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The problem of estimating wind drift in migrating birds

Green, Martin LU and Alerstam, Thomas LU (2002) In Journal of Theoretical Biology 218(4). p.485-496
Abstract
Whether migrating birds compensate for wind drift or not is a fundamental question in bird migration research. The procedures to demonstrate and quantitatively estimate wind drift or compensation are fraught with difficulties and pitfalls. In this paper, we evaluate four methods that have been used in several studies over the past decades. We evaluate the methods by analysing a model migratory movement with a realistic scatter in flight directions, for the ideal cases of full drift and complete compensation. Results obtained with the different methods are then compared with the "true behaviour" of the model movement, illustrating that spurious patterns of drift and compensation arise in some cases. We also illustrate and evaluate the... (More)
Whether migrating birds compensate for wind drift or not is a fundamental question in bird migration research. The procedures to demonstrate and quantitatively estimate wind drift or compensation are fraught with difficulties and pitfalls. In this paper, we evaluate four methods that have been used in several studies over the past decades. We evaluate the methods by analysing a model migratory movement with a realistic scatter in flight directions, for the ideal cases of full drift and complete compensation. Results obtained with the different methods are then compared with the "true behaviour" of the model movement, illustrating that spurious patterns of drift and compensation arise in some cases. We also illustrate and evaluate the different methods of estimating drift for a real case, based on tracking radar measurements of bird migration in relation to winds. Calculating the linear regression of mean geographic track (resulting flight direction) and heading directions (directions of the birds' body axis) of a migratory movement under different wind conditions in relation to the angle a (the angle between mean track and heading) always provides robust and reliable results. Comparing mean flight directions between occasions with winds from the left and right of the mean flight direction of the whole migratory movement also always provides expected and correct measures of drift. In contrast, regressions of individual flight directions in relation to alpha (the angle between track and heading for the specific individuals or flocks) are liable to produce biased and spurious results, overestimating compensation/ overcompensation if following winds dominate in the analysis and overestimating drift/ overdrift if opposed winds are dominating. Comparing mean directions for cases with winds from the left and right in relation to individual flight directions also gives biased and spurious results unless there is full variation in wind directions or an equal distribution of crosswinds from left and right. The results of the methodological evaluation and the analysis of the real case indicate that some earlier analyses of wind drift may have to be re-evaluated. (C) 2002 Elsevier Science Ltd. All rights reserved. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Journal of Theoretical Biology
volume
218
issue
4
pages
485 - 496
publisher
Academic Press
external identifiers
  • wos:000179120800008
  • pmid:12384051
  • scopus:0036406360
ISSN
1095-8541
DOI
10.1006/jtbi.2002.3094
language
English
LU publication?
yes
id
218c5bfa-dd8a-4a43-a054-4e030e317328 (old id 145465)
date added to LUP
2007-06-25 08:01:04
date last changed
2017-12-10 04:27:15
@article{218c5bfa-dd8a-4a43-a054-4e030e317328,
  abstract     = {Whether migrating birds compensate for wind drift or not is a fundamental question in bird migration research. The procedures to demonstrate and quantitatively estimate wind drift or compensation are fraught with difficulties and pitfalls. In this paper, we evaluate four methods that have been used in several studies over the past decades. We evaluate the methods by analysing a model migratory movement with a realistic scatter in flight directions, for the ideal cases of full drift and complete compensation. Results obtained with the different methods are then compared with the "true behaviour" of the model movement, illustrating that spurious patterns of drift and compensation arise in some cases. We also illustrate and evaluate the different methods of estimating drift for a real case, based on tracking radar measurements of bird migration in relation to winds. Calculating the linear regression of mean geographic track (resulting flight direction) and heading directions (directions of the birds' body axis) of a migratory movement under different wind conditions in relation to the angle a (the angle between mean track and heading) always provides robust and reliable results. Comparing mean flight directions between occasions with winds from the left and right of the mean flight direction of the whole migratory movement also always provides expected and correct measures of drift. In contrast, regressions of individual flight directions in relation to alpha (the angle between track and heading for the specific individuals or flocks) are liable to produce biased and spurious results, overestimating compensation/ overcompensation if following winds dominate in the analysis and overestimating drift/ overdrift if opposed winds are dominating. Comparing mean directions for cases with winds from the left and right in relation to individual flight directions also gives biased and spurious results unless there is full variation in wind directions or an equal distribution of crosswinds from left and right. The results of the methodological evaluation and the analysis of the real case indicate that some earlier analyses of wind drift may have to be re-evaluated. (C) 2002 Elsevier Science Ltd. All rights reserved.},
  author       = {Green, Martin and Alerstam, Thomas},
  issn         = {1095-8541},
  language     = {eng},
  number       = {4},
  pages        = {485--496},
  publisher    = {Academic Press},
  series       = {Journal of Theoretical Biology},
  title        = {The problem of estimating wind drift in migrating birds},
  url          = {http://dx.doi.org/10.1006/jtbi.2002.3094},
  volume       = {218},
  year         = {2002},
}