Female western bluebirds that have to compete for nesting sites produce more early-hatching male chicks than do females with fewer competitors. The chicks are also likely to be more aggressive. This has long-term effects on the range and behaviour of subsequent generations.

Credit: Nobuo Iwata/Moment Open/Getty

Renée Duckworth and her colleagues at the University of Arizona in Tucson discovered that female western bluebirds (Sialia mexicana; pictured) that live in areas with many neighbours and few nesting sites laid eggs containing more androgen — a hormone that boosts aggression in the offspring — than females facing less competitive pressure. Those first eggs also tended to produce more males, which can compete for and colonize new territory. When the researchers increased the number of nesting sites in study areas in western Montana, however, the females produced eggs with less androgen, and fewer male offspring in the early eggs.

This eventually allowed the western bluebird to boost its numbers and displace its competitor, the mountain bluebird (S. currucoides).

Science 347, 875–877 (2015)