Weather induced variations in raptor migration : a study of raptor migration during one autumn season in Kazbegi, Georgia, 2010

Abstract: Raptors migrate by using either soaring and gliding flight or a combination of gliding and powered flight. The energy consumption when using powered flight increases rapidly with body mass and is therefore mainly used by the smaller raptors, such as Falcons, Sparrowhawks and Harriers. Soaring and gliding flight are mainly used by the larger raptors such as Buzzards, Kites and Eagles. When using soaring and gliding flight, birds are dependent on updrafts from thermals or orographic winds to gain height. This dependence makes them vulnerable to environmental conditions which would inhibit the creation of these updrafts and result in variations in the amount of migrating raptors that can be seen from a given point. Thermal updrafts are created by uneven solar heating of the ground where certain spots are heated more quickly than others due to its low albedo. The warm surfaces then conduct heat to the overlaying air which expands and rises. Orographic updrafts come from horizontal winds that are deflected upwards by for example a mountain ridge. Large mountain chains act as physical barriers for migrating raptors due to the different environmental conditions such as precipitation, fog and strong winds that affect the creation of thermals and thereby the energy consumption during migration. In this study a number of meteorological variables are tested for correlation to raptor migration data from one autumn season in Kazbegi, Georgia. The results show mostly weak correlations, but three correlations are significant at α=0.05; Black Kite – cloud height, Honey Buzzard – wind speed and Levant Sparrowhawk – wind speed. Some other general trends can also be seen. When investigating the weather conditions during the days prior of the first pronounced migration peak it is obvious that the migration is held back by unfavorable conditions with low cloud height, low visibility, low temperatures and winds from the north sector. On the day of the peak there is a pronounced change of weather with maximum cloud height, maximum visibility, higher temperatures and winds from the south sector which results in the largest migration peak of both soaring and flapping raptors.