Page 118 - Panuccio2012

Weather conditions and the visible migration of Short-toed Snake Eagles
Most raptors migrate using two basic flight styles:
flapping flight and soaring-gliding flight. When
facing different weather conditions, migrants show
a high degree of plasticity in their behavior (Vardanis southwards during spring and northwards during
et al. 2011). In particular, the daily movement of
raptors and their detectability are strongly affected
by wind direction and strength, temperature,
barometric pressure and relative humidity
(Alerstam 1990, Kerlinger 1989, Maransky et
al. 1997, Agostini et al. 2002b, Panuccio 2011,
Panuccio et al. 2010). Flight strategies have
been shaped by selection to minimize the costs
of migration, such as flight time and energy
expenditure (Hedenstrom 1993). The latter
factor shaped the migration strategies of broad-
winged species like eagles and vultures (Kerlinger
1989). The Short-toed Snake Eagle (Circaetus
gallicus) is a long-distance migrant that mostly uses
soaring-gliding flight to exploit thermal currents
and updrafts that reduce energetic costs. As a
result, migrating snake eagles avoid crossing large
bodies of water, where thermals are almost absent.
Instead, they concentrate at isthmuses, straits and
along mountain chains. Along these geographical
features, particularly mountain chains, soaring
birds can exploit stronger thermal conditions and
wave lifts. As a result, Short-toed Snake Eagles
sometimes undertake long detours over land
between their wintering grounds, located south of
the Sahara desert, and their breeding grounds in
Europe, in order to cross the Mediterranean Sea
at its narrowest points: the Strait of Gibraltar and
the Bosphorus. Birds breeding in central-southern
Figure 1. The study area.
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Greece and in the Italian Peninsula, use the so-called
“circuitous migration” that involves movements
that are opposite to the main migration direction,
autumn, at the end and at the beginning of their
movements, respectively (Agostini et al. 2002a,
Panuccio et al. 2011).
Since the migratory behavior of raptors changes
in relation to weather conditions and geographic
features, population monitoring methods that rely
on direct observation could be biased by these
factors. In addition, weather-related variables
could change between spring and autumn, affecting
migration counts in the two periods. In this study
we document the influence of wind, temperature,
barometric pressure, relative humidity and time
of the day on the numbers of Short-toed Snake
Eagles observed migrating along a mountain chain,
through systematic observations on the slopes of
Mount Olympus (northeast Greece), during both
autumn and spring movements.
Study area and methods
The mountain chain (approx. 40° 01’N, 22°
29’E) located in this area of north-eastern Greece
runs parallel and very close to the coast (approx.
7 km) forcing migrants to concentrate on a
narrow corridor (Panuccio et al. 2011). North
of Mount Olympus there is a large flat area
while the mountain chain continues southwards.
Observations were made with telescopes and
binoculars between 9 September and 1 October
2009 and between 6-26 March 2010, the peak
periods of the autumn and spring migration of
the Short-toed Snake Eagle in the Mediterranean
basin (Agostini et al. 2002b, Baghino et al. 2009).
The observations were interrupted only due to
heavy rain or snow. Each day was divided into four
two-hour periods: 9:00-10:59 hr, 11:00-12:59 hr,
13:00-14:59 hr, and 15:00-sunset hr (solar time).
Hourly weather data were obtained daily from the
Litochoro meteorological station.
In the statistical analysis we used hourly data. To
compare weather data across the two seasons we
used Mann-Whitney and Kruskall-Wallis tests,
in the latter case after a Levene test to compare
variances. When comparing the average numbers