Learned Behavior: Color Preference

Although the eggshell removal behavior of shorebirds is considered an instinctive behavior, their preferential association with color seems to be a learned response that enhances their instinctive behavior.

Observations by Tinbergen (1963)

Tinbergen conducted an experiment where the Common Black-headed Gulls were given dummy eggs of unnatural colors, such as blue or red, to incubate in their nests.  The birds were observed to preferentially remove the dummy shells of the same color as their own eggs – white.  This behavior enhances its anti-predation function, since by removing the white dummy eggs, the birds make their own eggs in their nests less conspicuous for the predators.

Predators of the Common Black-headed Gulls such as Herring Gulls and Carrion Crows were seen to have very little difficulty locating well-camouflaged eggs.  Tinbergen tested the Herring Gulls and Carrion Crows’ predation on different colors of eggshells to explain Common Black-headed Gulls’ tendency to remove white eggshells despite predators’ ability to locate the well-camouflaged eggs.  He found that although all different colors of the eggs were eaten by the predators, the white ones were discovered more frequently.  This result explains the Common Black-headed Gulls’ tendency to remove white eggshells over any other colored ones.

Observations by D.Max Snodderly Jr. (1978)

D. Max Snodderly Jr. investigated laughing gulls’ (Larus atricilla) visual preference for eggshell removal behavior, by preference tests where the shell models were matched to the darkness of the eggs in the nest.  The laughing gulls showed strong color preference, especially for orange.  Preference for natural models was observed, which supports Tinbergen’s findings.  However, no preference difference between the natural models and solid white models was observed, which highlights the importance of color, and not the pattern, of the eggshells.

In the same experiment, D. Max Snodderly Jr. also found that the laughing gulls showed no preference for the brightness of eggshells.  The control tests with white, grey and black models showed no difference in preference.  Thus, the brightness plays a very little role in eggshell removal behavior of the laughing gulls; the preference of the eggshell removing behavior is heavily based on color of the eggshells.

Learned Behavior: Distance between the Eggshell Disposal from the Nest

Tinbergen further showed through his experiments that the further from an intact egg the empty shells were disposed, the safer the intact eggs.  To protect their chicks from getting attacked by predators, the Common Black-Headed Gulls learned to dispose the empty eggshells, after their chicks hatch, as far away from their nest as possible.  However, leaving the chicks unsupervised in their nests for too long increases their offsprings' vulnerability to the predators.  So the parent gulls cannot go too far away to dispose the eggshells and leave their offspring unattained for too long.

Black-headed gull hatching