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Weather conditions and the visible migration of Short-toed Snake Eagles
Figure 2a. Correlation between temperatures 258 birds on 24 March) Short-toed Snake Eagles
and numbers of Short-toed Snake Eagles seen were observed in autumn and spring, heading
migrating at the site during autumn. NNE and SSW, respectively. Of the weather
variables measured at the site, barometric pressure,
Figure 2b. Correlation between temperatures relative humidity (both higher in spring than in
and numbers of Short-toed Snake Eagles seen autumn) and temperatures (higher in autumn
migrating at the site during spring. than in spring) were significantly different across
of birds passing through the study area per hour the two seasons (Table 1). Winds from NE-NNE-
with different wind conditions, we used the z N-NNW-NW (with head component in autumn
test after the F test run to compare variances. We and tail component in spring) and SE-SSE-S-
used logarithmic transformation of original data SSW-SW (with tail component in autumn and
when variances were not homogeneous. Finally, head component in spring) commonly occurred
to correlate the passage of eagles with barometric in both periods, while in autumn prevailing winds
pressure, relative humidity and temperatures we were also from WNW-W-WSW (lateral; Table 2).
used Spearman’s rank correlation tests (see Fowler Wind speed was weak (< 15 km/h) for all wind
and Cohen 1996). directions during both spring and autumn. In
Results autumn, there was no significant difference in
A total of 367 (max. one day count: 107 birds the strength of headwinds, tailwinds, or westerly
on 19 September) and 602 (max. one day count: winds (KW = 10.02, P > 0.05). In spring, tailwinds
were stronger than headwinds (U-test = 2049, P <
0.01). The comparison of the mean hourly passages
of eagles during different wind conditions (Table
4) showed that wind direction did not affect the
number of raptors detected at the site during either
autumn (headwind vs. tailwind: z = 0.11, P > 0.05;
westerly wind vs. headwind + tailwind: z = 1.79,
P > 0.05) or spring (headwind vs. tailwind: z =
0.32, P > 0.05). The passage of Short-toed Snake
Eagles was positively correlated with temperature
in both seasons (Fig.1; spring: rho = 0.36, P <
0.001; autumn: rho = 0.14, P < 0.05). However,
during autumn, the number of migrants observed
decreased drastically with temperatures above 23°C
(Fig.2a). The number of migrants recorded did not
show a daily peak in autumn. During spring, the
number of migrants recorded appeared to peak
in the early afternoon (Contingency table: χ² =
191.78; d.f. = 3; P < 0.001; fig. 3). Barometric
pressure was positively correlated with the number
of raptors observed during autumn (rho = 0.15, P <
0.05) but not during spring (rho = 0.13, P > 0.05).
Barometric pressure was negatively correlated with
temperature in autumn (rho = - 0.26; P < 0.01) but
positively correlated with temperature in spring
(rho = 0.23; P < 0.01). Finally, relative humidity
did not affect the passage of Short-toed Snake
Eagles in either season (autumn: rho = -0.12, P >
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