MIGRATION ACROSS THE MEDITERRANEAN OBSERVED BY RADAR

Abstract

SUMMARYBird migration across the Mediterranean was studied by radar from an aircraft carrier for brief periods during the springs of 1961, 1962 and 1963 and in the autumn of 1963.The radar displays were photographed and analysed to give intensity (scales 0 to 6) and tracks have been plotted on the accompanying Maps together with Tables setting out the weather data and records of bird sightings. Most echoes that could be measured were between 4,000 and 6,000 ft. Small (passerine) echoes usually moved at 20–30 knots, large ones (presumed to be waders, ducks, etc.) at up to 47 knots.The analyses of each of the nine sets of observations are discussed in detail from the Atlantic to the south of Crete, including the Aegean. There is clear evidence, within the limitations of the stated methods used and the dates of observations, that migration takes place along the whole length of the Mediterranean at almost uniform density except across the Ionian Sea, where it is greatly reduced. This is possibly because the long desert crossing and the long sea crossing are separated by only a very narrow coastal strip of vegetation and there is obvious survival value in crossing further east or west. There is no evidence for concentration across the Sicilian narrows, through Malta or across the Straits of Gibraltar. The direction of migration west of the Ionian Sea is N.E./S.W. and to the east is N.N.W./S.S.E.A large‐scale movement was observed coasting 20 miles offshore from Portugal in autumn tracking S.W. off Cape St. Vincent. This area in particular deserves further investigation as also does the area south of Crete.Migration is considerable at least from 18 August‐24 October. The main departure of small migrants from land starts 40–50 minutes after sunset and tails off with the dawn. The diurnal pattern of intensity observed at sea is noted and it is suggested that the shape of the build‐up and the time of the peak can provide a clue as to the point of departure. There are indications that in spring, especially south of Crete, some migrants do not rest in the narrow coastal strip but cross the Sahara Desert and the Mediterranean Sea non‐stop.The significance of the results is discussed in the light of further visual observations made in 1964 and 1965.