But HOW do they do it?

Bird Moms Manipulate Birth Order To Protect Sons:

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Since 2002, Badyaev, Oh and their colleagues have been intensively documenting the lives of a population of house finches (Carpodacus mexicanus) on the UA campus.

Throughout the year, the researchers capture birds several times a week to band and measure them and to take DNA and hormone samples. During the breeding season, the researchers locate the nests, keep track of activity in the nest, follow nestling growth and development, and take DNA samples from the chicks.

The researchers have also been counting the numbers of mites on the birds and documented a seasonal pattern. When breeding starts in February, the mites are absent. As winter turns to spring, mites start showing up on the adult females, in their nests and on their nestlings. The exact timing depends on the year.

Mites can kill nestlings.

"When it is safer inside the nest than outside, then there's no need for young to leave the nest until growth is complete, but when mortality risk of staying in the nest is great, chicks need to complete their growth fast and get out as soon as they can," Badyaev said. "What should a mother do in the face of shifting mortality risk?"

"To leave the nests sooner and still survive outside of nests, the kids need to grow faster," Badyaev said. "But the mechanisms which regulate nestling growth in relation to changing mortality were not known."

So the researchers looked to see how finch moms changed their child-rearing strategy so as to always do best by their kids.

The birds lay one egg per day. To successfully raise baby finches in the presence of mites, the mothers altered the order in which male and female eggs were laid.

When mites were absent, the chances of any particular egg being male or female were even. But once mites came into the picture, the mothers laid female eggs first and male eggs last.

Males that grew during mite season did more of their development in the egg before hatching. Their mothers accelerated their sons' growth, both in the egg and after they hatched.

"Mothers essentially hid their sons in the eggs," Badyaev said.

It's remarkable that the fledglings have such similar morphology with or without mites, he said. "Mothers did that by modifying the order of laying of male and female eggs and the pattern of their growth."

This is cool ecology and evolution. But where is the physiology, i.e., the mechanism of birth-order of sexes?