Origin of passerine migratory waves: Evidence from the blackcap at a stopover site

Hundreds of millions birds migrate through Israel every spring, usually in several waves during which a large number of birds are seen flying over Israel for several days, followed by periods in which only a small number are spotted. The aims of this study were to (1) determine whether the spring migration of the blackcap demonstrates a wave pattern, (2) determine the effect of weather conditions en route on the number of blackcaps arriving and forming each wave, and (3) use morphological characteristics and stable isotope composition of the feathers to examine the hypothesis that early arriving blackcaps represent populations breeding in low latitudes while later ones represent northern populations.We found that (1) our first hypothesis was fully supported, and blackcaps arrive to Israel in "waves"; (2) weather conditions, and in particular wind speed and direction, were significantly related to the number of blackcaps captured; and (3) our morphological and δ C-13 data suggest that birds from the early season originated from more southern latitudes and/or more western longitudes within the breeding range of blackcaps.