Gull behavior part 3: Egg shell removal by a Black-headed Gull
While observing a Black-headed Gull colony today, my eye fell on an adult Black-headed Gull (Chroicocephalus ridibundus) picking at an egg shell. It quickly dawned on me that it was in the process of removing the shell from the nest, a process which is performed shortly after the hatching of a chick.
It is thought that the removal of shells at or near the nest reduces the risk of predation of eggs that have not yet hatched (the light color of the inside of the egg shell attracts attention and defeats the purpose of the outside of the eggs being camouflaged).
Before removing it, the shell may be nibbled and any material left inside might be eaten. The shell is removed by holding the thin edge and carrying it a short distance from the nest.
This behavior can be clearly seen in this short video that I was able to shoot: the adult Black-headed Gull picks up the shell, flies a short distance to drop the shell in the water, cleans its bill and immediately returns to the nest on which 2 chicks can be seen.
The Animal in Its World – Niko Tinbergen, third printing 1975
Bird Study – Egg shell removal by the Black-headed Gull (Larus r. ridibundus L.) II. The effects of experience on the response to colour, Volume 9, Issue 2, 1962