Improved decision making by migrating raptors during peak period of migration

Abstract

Earlier studies had shown that raptors on autumn migration at Buskett, Malta, tended to appear when wind strength in the early morning was low (below 10 knots; GOOD conditions). Only a small proportion of rap tors migrated when wind strength in the early morning was high (above 10 knots; BAD conditions). All raptor species except the kestrels showed such behaviour, the principal species involved being Honey Buzzard Pemis apivorus, Marsh Harrier Circus aeruginosus and Hobby Falco subbuteo. Data for three migration seasons (1976-78) were divided into three periods: EARLY (1-15 September), PEAK (16-30 September) and LATE (1-15 October), and the fraction of rap tors (all species except the kestrels) migrating in BAD conditions calculated. The fraction migrating during BAD conditions was significantly lower during the PEAK period. The average wind speed in which raptors migrated was calculated for each of the three periods. Raptors migrating during the PEAK period flew in an average wing speed which was lower than raptors migrating EARLY or LATE. It is suggested that socialisation during the PEAK period when many birds were on migration simultaneously might have improved the quality of decision making. Unfortunately, uncertainty about what actually takes place during strong northwesterly winds greatly reduces the value of this study and it is possible that the reported effect is an artefact.