Empty Shells: It seems that birds dislike empty eggshells. Shells are usually found and then picked up with the bill, flown out of the nest, and dropped at a distance. Grebes release their eggshells far from the nest by pushing them beneath the water. Adult hawks usually eat the shells. Numerous birds with early onset of puberty abandon their nests and eggshells, moving their young to other locations. Niko Tinbergen, a pioneering ethologist who studied the shell-disposal behavior of Common Black-headed Gulls, a European species that is beginning to colonize eastern North America, was drawn to these devices for separating chicks from the egg remains. After a chick hatches, Common Black-headed Gulls typically take off with the eggshell in a few hours; occasionally, they do so in just a few minutes. Tinbergen postulated that predators would find the nest more easily because of the shell’s stark white lining. However, predators like carrion crows and Herring Gulls appeared to have little issue finding the blotched, khaki-colored eggs, which appear to be well camouflaged to the naked eye. Furthermore, there are risks involved in shell disposal. The chicks and any remaining eggs are exposed for up to ten seconds after the gull departs to discard the shell, which is more than enough time for a winged predator to swoop in, seize one, and fly away. Tinbergen tested his hypothesis in several ways. He dispersed a mixture of gull eggs, some painted white and some unaltered, in a region that was frequented by raptors. The findings were clear-cut: while both types of eggs were found and consumed, the white ones were found more often. Then, two sets of unaltered gull eggs were released by him and his colleagues, one set without an eggshell and the other with an eggshell placed about four inches away. A few grass straws were used to cover the eggs in order to help them blend in; the eggs that had nearby shells were slightly better covered than the lone eggs. Once more, the findings were unmistakable: eggs near shells had a threefold higher chance of being discovered and consumed by gulls and crows than lone eggs, despite their superior camouflage. Subsequent research revealed that an intact egg was safer the farther its eggshell was placed from it. Furthermore, the gulls were most likely to remove the eggshell “dummies”bent strips of metalthat resembled real eggshells from their nests when they were shown the identical but differently colored eggshells. Green dummies that blended in nicely with the surrounding grass were not as likely to be removed as dummies in highly noticeable colors like red or blue. It turned out that color, not shape, was the key cue that indicated shell disposal. The results of these two studies supported the theory that removing the eggshell enhanced predator protection. The dummy experiment also showed that evolution had resulted in a response in the gulls that lessened the nest’s prominence by keeping vegetation that could help conceal eggs and young in addition to removing shells and other noticeable items. Tinbergen continued to learn a lot about the gulls’ ability to distinguish between an egg, a partially hatched chick, and an empty eggshell. He conducted a series of tests using modified eggs to ascertain whether the thin edge of a broken shell was the primary feature informing the adult that it was not an egg. These tests included blown eggs that were empty but had the shell intact, blown eggs that had extra broken eggshell flanges glued to them, eggshells that were open and filled with plaster or cotton wool, and eggshells that were open and filled with lead weighing as much as a chick. According to his findings, the chick’s weight appears to be what keeps gulls from tossing a hatching egg with a thin edge before the chick is free. A gull would instantly quit picking up a shell that contained a lead weight. Not a single “chick-weighted” shell was removed from the nest. The way that gulls discard their shells appears to be based on both instinct and education. Even before they lay their first egg, first-time breeders remove experimentally placed shells from their nestspossibly a genetically programmed behavior. However, birds that are incubating dummy eggs of unusual colors, such as black, tend to remove dummy shells of the same color more frequently. Such preferential association appears to be a result of learning how to “fine-tune” their innate reaction to remove the egg. Tinbergen noted that gulls removed eggshells from their nests considerably more slowly than oystercatchers and Ringed Plovers did. He came to the conclusion that the gulls’ slowness was caused by the way they built their colonial nests. Some Common Black-headed Gulls will devour freshly hatched chicks or pipped eggs left by their neighbors. It seems that parent gulls are compensated for remaining with the chicks until they are dry and fluffy, presumably to keep cannibal gulls away from them. Because they are solitary nesters, oystercatchers and plovers do not face the same danger when they abandon their nests early to discard their shells. SEE: Gull Development; Parental Care. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.
FAQ
Do birds remove egg shells from their nests?
Do birds throw eggs out of the nest?
What happens to the egg shell after the bird hatches?
What do mother birds do with the egg shells?