Geoscience Reference
In-Depth Information
subsistence economy. The pearly gray feathers of gulls were also sought after by the thriving millinery trade in
the late 19th century.
Two events—one deliberate, the other fortuitous—converged to reverse the herring gulls' fate. Around the
same time most settlers abandoned the bird islands for the convenience of life on the mainland, the Migratory
Birds Convention Act was enacted to protect the then-endangered gulls as well as many other bird species.
Perhaps the most important factor in the subsequent growth of gull populations was the waste produced by a
mechanized fishing industry and an increasingly wasteful consumer society.
Herring gulls are adaptable opportunistic feeders, as are we, and quickly capitalized on these human-gener-
ated food sources. As a result, by 1965 the census showed gull populations had ballooned to 100,000 pairs in
240 colonies from New York City to Grand Manan. The breeding population has since spread as far north as
Labrador and as far south as northeast South Carolina. That trend has reversed in the recent decade, however,
because of the sudden collapse of the groundfish industry as well as the consolidation of municipal dump sites.
The larger great black-backed gulls are found mainly on the Atlantic Coast, while herring gulls, which are
highly adaptable, have a breeding range that extends far inland, including every province and territory of
Canada and large inland lakes and rivers throughout New England. A large part of their success, while abetted
by human factors, is related to their innate behavior. Studies carried out by Nobel laureate Niko Tinbergen
demonstrated that gulls are doting parents, boosting survival of their young. Most important, gulls vigorously
defend their breeding territory, and during incubation they gently turn their eggs with their bills, a behavior
that encourages even development of the embryo. Once the eggs hatch, the gulls immediately remove the
broken eggshell, whose shiny white inner surface might attract predators. Parent gulls also guard chicks
against attacks by neighbors, since some herring gulls are cannibalistic. Once the chicks begin to explore the
feeding territory beyond the breeding ground, they must learn to mitigate the adult's territorial aggressiveness.
They do so by assuming a hunched posture, pumping their heads, and voicing shrill calls—the same set of be-
haviors that they invoke to stimulate the parents to feed them while on the breeding grounds. The behaviors,
innate and learned, of both adults and chicks combine to reduce the mortality of the young and contribute to
the herring gull's obvious success.
