divorced his current partner who was also his daughter. Could that possible be right?

I contacted Hidalgo Aranzamendi to check. Apparently I had interpreted the paper correctly. She described it as “an interesting example of the purple-crowned soap opera.” It seems that other species of Australian fairy-wrens have an even higher incidence of parentage by males other than the social fathers.

Hidalgo Aranzamendi described her feelings about field work to me. “Watching and following these birds and discovering their partnerships, new romances and infidelities, was a great adventure.” As a field biologist, I knew just what she meant. “I recall my best moments in the Kimberleys as those watching the most beautiful sunsets at the end of a long-day of work while drinking a cold beer in the company of great friends. Nothing can beat that feeling.”

Why would anyone want to do anything other than study birds?

Hidalgo Aranzamendi, N., M. L. Hall, S. A. Kingma, P. Sunnucks, and A. Peters. 2016. Incest avoidance, extrapair paternity, and territory quality drive divorce in a year-round territorial bird. Behavioural Ecology 27:1808-1819.

Photo credits: Purple-crowned Fairy-wrens, © Michelle L Hall – phys.org; Purple-crowned Fairy-wren – animalia-life.club