Social Factors

Seven of the 35 soaring raptor species that migrate through Israel form long drawn-out flocks, as do White Storks Ciconia ciconia, Black Storks C. nigra, Great White Pelicans Pelecanus onocrotalus and Common Cranes Grus grus. One presumed advantage of soaring birds migrating in such concentrations is that it makes finding thermals easier, thus conserving energy. By watching the birds ahead that are already circling upward, a bird can head for them without wasting time and energy in thermal location. The longer is the migration to be performed, the greater is the amount of time and energy saved by this behaviour. Observations made by radar and by use of a glider in Israel revealed that, on peak migration days, the lines formed by flocks extended up to 200 km, so that most individuals had before them a continuous route marked out by their predecessors (Leshem & Bahat 1999).

It is difficult with raptors to tell whether the birds migrate in flocks simply because they share the same narrow migration route, and the same thermals and updrafts within it, or whether they are attracted to one another for other reasons.

Not surprisingly, the biggest flocks are seen in relatively numerous species which migrate within a short time period, such as the Western Honey Buzzard Pernis apivorus and Levant Sparrowhawk Accipiter brevipes in the Old World, or the Broad-winged Hawk Buteo platypterus and Swainson's Hawk B. swainsoni in the New World, all of which can be seen in concentrations exceeding 1000 individuals. Once they leave a thermal, the birds seem to behave as individuals, and head off for the next thermal, whose position is marked by the presence of other circling birds. In this sense, the birds clearly benefit from travelling on the same days as others, and from watching one another, but in some species, such as the Mississippi Kite Ictinia mississipiensis and Lesser Kestrel Falco naumanni, the social bonds seem stronger. These latter species are mostly insectivores and normally feed in flocks.

Pelicans tend to migrate in their own flocks and seldom intermix with raptors. They use a somewhat different system to locate thermals. After leaving one thermal, they split into dozens of secondary flocks, flying in V-formation on a broad front that can extend for a kilometre or more. This allows the pelicans to randomly sample air currents, so when part of a flock locates a thermal and begins circling upward, the other pelicans immediately glide towards them, and in this manner the flocks proceed with maximal efficiency. This behaviour is especially important for pelicans because they migrate in individual flocks over a period of about two months, and are not concentrated within a short interval, as are storks and most raptors.

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