A study of house finches has demonstrated that in just 30 years, finches newly settled in Montana and Alabama begin to act quite different from each other, despite being close kin. Females in one region produce male chicks first, then females; in another region, this pattern is reversed. Controlling the sex of their eggs as they lay them allows mothers to influence the size of their offspring, an important survival trait that appears to improve these avian pioneers' chances of success in a new environment.
In 1939, many house finches were released in New York. From there, the finches headed south, reaching Alabama about 25 years ago. At about the same time, some native California birds colonized Montana. To see how the new habitats affected these populations, evolutionary ecologist Alexander Badyaev of Auburn University in Alabama and colleague Geoffrey Hill tagged thousands of birds at each site and followed their offspring from hatching through adulthood.
Badyaev found that females were controlling the sex of their offspring. Alabaman females lay males first; the final egg laid is female. The opposite is true in Montana. As in some other species, the first hatchlings grow the biggest. Thus in Alabama, males get a jump on their nest mates and grow bigger, while in Montana, females have the growth advantage. By replacing early and late eggs in field experiments, the team found that by biasing the sex of the eggs and laying them in a particular order, the mother increased chick survival by 10% to 20% over chicks from eggs laid in no particular order, they report in the 11 January issue of Science. Badyaev speculates that this advantage helped the birds colonize their new ground.
The amazing thing is that the birds have adopted these different strategies only in the past few decades. "The idea that the [divergence] could be that rapid is really remarkable," says Ben Sheldon, an evolutionary biologist at Oxford University, United Kingdom.