And sure enough, she saw her first swallows on the same day. Caterpillars Many moth and butterfly eggs synchronize their hatching with leaf-out. Plants have cell walls that protect the nutritious insides. Many tiny, newly hatched caterpillars would have trouble eating through the cell walls of full-grown tree leaves to get at the nutritious food inside, but right when the leaves emerge, before the cell walls are firm and strong, these tiny caterpillars can munch on them easily. As the leaves grow and get tougher, the caterpillars are growing, too, their jaws becoming stronger.
Warblers fuel their migration in part on hatching caterpillars.
If they arrive a day or two before leaves have emerged and caterpillars hatched, they concentrate near water, where the abundance of emerging aquatic insects can sustain them as they wait for their real food supply. But most warblers arrive just as trees are leafing out. Fortunately, they can both eat small berries when insects aren't available. These two exceptions are exceptional in another way-they each have longer intestines than the other members of their families. The Barn Swallow is one native American species that has become very dependent on humans.