Regional and seasonal flight speeds of soaring migrants and the role of weather

Submitted by avianbiology on 4 August 2014.

Wouter Vansteelant holding a male Honey Buzzard that carried GPS-logger #56 from July 2009 till July 2011. Two spring migrations and one autumn migration of this bird were used for analyzing weather influences on the bird’s travel performance (photo by Willem Bouten).

A big challenge for migration ecologists is to resolve how environmental conditions along the migration routes of birds affect their migration timing and individual fitness and this is one of our key research aims in the Computational Geo-Ecology group at the University of Amsterdam. Soaring migrants are an interesting case among migrating birds in the extent to which they depend on suitable weather conditions to travel long distances. Having spent many hours monitoring raptor migration with Batumi Raptor Count at the at the Black Sea coast of southwestern Georgia, I´ve seen very clearly that thermal convection and wind conditions strongly affect the migration itineraries of soaring birds. However, to be able to study the ecological relevance of such weather influences at the scale of full migrations one has to follow individual birds along their entire migratory journeys.

In the Netherlands, two raptor species are being studied with high-resolution GPS loggers (www.UvA-BiTS.nl). Researchers at the University of Amsterdam in close collaboration with Treetop Foundation (www.boomtop.org) have been tracking Honey Buzzards since 2008 and collaboration with the Montagu´s harrier foundation (www.werkgroepgrauwekiekendief.nl) started in 2009. Because the breeding success of these migratory species is expected to depend on environmental conditions during migration and wintering stages, a key aim of both foundations is to study the birds´ ecology across their entire annual cycle. By using the same tracking system on both species we could compare their responses to weather relatively easily.

Co-authors Almut Schlaich and Raymond Klaassen deploy a new generation GPS-logger on a male Montagu’s Harrier breeding in eastern Groningen (Netherlands) in spring 2014. Research on harrier migration with UvA-BiTS still continues today.

 

 

Locating nest sites and catching and trapping raptors takes a lot of perseverance and we are deeply indebted to many volunteers who assisted in the process. During the breeding season, all individuals were being monitored meticulously by researchers, both in the field and behind the computer, such that we could upload measurement programs designed specifically for the migration and wintering areas just before individual birds departed southward. Every spring, we were awaiting the return of the birds in great anticipation. When the first migration data were retrieved from returning birds, we were terribly excited to see a great amount of variation in migration patterns between individuals, seasons and regions, which suggested weather conditions en route were playing a key role in the migration itineraries of these migrants

For this paper we set out to determine to what extent the hourly speed and daily mean speed and daily travel distance of honey buzzards and Montagu´s harriers was determined by the convective and wind conditions which they encountered en route. We wanted to compare how the effects of weather conditions differed between the two species, because honey buzzards are known to have a stronger predisposition for soaring flight whereas marsh harriers use both soaring and flapping flight regularly. Moreover, we wanted to check if weather conditions during migration could explain regional and seasonal patterns in migration speed of these two species, because such patterns have been reported regularly for other long-distance soaring migrants. While regional and seasonal migration speeds of soaring migrants are often suggested to be due to adjustments in flight behavior, we found that both wind and thermal convection strongly influence migration speed along the entire migration route, explaining nearly all regional and seasonal variation in flight performance during travel.

 

In summer 2012, Honey Buzzard #182 was retrapped by field researcher Peter van Geneijgen after three years of tracking. The logger had gone pale but was still functional after six Sahara-crossings on the back of this male bird. The logger caused no harm to the bird’s plumage nor body.

 

 

The paper is openly available to all interested readers at http://onlinelibrary.wiley.com/doi/10.1111/jav.00457/abstract

Wouter Vansteelant
Ph.D. student, Computational Geo-Ecology (IBED), University of Amsterdam

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