Page 5 - Agostini_alii_2007

60 SHORT COMMUNICATIONS VOL. 41, NO.1

autumn migration (Agostini et al. 2005b). In particular, winds, the difference in energetic costs of powered flight
those passing via Pantelleria used a curvilinear flyway, ap- over water vs. soaring flight over land is likely lower than
parently choosing the shortest route between North Africa the difference during head winds. Finally, the location of
and this central Mediterranean island, where they changed breeding areas might also play a role in the decision of
their orientation and flew NE toward western Sicily. They whether or not to cross the Tyrrhenian Sea; in particular,
probably applied true navigational abilities, as Pantelleria birds breeding in southernmost Europe may be less reluc-
is nearly always out of sight from Tunisia (Agostini pers. tant to make the sea crossing because they are nearing the
obs.). Flocks using this flyway with W-WNW-NW winds flew end of their migration and flying in large numbers over
in tailwinds between the Cap Bon Peninsula and Pantel- wide water surfaces (Agostini and Panuccio 2005). Obser-
leria, and then moved in lateral winds from Pantelleria to vations of thousands of European Honey-buzzards seen
western Sicily. In contrast, during N-NNE winds, birds per- undertaking further sea crossings between the Italian Pen-
haps allowed themselves to drift ESE toward Pantelleria, insula and the former republic of Yugoslavia and Albania
thus avoiding flying directly into the wind, and later flew are consistent with this hypothesis (Gustin and Sorace
about 110 km in head winds between that island and west- 2004, Premuda et al. 2004). In conclusion, our study sug-
ern Sicily. As a result, during both W-WNW-NW and N- gests that prevailing winds, geography, navigational abili-
NNE winds, birds saved energy during the first stage of ties, and the time of the day interact to shape the migra-
the crossing; upon reaching Pantelleria, they interrupted tory paths of European Honey-buzzards.
their non-stop powered flight over water, exploiting ther-
mals over land before undertaking the final sea crossing
FACTORES QUE DETERMINAN LAS RUTAS DE PERNIS
from Pantelleria to western Sicily at a higher altitude. As
APIVORUS DURANTE LA MIGRACIO ´ N DE PRIMAVERA
during their autumn migration, this flight pattern of inter-
EN LA DEPRESIO ´ N CENTRAL DEL MEDITERRA ´ NEO
rupting the powered flight over water could lower the risk
of unpredictable weather changes during the water cross- RESUMEN.—Observamos a individuos de la especie Pernis
ing, and may perhaps result from a conservative strategy apivorus en puntos migratorios cerca del Mar Mediterra´-
(Agostini et al. 2005b). Moreover, passing mostly in the neo central. Se realizaron observaciones simulta´neas entre
afternoon, the birds at Pantelleria were probably more el 25 de abril y el 20 de mayo de 2004 en las islas Maret-
fatigued than those undertaking the sea crossing in the timo, Pantelleria (localizadas a los costados norte y sur del
morning and thus more reluctant to fly directly toward canal de Sicilia, respectivamente), Panarea y en los estre-
western Sicily from the Cap Bon Peninsula. Conversely, chos de Messina (ambos al el este de Sicilia). La bandada
during WSW-SW-SSW winds flocks passed mostly over Mar- se utilizo´ como unidad de muestreo. Un total de 216 ban-
ettimo, probably exploiting the tailwind to minimize ener- dadas pasaron vı´a Pantelleria, la mayor parte durante vien-
getic costs. However, during south-westerly winds, a lower
tos N-NNE y W-WNW-NW con un pico evidente a finales de
proportion of flocks was seen at Marettimo–Pantelleria
la tarde y durante el atardecer. Las aves escogieron cruzar
compared to that observed in eastern Sicily (Panarea–
por la ruta ma´s corta entre el norte de A ´ frica y Pantelleria
Straits of Messina). Probably, during such winds, many utilizando una migracio´n curvilı´nea sobre el agua. Durante
flocks used a more direct path between North Africa and los perı´odos de viento con direccio´n sur-oeste, P. apivorus
western Sicily, bypassing both Marettimo and Pantelleria. se registro´ principalmente en Marettimo a lo largo de la
During observations made at the Ustica Island (Fig. 1), ruta ma´s directa hacia el oeste de Sicilia; estas aves proba-
European Honey-buzzards were seen flying quickly with
blemente aprovecharon los vientos de cola. Con vientos
tailwinds, sometimes at a very low altitude over the water,
con direccio´n sur-oeste y durante las horas de la man˜ana,
which was in agreement with our observations made at
P. apivorus mostro´ un frente de migracio´n ma´s amplio
Panarea (Agostini et al. 2005a).
entre el este de Sicilia y la Penı´nsula Ita´lica (Estrechos
Between eastern Sicily and Italian Peninsula (Straits of
de Messina–Panarea). Nuestro estudio sugiere que los
Messina-Panarea) European Honey-buzzards migrated on vientos predominantes, la geografı´a, las habilidades de na-
a broader front in the morning and during south-westerly
vegacio´n y la hora del dı´a determinan las rutas migratorias
winds, when a higher proportion of flocks crossed the
en esta especie.
southern Tyrrhenian Sea via the Lipari Islands. Conversely, [Traduccio´n del equipo editorial]
flocks concentrated their passage via the Straits of Messina
with N-NNE winds and during midday. These results might
be explained by the fact that the thermals presumably are
weaker in the morning than during midday, and, conse- ACKNOWLEDGMENTS
quently, the difference between energetic costs of flight
We thank Andrea Corso, Giuseppe Campo, Giovanni
over land and over water is less consistent during the
Cumbo, Elisabetta Cutini, Guido Premuda, Giuseppe
morning. Moreover, as we found for the Pantelleria–Mar-
Rossi, and Antonino Provenza for the field assistance. Fi-
ettimo area, during south-westerly winds a higher propor- nally, we thank F. Ziesemer and K. Huang for their useful
tion of flocks might choose the more direct route to reach comments on earlier drafts of the manuscript. The work
the Italian Peninsula, exploiting the tailwinds. During tail- was supported by grants of LIPU-UK.