| Popis: |
Summary Migration over Puerto Rico was recorded by time-lapse filming of the display of a long-range surveillance radar on 108 days and 93 nights in the period 13 August-13 November in 1970 and 1971. Altitudes were recorded on nine days. Passerine movements to the E-SSE from Puerto Rico and Hispaniola occurred on many nights. Their density was greatest between late September and mid-November, especially when the easterly Trade Winds abated. Shorebirds and passerines moving SE, S and SW on a broad front approached from over the ocean intermittently throughout the period, with largest numbers from mid-September to early November. Arrivals occurred at all hours of the day and night. Many were at altitudes of 3 to 6 km, including some believed to be passerines. Birds that flew high by day descended after sunset. Many continued southwards over or past Puerto Rico. The tracks of birds arriving from over the ocean were correlated with wind direction, but were rarely downwind. The variance in tracks was slightly greater on cloudy than on clear nights, but there was no evidence of serious disorientation in any situation. There was a negative correlation between the density of arrival from the north and the easterly component of the wind. This, together with evidence that the eastern edge of most passerine flights was near Puerto Rico, suggests the existence of lateral wind drift. The relationships of estimated headings to wind and to tracks also suggested that the birds were drifted, contrary to the situation in spring. The shorebirds and passerines arriving from the NW-NE were concluding overwater flights of 2000–3000 km from SE Canada and the eastern U.S.A., often along clockwise-curved routes. Some that continued to South America flew 4000 km or more non-stop. |
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