Correlations between long distance migrants in year-to-year fluctuations of arrival timing

This study investigated the year-to-year fluctuations in arrival timing in the UK of 23 species of summer migrants. Median arrival dates, estimated from BirdTrack survey data from 2002-10, and Cheshire and Wirral first-arrival dates were analysed using Pearson correlations. Highly statistically significant correlations were found for pairings within a group comprising some warblers and the Pied Flycatcher Ficedula hypoleuca, in particular for pairings of species which have similar average arrival dates. In contrast, no significant correlations were observed for pairings involving the earlier-arriving Willow Warbler Phylloscopus trochilus and later-arriving Spotted Flycatcher Muscicapa striata. No significant correlations with any other warbler species were apparent for a third species, Lesser Whitethroat Sylvia curruca, which, unlike the other species, follows an easterly migration path through the Middle East before travelling northwest across Greece, northern Italy and central Europe. It is suggested that these correlations in arrival timing may arise from the within-year similarities in local weather conditions encountered en route by species with similar spring migration strategies. Significant correlations were also identified between some other pairings, for example those of Common Cuckoo Cuculus canorus, Barn Swallow Hirundo rustica, House Martin Delichon urbicum and Tree Pipit Anthus trivialis with various other species, but were generally lacking for some other species.