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Figure 4.3 Passage rates of nine species of raptor at Hawk Mountain in Pennsylvania, 1934-91, in relation to the passage (NW-SE) of cold fronts. The height of each bar is expressed as a percentage of the highest passage rate for each species. Passage rates on days with shaded bars are significantly higher than those on other days (Tukey's Studentized range test in association with one-way analysis of variance for species in which ANOVA probability was <0.05).

Using pooled data from a 55-year period of observations, cold fronts passed this area, on average, every 4-5 days in autumn, and raptor migration increased in the first three days after the passage of each front. Three basic patterns emerged, related to the flight mode of the species involved. Falcons (top three) had their highest rates of passage on the same day of frontal passage; accipiters (middle three) had their highest rates on the first day after the passage; and buteonine hawks (bottom three) on the first three days after the passage. The falcons migrated mainly by direct flapping flight, a mode suited by the high-speed winds that typically occurred soon after frontal passage. Accipiters flap less and soar more, so are better suited by the lighter updraft-producing northwesterly winds and light thermals that begin to form within a day after frontal passage. Finally, buteos soar more and fly higher on migration than do most other raptors, making them suited by the weather conditions that occur 2-3 days after frontal passage.

During the 55-year study period, 10-20 cold fronts passed Hawk Mountain each year between 1 September and 23 November (the peak migration period). In years with few fronts, more raptors passed after each one, reflecting the greater build-up of birds between fronts. The number of cold fronts did not affect the overall numbers passing each year. From Allen et al. (1996).

other method. Nevertheless, observations show that some low-level flight (below the radar horizon) occurs under headwinds and other conditions that inhibit higher level flight. In these conditions, many small birds migrate by 'bush-hopping' or 'tree hopping', in which they flit from bush to bush or from tree to tree, feeding as they go, but travelling continually in the same direction. Warblers, which normally migrate at night, sometimes pass in daytime through extensive bushy or reedy areas in this way, as do tits and other canopy-feeding passerines as they travel through wooded terrain. Many small birds also fly low for short distances in the gaps between showers. Such low-flying birds take more account than high-flying birds of topographical features, being more deflected by hills and water bodies, and in strong winds taking shelter provided by ground contours or forest edges. In these ways, small birds can maintain some progress on days when they might not otherwise travel because of strong headwinds or insufficient body reserves. By their combination of flying and feeding, these small passerines resemble swifts and hirundines that pick off insects as they go. But as headwinds increase, fewer and fewer birds take part in the migration, as species after species drops out.

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