Aerial Estimation of the Size of Gull Breeding Colonies

Abstract

Counts on photographs and visual estimates of the numbers of territorial gulls are usually reliable indicators of the number of gull nests, but single visual estimates are not adequate to measure the number of nests in individual colonies. To properly interpret gull counts requires that several islands with known numbers of nests be photographed to establish the ratio of gulls to nests applicable for a given local census. Visual estimates are adequate to determine total breeding gull numbers by regions. Neither visual estimates nor photography will reliably detect annual changes of less than about 25 percent. The herring gull (Larus argentatus) population in northeastern United States and adjacent Canada has been growing at an exponential rate since the early 1900's (Kadlec and Drury, ms. in preparation). This increasing population has created management problems by posing a hazard to aircraft at coastal airports, polluting municipal water supplies, and putting increased pressure on other species, such as terns and eider ducks. We have used aerial estimation techniques since 1961 to (1) inventory the population, (2) to attempt to detect total and regional population trends, and (3) to record, among 20 islands in Massachusetts Bay, the shifts of breeding adults that result from experimental efforts at population control. This paper reports our analysis of the accuracy of aerial censuses and aerial photography as measures of gull breeding populations. The use of aerial estimation and photography to measure populations of wild animals is not new. The technique has been used primarily to estimate waterfowl (Spinner 1946, Low 1947, Chattin 1952, Anderson and Murdy 1953, and Jessen 1957, 1Permanently assigned to a substation headquartered at Massachusetts Audubon Society, Lincoln, Massachusetts. 2 Contribution No. 61 from the Hatheway School of Conservation Education, Massachusetts Audubon Society, Lincoln, Massachusetts. among others), and more recently to estimate pairs or nests of seabirds in large colonies (Barrett and Harris 1965, Patterson 1965:456). Photographs have obvious value, but their accuracy has seldom been measured.