Migration and orientation of passerine night migrants in northeast England

Evening departures of passerine migrants were watched by radar in northeast England on 193 nights in the months of August to November 1961 to 1963. It was found that migrants were not departing each night on the same heading, from which they were drifted passively by the wind, but rather were departing on similar tracks night after night. On ten nights they maintained their tracks even when the upper-air wind changed during the night. This indicates that migrants can, at least sometimes, compensate completely for the effect of the wind and fly on a fixed track, but calculations show that there are certain wind conditions when this becomes impossible.
Movements were grouped according to the months in which they took place and the tracks which they followed, and then related to variousweather factors. Migration was seen most frequently with light winds favourable to the birds'track, after a marked drop in temperature (in October/November, but not August/September), and withabsence of cloud. However, it was also recorded on 17 nights when the cloud cover was complete.
The departures of small passerines from Britain in August and September follow tracks east of south, yet the same birds reach western Iberia later in their journey. It is suggested that the direction of departure from Britain has been evolved to take advantage of the prevailing (westerly) upper-air winds. Certain warblers leave their breeding areas in Northumberland without laying down much migratory fat, yet the same species make long flights non-stop (across the Mediterranean and Sahara) later in their migrations. These migrants change froman insect to a fruit diet in autumn, and can presumably IaKdown fat more quickly the further south they travel in Europe, as the soft fruit ripens earlier in southern areas.