The female Gouldian finch can control the sex of her egg to produce the strongest possible offspring, the BBC reports. The species comes in two head colors, red and black, and same-color mates are more compatible. If a female finds herself with a different color mate, her offspring will be 70% male, the heartier gender in that case. A same color match produces an equal number of males and females.
In birds, the sex of the offspring is entirely determined by the female egg cell. Scientists demonstrated the “sex bias” by artificially switching male head colorings, and came up with the same lopsided result. “It is pretty amazing to think that the female herself has so much control—subconsciously of course—over this basic physiology,” one researcher said. How the birds accomplish this is a “big mystery.”