Outside raking old leaves again this weekend I almost stepped on a surprising sign of spring. Only a few days after the last snowfall, half of a tiny, paper-thin eggshell was resting on the ground amid the sticks and debris of winter.

The shell had been cracked open to form a tiny cup, the top half severed cleanly from the bottom. It's hard to picture how a baby bird extricates itself like this, but every discarded eggshell is broken the same way.

Above the eggshell there was open sky; it didn't just get shoved out of the nest. When I went inside I did a little research, and found that many birds don't push empty eggshells out of the nest, but instead carry them far away from the nest location. And they don't drop both shell halves in the same place.

Biologists have done experiments showing that empty eggshells in the nest attract predators. In one study, whole eggs near empty eggshells were four times more likely to be found by predators than eggs with no broken shells nearby. It's not clear exactly why this is so, but a theory is that the bright white inside of an eggshell is like a beacon, allowing animals that prey on eggs and baby birds to spot a nest and its contents more easily.

Although flying off from a newborn chick to get rid of its shell is also fraught with danger (especially when it's still cold out) most birds seem to have a genetic aversion to keeping shells around once the eggs have hatched. In one study it was found that birds breeding for the first time removed empty eggshells from their nests, even before they laid their first egg.

A few days after coming across this unequivocal signal of changing seasons- on Easter, no less- I wished I had taken a photo. Rooting around the yard, I eventually found it, a little more crushed but still generally intact. Then it rained and the next day it was mush. Which must be why we're not walking on more eggshells this time of year.