Migratory bird species get a different early start than non-migratory species
In all vertebrates, mothers transfer variable amounts of hormones into eggs and embryos, which influence development and traits of offspring in later lives. In a recent study the researchers looked at the transferred hormones in different bird species eggs, and found a hundredfold difference in the thyroid hormones, which control development and growth in birds. Migratory and precocial bird species provide the highest levels of thyroid hormones to their offspring.