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 t