title: Flying with the wind: scale dependency of speed and direction measurements in modelling wind support in avian flight.
abstract: BACKGROUND
  Understanding how environmental conditions, especially wind, influence birds' flight speeds is a prerequisite for understanding many important aspects of bird flight, including optimal migration strategies, navigation, and compensation for wind drift. Recent developments in tracking technology and the increased availability of data on large-scale weather patterns have made it possible to use path annotation to link the location of animals to environmental conditions such as wind speed and direction. However, there are various measures available for describing not only wind conditions but also the bird's flight direction and ground speed, and it is unclear which is best for determining the amount of wind support (the length of the wind vector in a bird's flight direction) and the influence of cross-winds (the length of the wind vector perpendicular to a bird's direction) throughout a bird's journey.
  
  
  RESULTS
  We compared relationships between cross-wind, wind support and bird movements, using path annotation derived from two different global weather reanalysis datasets and three different measures of direction and speed calculation for 288 individuals of nine bird species. Wind was a strong predictor of bird ground speed, explaining 10-66% of the variance, depending on species. Models using data from different weather sources gave qualitatively similar results; however, determining flight direction and speed from successive locations, even at short (15 min intervals), was inferior to using instantaneous GPS-based measures of speed and direction. Use of successive location data significantly underestimated the birds' ground and airspeed, and also resulted in mistaken associations between cross-winds, wind support, and their interactive effects, in relation to the birds' onward flight.
  
  
  CONCLUSIONS
  Wind has strong effects on bird flight, and combining GPS technology with path annotation of weather variables allows us to quantify these effects for understanding flight behaviour. The potentially strong influence of scaling effects must be considered and implemented in developing sampling regimes and data analysis.
authors.affiliation: Department for Migration and Immuno-ecology, Max Plank Institute for Ornithology, Am Obstberg 1, 78315 Radolfzell, Germany ; Department of Biology, University of Konstanz, Konstanz, 78464 Germany.
authors.forename: Kamran
authors.initials: K
authors.lastname: Safi
authors.affiliation: Department for Migration and Immuno-ecology, Max Plank Institute for Ornithology, Am Obstberg 1, 78315 Radolfzell, Germany ; Department of Biology, University of Konstanz, Konstanz, 78464 Germany.
authors.forename: Bart
authors.initials: B
authors.lastname: Kranstauber
authors.affiliation: Am Fügsee 29, Seehausen am Staffelsee, 82418 Germany.
authors.forename: Rolf
authors.initials: R
authors.lastname: Weinzierl
authors.affiliation: Wildfowl & Wetlands Trust, Slimbridge, Gloucestershire, GL2 7BT UK.
authors.forename: Larry
authors.initials: L
authors.lastname: Griffin
authors.affiliation: Wildfowl & Wetlands Trust, Slimbridge, Gloucestershire, GL2 7BT UK.
authors.forename: Eileen C
authors.initials: EC
authors.lastname: Rees
authors.affiliation: Environmental Consultancy Services, White Strand, Killadoon, Louisburgh, Westport, Co. Mayo Ireland.
authors.forename: David
authors.initials: D
authors.lastname: Cabot
authors.affiliation: Department for Migration and Immuno-ecology, Max Plank Institute for Ornithology, Am Obstberg 1, 78315 Radolfzell, Germany ; Department of Biology, University of Konstanz, Konstanz, 78464 Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Cruz
authors.affiliation: Department for Migration and Immuno-ecology, Max Plank Institute for Ornithology, Am Obstberg 1, 78315 Radolfzell, Germany ; Department of Biology, University of Konstanz, Konstanz, 78464 Germany.
authors.forename: Carolina
authors.initials: C
authors.lastname: Proaño
authors.affiliation: U.S. Geological Survey, Western Ecological Research Center, 505 Azuar Drive, Vallejo, CA 94592 USA.
authors.forename: John Y
authors.initials: JY
authors.lastname: Takekawa
authors.affiliation: Emergency Center for Transboundary Animal Diseases, Animal Production and Health Division, Food & Agriculture Organization of the United Nations, Rome, 00153 Italy.
authors.forename: Scott H
authors.initials: SH
authors.lastname: Newman
authors.affiliation: Centre for Ecology and Evolution in Microbial Model Systems (EEMiS), Linnaeus University, Kalmar, SE-391 82 Sweden.
authors.forename: Jonas
authors.initials: J
authors.lastname: Waldenström
authors.affiliation: Centre for Ecology and Evolution in Microbial Model Systems (EEMiS), Linnaeus University, Kalmar, SE-391 82 Sweden.
authors.forename: Daniel
authors.initials: D
authors.lastname: Bengtsson
authors.affiliation: School of Natural Resources, North Carolina State University, 3118 Jordan Hall, Raleigh, NC 27695 USA ; North Carolina Museum of Natural Sciences, 11 West Jones St, Raleigh, NC 27601 USA.
authors.forename: Roland
authors.initials: R
authors.lastname: Kays
authors.affiliation: Department for Migration and Immuno-ecology, Max Plank Institute for Ornithology, Am Obstberg 1, 78315 Radolfzell, Germany ; Department of Biology, University of Konstanz, Konstanz, 78464 Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Wikelski
authors.affiliation: Department of Civil, Environmental & Geodetic Engineering, The Ohio State University, Columbus, OH 43210 USA.
authors.forename: Gil
authors.initials: G
authors.lastname: Bohrer
keywords: Aves; Doppler-shift; ECMWF; Flight direction; Flight speed; GPS; Measurement error; NOAA; Scaling