Patterns in raptor migration in relation to weather: Implications for migration monitoring

More than 800.000 raptors of ca. 30 species migrating over Batumi every autumn. Ten species observed in numbers constituting >1% of their world population (see Verhelst et al. 2011). Several others in (one of) the highest concentration known to ornithologists anywhere in the world. The list of arguments to show the importance of the Batumi bottleneck in an international context is quite impressive. So for the casual observer, the need for implementing a long-term monitoring of migratory raptors at the eastern Black Sea seems clear. However, many factors affect the efficiency and precision with which a monitoring can be conducted and therefore the reliability of the obtained results for interpretation by conservationists.

Indeed, if the effort with which we count birds is variable among years, then we should also expect the counted numbers of raptors to fluctuate accordingly. Over the course of our experimental counts of 2008 up to 2010 BRC has implemented and improved a protocol that deals as well as possible with standardizing count effort for a long-term monitoring. In this way, we aim to deal with any counting bias as good as possible.
However observed numbers of birds are also prone to plenty of natural sources of variation which can not be controlled by any protocol. Furthermore, these natural conditions may affect species differently. If this is true for migrants observed in Batumi, then this needs to be considered carefully before ground-based counts are to be used in conservation practice.
One of the most crucial, but also variable, factors determining flight paths of migratory birds is weather! Wind, rain, atmospheric stability, … may all affect migrating birds differently, and the effect will depend on the movement strategies employed by any species. For monitoring purposes this is important as regional but also local weather conditions may cause migrating birds to choose another route, where they are not picked up by counters.

For these reasons one of the first crucial steps in any migration monitoring program should be to assess the impact that weather has on the counted species in order to determine for which species the huge efforts that are required are actually worthwhile. Thus, for Batumi we need to determine:

  1. for which species the route along the eastern Black Sea coast is ‘crucial enough’ to assume that a relatively constant part of trans-Caucasian migrants will use it in any given year and
  2. how local routes close to Batumi may shift out of sight of the observers as a result of local weather conditions.

These research avenues are now (2011-2012) being pursued in a cooperation of thesis students and researchers affiliated with BRC and MEDRAPTORS.