Spatial variation in spring arrival patterns of Afro-Palearctic bird migration across Europe

Migratory species vary in the timing of their return to breeding areas, but geographical patterns of arrival and the factors governing them have been little studied, despite potentially affecting responses and vulnerability to global change. Using data from EuroBirdPortal, a project combining citizen science data from across Europe, we quantify continent-wide inter-specific variation in the spatio-temporal patterns of spring arrival of 30 common migrant bird species and relate these to species characteristics and environmental conditions during arrival.

Birds arrived earliest in southern Europe and last in the north-east, where the duration of the arrival period was shortest (<20 days). The average speed of spread northwards was 2.6km/h (range: 1.7-6.0). For 18/30 species, arrival duration was negatively correlated with the date spring arrival starts, offering some support for the green-wave hypothesis. Species arrived at colder temperatures in the north and within a narrower temperature window than the timing of spring, as measured by green-up temperature. Four species arrive after the start of spring at the highest latitudes, with uncertain impacts on their populations.

Arrival phenology could be split into three clusters splitting species across a major gradient running from species which arrived earlier, least synchronously and in colder temperatures and progressed most slowly northwards, through to species arriving later, most synchronously and in warmer temperatures, advancing quickly through Europe. Arrival of the former, comprising a) species wintering in the Sahel or north of the Sahara and b) aerial screening species, may be limited by temperature in Europe, whilst arrival of the latter may be determined in part by conditions in Africa, necessitating rapid advance to fill already-suitable breeding locations. There was, however, no evidence that spring phenology patterns affected long-term Europe-wide population trends, suggesting that any impacts of responding to warmer springs may be spatially variable or interact with other factors.