Of baby birds and their grandparents

In addition to homo sapiens, there are several animal species in which grandparents likely play a significant role in the survival of juveniles.

Elephant families often comprise a grandmother, her younger sisters, her daughters, her nieces, and her grandkids. The grandma is in charge, and her ability to protect the family from lions and other predators seems to grow keener with age.

Similarly, many orca pods are led by elderly matriarchs, who probably know more than their grown offspring do about where to find salmon during times of scarcity.

I’ve written about both elephant and orca grandmas, and you may have read about them elsewhere, too. Less famous are the grandmothers of the avian kingdom.

So far, grandparent care has been observed in only one bird species: the Seychelles Warbler, which inhabits five small islands in the Indian Ocean. Like humans, the birds are cooperative breeders; parents depend on “helpers” to assist in the care and feeding of young.

Most of these helpers are older sisters who haven’t begun breeding themselves. But others are grandmothers, scientists discovered a little over a decade ago.

David Richardson of the University of East Anglia and his colleagues spent decades on Cousin Island collecting data on every single warbler there. They learned that a typical “breeding unit,” or family, comprises a dominant female, a dominant male, and one to three subordinates of either sex who help the dominant pair protect and provision its chicks. The family nests in the same territory year after year.

While most dominant females retain their position until they die, a significant minority—14 percent—are “deposed,” often by one of their daughters, sisters, or nieces. Some of these deposed females leave the territory, but 68 percent remain as subordinates, helping the very females who usurped them. In a paper, Richardson theorizes that the deposed females are making the best of a bad situation: since their demotion precludes them from bearing more young, they might as well maximize their reproductive fitness by ensuring the survival of their grandkids, nieces, and nephews.

But what about the guys?

Very few dominant males are deposed, the researchers found, and rather than being loyal to their mates, they’re loyal to their territories. So when a female is deposed, her male partner generally stops mating with her and starts mating with the new dominant female. Which means, of course, that the male’s new mate is sometimes his daughter or his niece.

This isn’t too common, however, thanks to widespread infidelity among female Seychelles Warblers. About 40 percent of a female’s chicks result from copulations with someone other than her long-term mate, which means that a significant number of her offspring are unrelated to her mate.