A comparison of two methods for monitoring migrating broad-winged Raptors approaching a long water crossing

Observations on raptor migration were carried out at the island of Marettimo (Central Mediterranean) during the first half of October 2007. Raptors concentrate at this site before crossing the Channel of Sicily en route to Africa (130 km). We used and compared two different census methods in order to limit the problem of double-counting of migrating raptors. A total of 510 and 299 individuals were respectively counted using two different methods: the first one is the standard method in use to count migrating raptors (all-occurrence), the second one consists in considering the maximum number of raptors observed together for each observation day (maximumdaily-count). Raptors observed were almost juveniles and immatures (inexperienced individuals). For short-toed snake eagle Circaetus gallicus, booted eagle Aquila pennata and common buzzard Buteo buteo, the results show signifcant differences in counts. Weather conditions affected observed numbers of these species at the site, although they did so similarly for both methods. Results show that, at Marettimo, it is not possible to make an accurate count of inexperienced broad-winged raptors approaching the long water crossing. For this reason we suggest to consider as an activity index the daily count of the maximum number of birds of the same species observed together during each day of field work