MIGRATION FROM MOROCCO INTO SOUTHWEST SPAIN IN RELATION TO WEATHER

Abstract

“We must not, I think, deny migration in general: because migration certainly does subsist in certain places, as my brother in Andalusia has fully informed me. Of the motions of these birds he has ocular demonstration”.–Gilbert White, 12 February 1771.Summary1. Migration was stuaied on the south coast of Spain between Gibraltar and Huelva during five successive springs. Systematic observations were made during a total of 11 weeks by four parties, and casual observations made by other observers have also been used.2. West of Cape Trafalgar, grounded passerine night migrants were seen in numbers only during east winds. Observations of birds crossing the moon indicated that this correlation was due to drift, and that in the absence of east winds many more birds pass over the Straits of Gibraltar than over the wider stretch of sea to the west, probably after diversion towards the Straits by the west coast of Morocco.3. The mean heading of nocturnal migrants seen crossing the moon at the Straits of Gibraltar was usually between north and N.N.E., but was N.W. on one night when clouds obscured the stars.4. Within the periods of east winds, observed migration was correlated with dry weather and rising night temperature in Morocco, but not merely with high temperature. It may also have been correlated with clear skies, but there is slight evidence that cloud cover also caused increased drift.5. Migration of raptors and storks was seen on most days. Largest numbers were seen at the east end of the Straits in west winds, and vice versa; this is probably due to drift.6. Other day‐migrants seen included Turtle Dove Streptopelia turtur, Bee‐eater Merops apiaster, Swift Apus apus, Alpine Swift A. melba, Swallow Hirundo rustica, Sand Martin Riparia riparia, and Short‐toed Lark Calandrella cinerea. Many were drifted west of the Straits, retaining their N.E. heading, but others–perhaps those breeding in western Iberia–were seen flying N.W. after crossing the Straits.7. Large numbers of Black Terns Chlidonias niger were seen flying east or N.E. on a few days of strong east wind.