What determines the speed at which birds fly?

To: Birding Aus <>
Subject: What determines the speed at which birds fly?
From: L&L Knight <>
Date: Tue, 17 Jul 2007 18:53:25 +1000

Public release date: 16-Jul-2007
Contact: Natalie Bouaravong

Public Library of Science

What determines the speed at which birds fly?

Aerodynamic scaling rules that explain how flight varies according to
weight and wing loading have been used to compare general speeds of a
wide range of flyers, from the smallest insects to the largest
aircraft. In a paper published this week in the open access journal
PLoS Biology, Thomas Alerstam, Mikael Rosen, and colleagues from the
University of Lund in Sweden analyze the flight speeds of 138 bird
species and overturn the general assumption that maximum flight speed
of a species is solely determined by such rules. Flight speed doesn’t just depend on the size of the bird (mass and wing loading), but also
reflects functional constraints and the evolutionary lineage of the
species in question.

The authors argue that only empirical measurements of flight speeds
enable you to evaluate how general such aerodynamic rules really are.
They used tracking radar measurements of the cruising speeds of
migrating birds (collected by themselves and others) to do the analysis and provide the comprehensive dataset with the paper (e.g. this
contains the flight speed of approximately one-third of all European
bird species). Their analysis reveals that the difference between the
speed of small and large birds is not as great as expected; they
suggest that this surprising result is likely to be the result of
disadvantages associated with very slow speeds among smaller birds and with very fast speeds for larger birds. They also show that the evolutionary history of the species helps explain much of the variation in flight speed: species of the same group tend to fly at similar
characteristic speeds. For example, birds of prey and herons had slow
flight speeds, on average, given their mass and wing loading, whereas
the average speed for songbirds and shorebirds was faster than would be predicted.

This study suggests that there are different functional adaptations
affecting flight differently among different types of bird, and that
there exists a diversity of cruising flight characteristics among birds that remain to be explored and understood.

Citation: Alerstam T, Rose´n M, Ba¨ckman J, Ericson PGP, Hellgren O (2007) Flight speeds among bird species: Allometric and phylogenetic
effects. PLoS Biol 5(8): e197. doi:10.1371/journal.pbio.0050197.

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