In Montana, it is an evolutionary advantage for the females to be big and the males, small. In Alabama the reverse is true. That is, if you are a finch.
In less than 30 years finches have undergone a remarkable adaptation. Montana finch populations have adapted to produce large females and small males. In Alabama, by contrast, finches produce large males and small females.
Most people think of evolution as a process that takes millions of years, said evolutionary biologist Alexander Badyaev of Auburn University in Alabama, who led the study. But here is an example of real-time evolution in which two populations of finches developed characteristics to match their new environments in just a few decades, he added.
It turns out that finches are able to influence the size of their offspring by controlling the sex of their eggs according to the hatching order.
It has been well documented, particularly in the poultry industry, that the first laid egg tends to produce the biggest chick. In Montana the first-born is more often female, and thus the largest. In Alabama the first-born tends to be male.
"At first we thought that first-born chicks might just grow big because they get more food by hoarding their parents' attention," said Geoffrey Hill, of Auburn University in Alabama, who collaborated on the study.
But transferring first-born chicks to foreign nests where they were the youngest revealed that their growth rate was predetermined and had little to do with the amount of food they received in the nest.
"Chicks tend to grow according to the position in which they were laid," said Hill. A first-laid chick tends to grow as a first-born even when placed in a foreign nest of older chicks.
"Something is done to the first egg as it is being laid that ensures that its chick will have an advantage," said Hill. Eggs are basically nutrition capsules, but they also contain hormones, an immunity component, and carotinoid pigment molecules that color the yolk yellow and are also important for the chick's growth and development. How birds control the sex of the egg is not known. But Hill intends to study how these various components vary according to hatching order.
"Hatching order has very strong implications for the growth and survival of the offspring," said Badyaev.
In Montana large females and small males are better adapted to survive the cold, dry winters.
If a Montana female lays a male egg first, the hatchling will grow to be large and have a much lower chance of survival. If the chick dies the mother will have wasted its resources.
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