Influence of body condition and weather on departures of first-year European robins, <Emphasis Type="Italic">Erithacus rubecula</Emphasis>, from an autumn migratory stopover site

How migratory birds decide when to leave a stopover site is important to the understanding of bird migration strategies. Our study looks at how body condition and the weather affect the decision to depart on nocturnal migratory flight. During two autumn migration seasons (2002–2003), we radio tracked 51 first-year European robins, Erithacus rubecula, at a stopover site on the Courish Spit (Eastern Baltic) from the first day after landing until their migratory departure. The tagged robins stopped over for 1–14 days. There was no clear relationship between stopover duration and energetic condition on arrival. Weather conditions (wind, precipitation, and cloud cover) on departure differed measurably between years. In 2002, robins took off mainly under following winds and clear skies. In 2003, there were mainly light head winds and partially cloudy or overcast skies. This could be explained by the year-specific role of weather factors in making the decision to depart. In both years, robins making short (1–2 days) stopovers took off in more varied weather situations than those individuals with long stopovers. This suggests that robins from the former group were more inclined to continue with migration than longer-stay birds that, apart from re-fuelling, could be waiting for favourable weather. The lack of a relationship between stopover duration and body condition and some departures under unfavourable weather conditions suggest that endogenous spatiotemporal programmes may play an important role in controlling stopover duration in robins.     

The good and the bad stopover: behaviours of migrant reed warblers at two contrasting sites

Successful migration for passerine birds depends largely on the quality of stopover habitats, but we still lack complete knowledge of how migrants search for habitats en route and how they behave when landing at poor quality stopover sites. We compared the distance of exploratory movements and stopover durations of the reed warbler Acrocephalus scirpaceus, a reedbed habitat specialist, released at suitable (reed bed) and unsuitable (sand dune) stopover sites. Birds tape-lured during nocturnal migration to a sand dune were captured, radio-tagged, released and tracked at two sites of contrasting habitat quality. Lean birds were found to move further in the dunes (max. 300 m) than in reeds (max. 200 m), whereas ‘fat’ individuals at both sites remained stationary. Birds spent just 1 day in the dunes and up to 13 days in the reeds. Our results suggest that some nocturnal migrants with restricted diurnal exploratory movements depend on stopover site selection when ceasing nocturnal flight.