Empty Shells

Birds appear to have an aversion to empty eggshells. Upon discovery, shells are typically picked up with the bill, flown from the nest, and dropped at some distance. Grebes thrust their eggshells under the water, releasing them far from the nest. Adult hawks usually eat the shells. Many birds with precocial young desert both nest and eggshells, herding their chicks elsewhere. These devices for distancing chicks from the remains of the eggs attracted the attention of the pioneer ethologist Niko Tinbergen, who studied the shell-disposal behavior of Common Black-headed Gulls -- a European species that is starting to colonize eastern North America.

Common Black-headed Gulls normally fly away with the eggshell within a couple of hours after a chick hatches; sometimes they carry it off within minutes. Tinbergen hypothesized that the bright white lining of the shell would make the nest easier to detect by predators. But predators such as Herring Gulls and Carrion Crows seemed to have little trouble locating blotched, khaki-colored eggs that seem well camouflaged to the human eye. Furthermore, there are risks involved in shell disposal. When the gull leaves to dispose of the shell, the chicks and any remaining eggs are exposed for up to ten seconds, more than enough time for a winged predator to zoom in, grab one of them, and depart.

Tinbergen tested his hypothesis in several ways. In an area patrolled by predators, he distributed a mix of gull eggs, some unmodified and some painted white. The results were unambiguous: although both kinds of eggs were found and eaten, the white ones were discovered more frequently. Then he and his coworkers put out two sets of unmodified gull eggs, some alone and some accompanied by empty eggshells placed about four inches away. The eggs were covered with a few grass straws to help camouflage them, and those with the shells nearby were covered a little better than the lone eggs. Again, the results were clear: even though they were better camouflaged, eggs near shells were three times more likely than lone eggs to be found and eaten by gulls and crows.

Further experiments showed that the farther from an intact egg the eggshell was placed, the safer the intact egg. And the gulls, when presented with variously colored but otherwise identical eggshell "dummies" (bent strips of metal), were most likely to remove from their nests those resembling real eggshells. There was a lesser tendency to dispose of dummies with very conspicuous colors like red or blue, and no tendency to remove green dummies that blended well with the surrounding grass. Color, not shape, proved to be the crucial clue eliciting shell disposal. These two experiments reinforced the idea that eggshell removal improved protection from predators. The experiment with dummies indicated, in addition, that evolution had produced in the gulls a response that reduced the conspicuousness of the nest not only through removal of shells and other prominent objects, but through maintenance of vegetation that might help camouflage eggs and brood.

Tinbergen went on to discover a great deal about how the gulls differentiate between an egg, a half-hatched chick, and an empty eggshell. To determine whether the thin edge of a broken shell was the main characteristic telling the adult that it was not an egg, he did a series of tests using modified eggs -- blown eggs with the shell intact but empty; blown eggs with additional flanges of broken eggshell glued to them; eggshells open and filled with either plaster or cotton wool; and eggshells open and filled with lead weighing as much as a chick. His results showed that it is the weight of the chick that apparently prevents the gulls from disposing of a hatching egg with a thin edge before the chick is free. If a gull started to pick up a shell containing a lead weight in it, it stopped immediately. Not a single "chick-weighted" shell was removed from the nest.

The gulls' shell-disposal behavior seems to be grounded in both instinct and learning. First-time breeders remove shells experimentally placed in their nests even before they have laid their first egg, presumably an act programmed into their genes. But birds that have been given dummy eggs of unnatural colors (including black) to incubate, preferentially remove dummy shells of the same color. Such preferential association seems to be a learned response "fine-tuning" their instinctive egg-removal reaction.

Tinbergen observed that oystercatchers and Ringed Plovers removed eggshells from their nests much more rapidly than did the gulls. He concluded that the slowness of the gulls was related to their colonial nesting habits. Some Common Black-headed Gulls will gulp down their neighbors' pipped eggs or freshly hatched chicks. Apparently it pays parent gulls to stay with the chicks until they are dry and fluffy, in order to prevent cannibal gulls from attacking them. Oystercatchers and plovers, being solitary nesters, do not run the same risk by leaving the nest early to dispose of shells.

SEE: Gull Development; Parental Care.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.