When her nest is invaded by bloodsucking mites, the female house finch juggles the birth order of her future offspring. She first lays eggs that will bear daughters--which tend to be hardier--and saves the eggs of her more sensitive sons for last. The strategy ensures that vulnerable male chicks spend less time with the mites and may help explain why the house finch has been so successful in adapting to new environments.
The house finch (Carpodacus mexicanus) is the conquistador of the bird world. Confined to the western United States and Mexico until about 1940, the bird quickly set up shop around the country when a few individuals were set loose in New York. Each population has had to overcome the hardships of its new environment, and scientists have long suspected that the secret to the bird's success is an ability to adapt very quickly to small changes in its surroundings.
To find how these birds fare in the face of an aggressive adversary, evolutionary biologist Alexander Badyaev and his colleagues at the University of Arizona in Tucson turned to a group of house finches that has lived in Arizona for hundreds of years. During the breeding season in late spring, the bloodsucking nest mite (Pellonyssus reedi) attacks mothers and chicks. As in most bird species, the male chicks are especially vulnerable to danger when born.
Badyaev and his team observed more than 100 finch nests over two breeding seasons. In the winter, when the mites are not on the scene, the researchers found that mothers laid their clutch of four or five eggs in no particular order. Boys were born more sensitive to their environment than girls, and although mothers laid one egg a day, all chicks left the nest at about the same time.
When mites started popping up in the spring, however, things changed. The mothers laid daughters before sons. In addition, the sons were not as susceptible to the mites as they were in the winter. The team suspects that's because the mothers pay a bit more attention to these late bloomers, funneling extra hormones and nutrients to them while they're still inside the safety of the egg. Besides allowing boys to be born in better shape, the strategy minimizes the amount of time males are in the nest, because they take off around the same time their sisters do. The practice helps the mothers reduce the mortality of their sons by 12.5%, the team reports online this week in the Proceedings of the National Academy of Sciences.
Invasive finches in other parts of the country could be relying on similar mechanisms to adapt to other environmental challenges, says Badyaev. Evolutionary biologist David Harper of the University of Sussex in the U.K. agrees. The study takes some of the mystery out of the "mystifying flexibility" of house finches, he says.