ARRIVAL AND PROGRESSION OF THE SWALLOW HIRUNDO-RUSTICA THROUGH BRITAIN

Arrival dates of Swallows from 20 carefully chosen years of the phenol ogical reports of the Royal Meteorological Society (1883-1947) were ab stracted. These years represent observations from 1000 different local ities in Britain and from between 24 and 325 localities per year. Arri val dates of the Swallow were processed in two different ways: (II usi ng a Geographical Information System, contour maps were produced for e ach year and compared subjectively; (2) using climatic data from Brita in and Europe, between-year variation in average arrival date and in s peed of progression through Britain were analysed statistically. Swall ows progressed through Britain at an average speed of about 50 km day( -1). In 'early' arrival years, Swallows progressed along a southwest-n ortheast axis, entering Britain by the southwest (the region receiving the most accumulated heat by April). They travelled faster across Bri tain in 'late' arrival years along a south-north axis. Arrival dates w ere earlier during warmer Aprils in Britain and western France, and fo llowing a warmer March in Iberia. Although the timing of spring migrat ion of the Swallow is related to European temperatures, the possible f uture effects of climatic changes may not be predictable because envir onmental conditions in Africa and changes in population size must also be taken into consideration.