J. Zool. doi:10.1111/j.1469-7998.2009.00596.x (2009)

Big eggs risk being crushed by the big birds that sit on them. Eggshells must be sturdy, but not so sturdy that they entomb chicks. These conflicting demands placed on eggshells set the upper size limit for birds, according to Geoffrey Birchard of George Mason University in Fairfax, Virginia, and Charles Deeming of the University of Lincoln, UK. They are also, the authors suggest, the reason that females outweigh males in the largest of species.

The researchers analysed shell thickness, body mass and incubation behaviour for 968 species of bird. Having relatively small males do the incubating allows eggs to be bigger yet thinner-shelled. The largest specimens of extinct giants such as the 400-kilogram elephant bird might all be female, they speculate.