Sneaky Starlings

Some birds shrink their parental duty by leaving their eggs in another's nest to be warmed, hatched, and raised as part of the other's brood. The habit is known as brood parasitism. For the host bird, however, extra chicks can mean an extra burden, and some of its own chicks may starve. Recently, a group of biologists in New Jersey found that one bird species handles the problem by taking out a little insurance.

A parasitic egg often belongs to another species, but some birds can't even trust their own kind. The European starling, Sturnus vulgaris, is a case in point. Males try to cuckold and kill each other, and the females try to lay eggs in each other's nest. The optimal size for a starling's clutch of eggs - the size at which the most offspring hatch and mature - is six.

With more than six eggs there is not enough food to go around, and most or even all the chicks may die. If you start with five and pick up a parasitic egg, you can handle it; if you start with six, and get an extra one, you'll fall over the edge.