Page 121 - Panuccio2012

Weather conditions and the visible migration of Short-toed Snake Eagles
0.05; spring: rho = -0.06, P > 0.05).
Discussion
The lack of difference between numbers of
eagles observed during westerly (lateral) winds
and during other wind directions (headwinds
+ tailwinds) in autumn, suggests that migrants
compensated the drift effect of such lateral
winds when passing through the study area to
avoid to be blown off over the Aegean Sea. As
reported in previous studies, raptors are able
to compensate the drift effect of lateral winds
by changing their heading and/or limiting the
use of soaring-gliding flight in particular when
migrating through or near an ecological barrier,
such in our case study (Kerlinger 1989, Klassen
et al. 2010, Panuccio et al. 2010). Also, tail and
headwinds did not affect counts at the site during
both spring and autumn. This result could be
explained by the fact that such winds were weak
during the observation periods although, during
spring, tailwinds were significantly stronger than
headwinds. On the other hand, tailwinds would
have had to increase the cross-country speed of
migrants and consequently their passage at the site with temperatures higher than 23°C. In March,
during periods with such atmospheric conditions
(Kerlinger 1989, Spaar 1995).
During autumn (but not spring) migration, adult
and juvenile Short-toed Snake Eagles belonging
to the population breeding in Greece tend to
migrate at the same time, increasing the number
Figure 3
Variation of the migratory flow of Short-toed
Snake Eagles throughout the day (solar time)
during autumn 2009 (9 September – 1 October)
and spring 2010 (6 – 26 March).
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of migrants expected to pass at the observation site
during post-reproductive movements (Panuccio et
al. 2011). However, in our study, larger numbers of
migrants were seen during the spring than during
the autumn migration. Observer detection could
have been affected by different thermal conditions
in the two seasons along the mountain chain. In
particular, the temperature in northern Greece
was higher, and consequently thermal conditions
stronger, in September than in March. As expected
for a soaring bird (Maransky et al. 1997), the
number of Short-toed Snake Eagles observed was
positively correlated with temperature during
both seasons; however, larger numbers of migrants
were seen in spring, when the temperature was
lower. Short-toed Snake Eagles would have had
to increase their travelling speed during favorable
thermal conditions showing a daily peak in
early afternoon in both seasons (Kerlinger 1989,
Panuccio et al. 2010). However, during our study,
this pattern was reported during spring but not
during autumn migration when, conversely, the
number of migrants detected drastically decreased
large areas of the mountain chain were covered
with snow. Perhaps during autumn migration,
when stronger thermal conditions occurred, a
higher proportion of Short-toed Snake Eagles
passed out of sight in early afternoon. Migrating
broad-winged raptors become difficult to see at
altitudes higher than 600 m above ground level
(Kerlinger 1989). Previous radar studies made
both in the United States and Israel showed that
the flight altitude of raptors increases linearly with
time of the day until the afternoon (Kerlinger
and Gauthreaux 1985, Bruderer et al 1994, Spaar
1995). Moreover, eagles (i.e. Steppe Eagles,
Aquila nipalensis) appear to be less likely to fly
at low altitudes than smaller species of raptors
(European Honey Buzzards, Pernis apivorus, and
Steppe Buzzards, Buteo buteo vulpinus, Spaar
and Bruderer 1996). Data collected by radar
in Israel also showed that migrating birds fly at
higher altitudes in autumn than in spring, and
in particular in early spring when flight altitude
was much lower (Spaar et al. 2000, Dinevich and
Leshem 2010). Bruderer et al. (1994) suggested