Biology
Parental Investment and Clutch Size Evolution in Altricial Birds
Quick fact
The small size of a songbird's clutch is not random; evolution fine-tunes it to the number of chicks that the parents can feed successfully, and this optimum is often fewer eggs than the bird can physically lay.
Why this is interesting
Most songbirds lay four or five eggs, but why not ten? After all, more babies could mean more survivors—yet nature favors a smaller family. What invisible force prevents birds from having larger clutches?