Autumnal Bird Migration Over The Windward Caribbean Islands

For more than two decades radar has been used to observe autumnal bird migration over the western North Atlantic Ocean and the Caribbean Sea (see Richardson 1980). Richardson (1976) used search radars to observe from the island of Puerto Rico from mid-August to early or mid-November, 1970 and 1971. Williams et al. (1977) made simultaneous short-term observations with a network of radars on the windward islands of Antigua, Tobago, and Barbados during each October from 1970 to 1973. Radar observations obtained over Antigua from 20 August to 30 September 1978 provided coverage of the fall migratory season comparable to that obtained at Puerto Rico. All data from the windward islands have been rescored and reanalyzed. The results presented here revealed an error in our previously published results and allow the development of a more unified theory of autumnal migratory behavior of birds in the Caribbean area. The FPQ-6, a long-range tracking radar (5-cm wavelength, 3-MW peak power, 0.40 beam width at 3 db points), was used on Antigua in 1970 and 1971, and the RC-32-B, a meteorological radar (10-cm wavelength, 500-kW peak power, 1.9? beam width), was used on Antigua-in 1972, 1973, and 1978 and Tobago and Barbados in 1973. Additional information on the radars may be found in Williams et al. (1977). Data were recorded from the radar Plan Position Indicator (PPI) screens on Polaroid type 107 film. In time exposures of 3-10 min slowly moving targets, such as birds, produced streaks on the film that were used to determine the direction, speed of movement, and density of migrants as described in Gauthreaux (1970) or Williams and Williams (1972). During the last minute of exposure the camera shutter was closed for 30 s and then reopened, producing a dot at the leading end of each complete track. Altitude was computed from range, angle of elevation, and correction factors for curvature of the earth and index of refraction of the atmosphere. Density of migration was determined from the maximum density observed in any PPI photograph during a 1-h period. Light, moderate, and heavy migrations were arbitrarily defined as fewer than 10 echoes in 4 min, 1020 echoes, and more than 20 echoes. Winds aloft were measured by radiosondes launched within 5 km of the radar sites, except at Tobago where the radiosonde site is located 40 km to the southwest. In almost all cases these wind data were measured within 4 h of our radar observations. Radars in the Caribbean usually operate from 0800 to 1600 local time (1200-2000 GMT). In 1978, our final observation at Antigua was at 2200 GMT daily. At all sites observations occasionally were interrupted by equipment failures or other projects with higher priority. These interruptions are shown in Fig. 1. Within the limitations of the work schedules at the radars, we obtained three observation periods/day with two or more photographs at different angles of elevation / observation period. The methods of analysis used here differ in several important respects from those used by Hilditch et al. (1973) and Williams et al. (1977). To insure uniform treatment for different years, I rescored all the original PPI photographs and rejected all tracks that could not be scored accurately for speed and altitude even if direction was unambiguous. This criterion may bias the data against small birds and those flying singly. The resulting data set of more than 2,000 tracks was used for all analyses unless otherwise indicated in the text. All statistical tests and correlations were calculated with individual tracks, not daily means. Track and ground speed refer to the velocity of a bird relative to the ground. Heading and airspeed refer to the velocity of the bird relative to the air mass and are calculated from track and ground speed by vector addition of the negative velocity of the wind measured at the altitude of the bird. Radar data show that autumnal migration over the island occurred as early as late August, with peaks in early September and again in October (Fig. 1). The heaviest migrations occurred in mid-October. Averaging tracks/observation over several years obscured the peak numbers observed during individual years. Heavy migrations in October commonly produced photographs with more than 40 complete tracks and at least 20 incomplete tracks. No photograph taken before October exceeded 14 complete tracks. Mean track by day varied from 1270 to 2070 with mean heading by day showing much less variation (128? 1660). Neither tracks nor headings were significantly (P > 0.05) correlated with date. I divided migrants into two groups, those observed before 15 September (early) and those observed after 20 September (late). There were no moderate or heavy migrations between those dates. Observations of the departure of migrants from North America (Drury and Nisbet 1964; Nisbet 1970; Williams et al. 1977; Richardson 1979, 1980) and ground observations in the Caribbean (McCandless 1961, Richardson 1976, Holland and Williams 1978) suggest this division corresponds to the peak migrations of shorebirds (early) and wood warblers (late). Histograms of the distributions of observed tracks