Recent Changes in the Ring-Billed Gull Population and Biology in the Laurentian Great Lakes

Abstract

SINCE World War II the Ring-billed Gull (Larus delawarensis) population of the Laurentian Great Lakes has been characterized by a massive increase in the number of nesting birds in all of the lakes. Published data on Ring-billed Gulls (Ludwig 1943) and Herring Gulls (Larus argentatus) (Paynter 1949; Hickey 1952; Ludwig 1962, 1966) were unable to indicate logical reasons for the enormous increases recorded; neither did published life tables suggest that such increases were possible in a Larus gull. Faced with the fact of an enormously increased gull population on the Great Lakes, it seemed clear that either parameters of population dynamics were incorrectly estimated, or the banding data on which the estimated mortality rates were based were inaccurate. Several workers (Hickey 1952, Hickey et al. 1966, Ludwig and Tomoff 1966) suggested that band loss could account for the unacceptable results. The incredibly rapid increase of this population that occurred between 1960 and 1965 simply could not fit even the most optimistically constructed model of population growth for the species based on field estimates of fledging rates and a reduced adult death rate (Ludwig 1966). Exploratory trapping data from 1965 showed that some birds raised in colonies of Lakes Erie and Ontario were recruited into Lake Huron colonies. This suggested that Great Lakes Ring-billed Gulls of breeding age may move from colony to colony and from lake to lake in some numbers. Ecologists have long recognized that most animal populations tend to remain relatively stable, fluctuating about a mean number from year to year in undisturbed ecosystems. Beeton (1965, 1966) cited an example of disruption of the Great Lakes in the virtual disappearance of the Hexagenia sp. mayflies from Lake Erie and the appearance of huge populations of formerly rare tubificid worms, nematodes, and fingernail clams; these effects he attributed to pollution. Thus ecosystem disruption in the Great Lakes is characterized by irruptions of formerly rare or absent species as well as extinctions or decimations of formerly abundant species. The recent explosive spread of the sea lamprey (Petromyzon marinus) and alewife (Alosa pseudoharengus) with the nearly simultaneous decline of the commercially valuable fish species underscores the fragile quality of these ecosystems (Miller 1956, Smith 1963). The species