John A. Kadlec

Structure of the New England Herring Gull Population

Measurements of the rates of population increase, reproduction, and mortality together with an observed age ratio, were used to analyze the population of the Herring Gull in New England. Data from sporadic censuses prior to this study, aerial censuses by the authors, and National Audubon Society Christmas Bird Count indicated that the New England breeding population has been doubling every 12 to 15 years since the early 1900s. This increase has involved founding new colonies and expanding the breeding range There is evidence that 15 to 30% of the adults do not breed in any given year. Sixty—one productivity measurements on 43 islands from 1963 through 1966, involving almost 13,000 nests, showed that from 0.8 to 1.4 young/breeding pair/year is the usual range of rate of production. The age distribution in the population was determined by classifying Herring Gulls by plumage category on an aerial census of the coast from Tampico, Mexico, to Cape Sable, Nova Scotia. Of the 622,000 gulls observed, 68% were adults, 17% were second— and third—year birds, and 15% were first—year birds. Mortality rates derived from band recovery data were too high to be consistent with the observed rate of population growth, productivity, and age structure. Loss of bands increasing to the rate of about 20%/year 5 years after banding eliminates most of the discrepancy. The age structure and rate of population increase indicate a mortality rate of 4 to 9% for gulls 2 years old or older, compared with the 25 to 30% indicated by band recoveries. The population structure we have developed fits everything we have observed about Herring Gull population dynamics, except mortality based on band recoveries.

The Current Status of the Herring Gull Population in the Northeastern United States

Kadlec and Drury (1968a) suggested that the Herring Gull (Larus argentatus) population breeding in the northeastern United States increased at an average rate of 4.5% 5% per year between about 1900 and 1965, except during the period of 1940-1955 when a combination of factors caused it to level off. Aerial censuses of breeding birds conducted by us, using the same methods in 1965 and 1972, suggest an increase of only 0.75% 1.5% per year during these seven years (Table 1). The accuracy of the census method (_+ 20%, Kadlec and Drury, 1968b) is insufficient to distinguish between two alternative hypotheses: (a) little increase in breeding birds occurred in the census area between 1965 and 1972; or (b) the increase continued through the period, but the estimate in 1965 was too high, or the estimate in 1972 was too low, or both.

Aerial estimation of the size of gull breeding colonies

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 1900s (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.

Effects of saline drinking water on early gosling development

Relatively high levels of saline drinking water may adversely affect the growth, development, and survival of young waterfowl. Saline drinking water was suspect in the low survival rate of Canada goose (Branta canadensis) goslings at Fish Springs National Wildlife Refuge (FSNWR) in western Utah. Hence, we investigated the effects of saline drinking water on the survival and growth of captive, wild-strain goslings from day 1-28 following hatch. We compared survival and growth (as measured by body mass, wing length, and culmen length) between a control group on tap water with a mean specific conductivity of 650 μS/cm, and 2 saline water treatments: (1) intermediate level (12,000 μS/cm), and (2) high level (18,000 μS/cm). Gosling mortality occurred only in the 18,000 μS/c treatment group (33%; n = 9). Slopes of regressions of mean body mass, wing length, and culmen length on age were different from each other (P < 0.05), except for culmen length for the intermediate and high treatment levels. We predict that free-ranging wild goslings will experience mortality at even lower salinity levels than captive goslings because of the combined effects of depressed growth and environmental stresses, including hot desert temperatures and variable food quality over summer.