Because male baby birds are more sensitive to mites than their sturdier sisters, when mites show up in a house finch's nest, the mommy bird responds by laying male eggs later.
Effectively, the mommy bird manipulates the birth order of her offspring!
Once breeding female finches are exposed to mites, their bodies make hormonal changes that affect the order of egg laying and accelerates the development of their sons while they're still in the egg.
"We've found a mechanism by which duration of growth can be adjusted to a changing risk of mortality," said Badyaev, a UA assistant professor of ecology and evolutionary biology. He added that this is the first documentation that maternal manipulation of both ovulation and growth influences the duration of development in birds.