Bluebird sitting on barbed wire
Bluebird Moms Can Make Competitive Sons
Female bluebirds can produce more or less competitive sons by influencing the amounts of hormones in their eggs, say biologists.
Futurity

Renee A Duckworth, a BIO5 member, studied populations of western bluebirds and mountain bluebirds, two closely related species that compete for nesting cavities in patches of forest recently ravaged by wildfires to understand how female bluebirds can produce more or less competitive sons by influencing the amounts of hormones in their eggs.