Squillions

A haze of pale blue flowers shimmers over the clifftop turf like a chiffon  scarf: spring squill making its brief appearance during the seabirds' nesting season. Amongst it was the broken shell of a gull's egg. Perhaps the embryo had been a victim of another bird or else it could have been the remnant of a successful hatching, the shell quickly cast away by the parent bird lest its white interior should draw attention to the presence of a vulnerable chick, a popular snack for hungry predators, or another egg (ditto).

80 years ago the Nobel prize-winning biologist and animal behaviourist Nico Tinbergen devised an experiment to test whether the removal of empty egg shells by black headed gulls really was protective, given that the procedure itself meant briefly leaving the nest.  By exposing eggs with and without accompanying broken shells he was able to establish that those 75% of the former were taken but only 25% of the latter. Thus the blotchy camouflage of the entire eggs was compromised by empty shells remaining in the nest. Removing them gives the hatched chick and the remaining eggs a better chance of survival.

Which shows that taking minor precautions makes sense even when  you're a bird.

The experiment is described in more detail here.