Shell shocker
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.
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