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DIRECTION, SPEED, AND COMPOSITION OF NOCTURNAL BIRD MIGRATION IN THE SOUTH OF ISRAEL

In: Israel Journal of Ecology and Evolution
Authors:
FELIX LIECHTI Swiss Ornithological Institute

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BRUNO BRUDERER Swiss Ornithological Institute

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Flight paths of more than 40,000 nocturnal migrants were collected by tracking radars simultaneously at two sites in spring and autumn. Winds were similar in autumn and spring: i.e., typical, rather weak trade winds with northerly directions below about 1500 m above sea level and with southerly or westerly directions higher up. The tracks of the birds were concentrated around 190° (between S and SSW) in autumn and around 5° (nearly N) in spring. Slight differences between the directional distribution of birds above the Negev Highlands and the Arava Valley could be attributed mainly to local winds and topography. Reversed migration occurring in autumn was at the altitudes of the southwesterly winds. Tracks and headings of different wingbeat classes did not differ significantly, except that birds with low wingbeat frequency and passerine-type flight had more westerly directions in autumn and more easterly directions in spring. Mean groundspeed in autumn was nearly 50 km/h, most birds profiting from following winds. In spring many migrants had to fly against the wind, thus, the groundspeed was on average lower (45 km/h) and its variation much wider than in autumn. Mean airspeed, however, was slightly higher in spring than in autumn, indicating a tendency to compensate for opposing winds. In weak winds airspeeds of wader-type birds were slightly higher than those of passerines; surprisingly, small wader-and passerine-type birds with higher wingbeat frequencies were faster than those with slower wingbeats. A high proportion of nocturnal migrants belonged to the wingbeat class of passerine-type birds. In autumn, the proportion of passerines with fast wingbeats increased from about one third in August to almost 50% at the end of October. During August and September, about one third of the identified birds had a wader-type wingbeat pattern. In spring, the proportion of passerines decreased slightly from March till May, correspondingly the proportion of wader-type birds slightly increased.

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