Impact of weather conditions

Any population may migrate through the Caucasus, but it is difficult to tell where exactly the birds will fly from year to year. From much of the migration literature it becomes clear that species that rely heavily on soaring flight - sustained by thermal lift or orographic updrafts (aka slope updrafts) - will have much more predictable routes compared to broad-front migrants that flap their way south. Some species may stop-over for the day when they encounter heavy rains, others may circumvents such bad weather systems and temporarily end up on a more easterly or westerly route. Etcetera. Such en route choices made by migrants will affect their timing of arrival at a given watch site, or even whether they arrive there at all.

For species seen in substantial numbers at Batumi a detailed analysis of migration timing in relation to local and regional weather conditions is now performed by Msc. Thesis student Jasper Wehrmann (University of Potsdam, Germany) in close cooperation and supervised by BRC researchers / Phd. students Wouter Vansteelant (IBED, University of Amsterdam, the Netherlands) and Brecht Verhelst (Edward Grey Institute for Field Ornithology, Oxford University, United Kingdom).

A similar analysis is also done on the 2010 full-season pilot count from Kazbegi (Greater Caucasus) by Bsc. Thesis student Michael Tholin (Lund University, Sweden) in cooperation with BRC.

For the collection and processing of meteorological satellite imagery the research team is getting substantial help and guidance by Phd. student Stijn Hantson (Departamento de Geografia, Universidad de Alcala, Spain).

The day-to-day migration for different species observed at Batumi and Kazbegi will be related to observed weather conditions for each day. These include cover by different cloud types (Terra and Aqua MODIS satellite imagery,fig 1) and a number of meteorological parameters observed from field stations throughout the Caucasus (NOAA database).
Raw image has been reclassified to show low (dark green), middle (blue) and high (light green) altitude clouds which have a different
physical/meteorological origin. In GIS the extent of cover can be caluculated for the different cloud types in seperate geographical regions
(polygones with names in redbrown) in the assumed 'flowshed' of migrating raptors to Batumi (red encircled white dot).

The approach consists of dividing the Caucasus area into regions where the effect of any set of weather conditions may have substantially different effects on migrating raptors because of important differences in regional topography and geography (fig1). By looking for periods of consistently bad weather in a given region followed by favourable flight conditions and comparing the resulting migration to ‘normal’ circumstances it will be possible to find out if raptors stop and wait for the better conditions (a stop in migration followed by a wave of raptors that accumulated over several days) or circumvent the bad weather fronts (a stop in migration in Batumi, followed by no substantial increase in numbers).

Based on the 2010 data for Kazbegi and Batumi we can also compare the day-to-day migration at both sites in order to find out more about how the timing of migration is correlated at either site. Because it is most unlikely that birds crossing the Caucasus in Kazbegi will end up at Batumi the comparison of the bottlenecks should yield important insights into the distribution of the migration of different species across the Caucasus.

Furthermore, from an inter-species comparison, the results of these analyses will contribute to the substantial literature on flight strategies of raptors and can link much of the theoretical concepts back to patterns in day-to-day migration as observed from ground-based counts.