Why Bird Eggs Are Not All the Same Shape

Eggs range from almost perfect spheres to sharp cones, and for a century the usual explanation was the nest. [A] A pointed egg laid on a bare cliff ledge rolls in a tight circle rather than off the edge, and the birds that nest on ledges do lay the most pointed eggs of all. [B] The account is satisfying and fits the best-known case, which is why it survived so long. [C] Measured across many hundreds of species, the correlation between nest type and egg shape is weak. [D]

A stronger relationship appears with flight. Birds that fly far and fast, whose bodies are narrow and whose flight muscles leave little room inside, tend to lay eggs that are long and asymmetric rather than round. A sphere holds the most volume for the least shell, but it is the worst shape to pass through a narrow body, and an egg that is elongated keeps its volume while becoming easier to carry and to lay.

This does not make the ledge story false, only local. Selection acts on the same egg from several directions at once, and in a species nesting on a cliff the rolling behavior may matter more than anything else. What the wider comparison shows is that the explanation which accounts for the most famous example need not be the one that accounts for most of the variation.