"The caterpillars that feed on trees are trying to match the hatching of their eggs to the timing of bud burst. The caterpillars want to feed on the juiciest and least chemically protected leaves. And it's not just the caterpillars, of course, that are important. But the knock-on effect is on nesting birds, which are also trying to hatch their chicks at the same time that there's the maximum number of caterpillars."