The brilliant ways of avian brood parasites

The pin-tailed whydah is a spectacular little bird. It’s also a parasite. And if you live near Los Angeles or some other parts of the United States, it could soon become a regular visitor to your backyard, says Mark Hauber, an evolutionary ecologist at Hunter College and the Graduate Centre of the City University of New York, USA.

In a study published recently in The Condor: Ornithological Applications, Mark and his colleagues used computer modeling to predict where you might spot them next. Their models suggest that potential sites for invasion include California’s

Orange County, southern Texas, southern Florida, Puerto Rico, Jamaica and many of the Hawaiian Islands. If the birds are introduced in great numbers to these areas, they could have a damaging effect on the birds you know and love.

During the mating season, a male pin-tailed whydah grows a plume of black feathers twice as long as his body. To impress a potential partner, he hovers in front her like a helicopter, flapping his wings and dangling his long tail feathers like luxurious locks of hair. He sings. After mating, the male leaves to breed more, and the female lays eggs — in another bird’s nest. “She doesn’t have to check. She doesn’t have to feed the babies. She doesn’t have to lead them to safety after fledging,” Mark said.

Foster mothers

The pin-tailed whydah is one out of only about 100 parasites of the 10,000 bird species in the world. In its native range in sub-Saharan and South Africa, it uses more than 20 other birds as foster mothers to care for its offspring. “These birds don’t look like a virus or bacteria, but they have the same impact,” Mark said. Brood parasites compete with their hosts. And the host birds must work harder to support themselves, their own young and the offspring they are tricked into fostering. Over time, it takes a toll on the hosts.

Another brood parasite, the brown-headed cowbird, does the same thing to about 200 hosts. Some people think that its parasitism, along with habitat loss, contributed to the decline of the endangered Kirtland’s Warbler in the Midwest, and other rare species. The cowbird expanded its territory naturally, but people introduced the whydah.

The whydah has successfully colonised Puerto Rico and is starting to make a home in California, and Mark is worried. Hosts that evolved with the whydah on the African continent have learned to recognise foster babies by the spots inside their mouths, what’s known as gape pattern recognition, and they feed them less than their own babies. But the whydah has also proven itself capable of switching hosts when its tricks don’t work. “It’s basically like a virus jumping from a pig to a human or a bat to some domestic animal,” he said.

After arriving in Puerto Rico in the 1960s and 1970s, for example, they learned to fool Orange-cheeked Waxbills. And in California, scaly-breasted munias have been found feeding young whydahs. Mark is worried they could target native birds that never learned to identify whydah babies by their spots.

People buy pin-tailed whydahs as pets. But males and females paired together make poor feathered companions. When not breeding the male loses his elaborate tail feathers. And when his displays are not well received, he will pick on the female. Bored or frustrated pet owners or shopkeepers who can’t sell them may release the birds into the wild, Mark said.

If enough birds are released, if the climate is right, and, more important, if a proper host is around, the whydah can persist. But the whydah is not a good flyer, does not migrate and may not be good at crossing bodies of water. Therefore, Mark thinks any invasion will remain somewhat localised.