Trends and temperature effects in the arrival of spring migrants in Portland (United Kingdom) 1959-2005

This paper examines the first arrival dates of 25 migrant bird species recorded at Portland, Dorset, UK for the period 1959-2005. To determine if trends over time existed, linear regression of first arrival dates on year was used; initially for the whole period and subsequently separately for 1959-1981 and 1982-2005. First arrival dates were compared to spring temperatures along migration routes in southern Europe and France and in the UK. The arrival of 11 species (e.g. Sand Martin, Blackcap and Pied Flycatcher) was significantly earlier with the mean trend for all species showing an advance of 1.6 days/decade over the study period. Over all species, trends in first arrival date were not significantly different between the two time periods but there were differences for Turtle Dove, Whitethroat and Lesser Whitethroat. Monthly mean temperatures along the European part of the birds' migration route were generally negatively correlated with, but can only explain < 30% of the variability in, first arrival dates. The consequences of both a changed phenology and differential phenological changes between species require urgent investigation.