Local flight strategies - routes and altitudes of raptor migration
Apart from the regional effects on migration as detailed above it is crucial to gain insight in how the detecteability of raptors by observers may change according to weather conditions. For Batumi this means we need to show empirically where (1) the local routes used by raptors are situated (e.g. coast – lowland – mountains) and how these may shift in relation to weather. But also (2) how high raptors are flying under differing conditions, since both the horizontal and vertical component of distance from observers will determine how easily the birds can be picked up during a monitoring program.
With each recorded raptor or flock seen during the BRC counts a distance code is reported that allows us to classify the locality of passage at any given time. Apart from the two stations that are constantly used throughout the monitoring there have also been short-term pilot counts at a third station further inland of the bottleneck for which the same type of distance codes have been used.
Unfortunately neither satellite telemetry nor reanalysis data are available at sufficient resolutions to determine differences in weather at a local scale for the bottleneck. However weather conditions have also been collected with sufficient accuracy to relate these to differences in local route location per half day of observations. From our on-site experience we gather that routes effectively shift closer to the coast throughout the day, probably due to the formation of dense, low clouds in the mountains at the eastern side of the bottleneck.
Similarly localised changes in weather conditions may affect the flight altitude of migrants. However it has been well described in the raptor migration literature that the development of thermal activity follows a predictable diurnal pattern that results in high flight altitudes for soaring migrants at mid day, when thermals are strong and abundant. These conditions are known to result in so-called ‘noonday lulls’ – hours at mid day when birds are flying too high to be detected by the eye or even optics - at other raptor watch sites. From our experience we presume this effect is of minor importance in Batumi, which would strongly promote the credence of our counts as a scientific tool for studying trends in numbers. It is however again important to show this empirically.
If you have any questions on weather-related patterns in migration or if you wish to contribute to the research done at Batumi please contact BRC at firstname.lastname@example.org or seek direct contact with Wouter Vansteelant who is coordinating such research on this site.