Abstract

The interannual variation in arrival timing of Whitethroat in England is strongly correlated with temperature and wind speed at locations to the south, along the migration route through Europe, with higher temperatures and lower wind speed, in particular at more northerly sites in France, associated with earlier arrival. Analysis of en route weather conditions demonstrates significant warming and wind stilling during the migration period in recent years, suggesting a potential role for amelioration of weather conditions in the observed advancement trend of arrival. When linked to the meteorological trends, quantified reaction norms derived from the interannual variations indicate that perhaps close to half of the recently observed arrival advancement for this species might be explained by plasticity of this sort. According to this mechanism, less favourable weather conditions that were previously more prevalent will have retarded progress during migration to a greater extent prior to recent climate change-dependent amelioration of the weather which now reduces delays to onward migration. However, the wider literature indicates that there has been advancement of Mediterranean passage of long-distance passerine migrants of a similar magnitude as the advancement of Whitethroat arrival in England, suggesting a limited role for this proposed mechanism. Earlier arrival in these European passage areas may offset the potential for further advancement due to warmer temperatures and lower wind speeds. It may mean that birds will continue to encounter similar meteorological conditions as was previously the case but, having advanced passage of the Mediterranean, slightly earlier in the year and so providing for progress through Europe at a broadly similar, meteorologically moderated rate. Amelioration of en route weather conditions may still make an important contribution to arrival advancement since, without it, earlier arrival in Europe could result in increased delays to onward migration.