The Migration of Common Loons Through Eastern New York

Tracking radar and direct visual observations were employed to study the migration of Common Loons (Gavia immer) through eastern New York. Single birds and groups of up to four individuals flew at altitudes between 1,500 and 2,700 m (above sea level), usually above the turbulent convective layer. These migrants were not readily visible without the aid of binoculars and the tracking radar. Loons flew downwind toward the Atlantic coast in autumn (1240) and toward the eastern end of Lake Ontario and the St. Lawrence River in spring (3470). Direction of migration was more variable in spring, possibly owing to the greater variability of winds on flight days and destination of migrants. Loons never flew into headwinds. Strong westerly winds in autumn promoted higher groundspeeds than in spring. Mean airspeed was faster than that reported for most other species and did not vary in relation to following wind component as did groundspeed. Estimated flight time between Lake Ontario and the Atlantic coast of southern New England was between three and four hours depending upon wind velocity. In Canada and the northern United States, Common Loons (Gavia immer) breed primarily on freshwater lakes, and winter along the Atlantic and Gulf coasts (Palmer 1962). Because these birds depend upon water for landing, taking off and foraging, they must fly long distances over vast terrestrial habitat during migration. Stop-overs at large bodies of water such as the Great Lakes are well documented (Bull 1974). This overland migration is not well studied, although the data available indicate that loons migrate (1) at moderate altitudes (Williams 1973), (2) as individuals or flocks of 2-15 (Preston 1956, Williams 1973), and (3) at speeds in excess of 25 mps (Meinertzhagen 1955). To determine how Common Loons accomplish these long-distance migrations over inhospitable habitat, I employed tracking radar and direct observations of the flight behavior of this species in eastern New York. The behaviors examined include (1) selection of altitude, (2) direction of flight in relation to wind, (3) flight speed, (4) flock size, and (5) daily timing of migration. STUDY SITE AND METHODS During the migration seasons of spring 1978-1980 and autumn 1978 and 1979, I used a 10-cm tracking radar (Able 1978) to study the in-flight behavior of individual diurnal migrants. The radar is situated at Berne, Albany County, New York (42035'N, 74007'W, elev. 507 m). Dates of radar operation included 31 August to 12 November and 19 April to 20 May, for a total of 189 h on 80 days. Observations normally began at 09:00 or 09:30. The radar was operated primarily to track migrating raptors, but many other species of birds were also tracked. Unlike birds in nocturnal radar studies, diurnal migrants can usually be identified by an observer working with the radar operator. While the migrant is tracked, the observer finds the bird by looking along the radar beam, using binoculars. When the target is located, the number of individuals, species and the general flight behavi r are noted. For loons I noted whether they were flying straight, turning, or circling in flight. Straight flight is defined here as flight without turns greater than 200. Data include azimuth, elevation and slant range at 10-s intervals and notes made by the observer. Only tracks 20 s or greater were used for the analyses. Wind velocity was measured at the same altitudes as migrating loons by tracking weather balloons equipped with radar-reflective material. Direction of surface winds was taken from local climatological data recorded by the National Weather Service. Percent cloud cover and precipitation were recorded at the study site and other weather data were taken from the morning radiosonde at the Albany County Airport. RESULTS AND DISCUSSION GENERAL BEHAVIOR AND FLOCK SIZE Twenty-four tracks of migrating loons were recorded. One additional migrant was not tracked, but was seen with binoculars. Twentythree tracks were of birds flying along straight (Rayleigh statistic, r > 0.90, Batschelet 1965) level tracks. The remaining track was of two loons gaining altitude while flying in circles.