Recent changes in body weight and wing length among some British passerine birds

  • Yom-Tov Y
  • Yom-Tov S
  • Wright J
 et al. 
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Abstract

We tested the prediction that global warming has caused recent decreases in body weight (Bergmann's rule) and increases in wing length (Allen's rule) in 14 species of passerine birds at two localities in England: Wicken Fen (1968–2003) and Treswell Wood (1973–2003). Predicted long‐term linear decreases in residual body weight occurred in four species: dunnocks (Wicken Fen), and great tits, blue tits and bullfinches (Treswell Wood). Non‐linear decreases also occurred in reed warblers and blackcaps at Wicken Fen, which also had a surprising linear increase in residual body weight in blackbirds. Residual wing lengths increased linearly, as predicted, in six of seven species at Wicken Fen. Whereas there were non‐linear long‐term increases in wrens, dunnocks and blackbirds in Treswell Wood. Unexpected linear decreases also occurred in residual wing lengths in willow warblers (Wicken Fen), and blue tits, great tits and chaffinches (Treswell Wood). The most parsimonious explanation for such long‐term changes in body weight is global warming, as predicted by Bergmann's rule. Greater site and species‐specific effects on wing length (e.g. non‐linear changes plus shorter wings in the woodland habitat) suggest a less straightforward conclusion concerning Allen's rule, probably because wing length involved variation in both bone and feather growth. Changes in residual body weights and wing lengths often differed between species and were sometimes non‐linear, perhaps reflecting short‐term modifications in selection pressures. Human‐induced influences are discussed, such as avian predator population densities and land‐use change. Short‐term variation in temperature had little effect, but rainfall did explain the unusual increase in blackbird body weight, possibly as a result of improving food (earthworm) availability.; We tested the prediction that global warming has caused recent decreases in body weight (Bergmann's rule) and increases in wing length (Allen's rule) in 14 species of passerine birds at two localities in England: Wicken Fen (1968-2003) and Treswell Wood (1973-2003). Predicted long-term linear decreases in residual body weight occurred in four species: dunnocks (Wicken Fen), and great tits, blue tits and bullfinches (Treswell Wood). Non-linear decreases also occurred in reed warblers and blackcaps at Wicken Fen, which also had a surprising linear increase in residual body weight in blackbirds. Residual wing lengths increased linearly, as predicted, in six of seven species at Wicken Fen. Whereas there were non-linear long-term increases in wrens, dunnocks and blackbirds in Treswell Wood. Unexpected linear decreases also occurred in residual wing lengths in willow warblers (Wicken Fen), and blue tits, great tits and chaffinches (Treswell Wood). The most parsimonious explanation for such long-term changes in body weight is global warming, as predicted by Bergmann's rule. Greater site and species-specific effects on wing length (e.g. non-linear changes plus shorter wings in the woodland habitat) suggest a less straightforward conclusion concerning Allen's rule, probably because wing length involved variation in both bone and feather growth. Changes in residual body weights and wing lengths often differed between species and were sometimes non-linear, perhaps reflecting short-term modifications in selection pressures. Human-induced influences are discussed, such as avian predator population densities and land-use change. Short-term variation in temperature had little effect, but rainfall did explain the unusual increase in blackbird body weight, possibly as a result of improving food (earthworm) availability.; Changes in the body size of passerine birds in the U.K. during the last 30 years or more were examined. Global warming should, according to Bergmann's rule, lead to a decrease in the body weight and, according Allen's rule, result in an increase in wing length of passerines. Wing length and body weight data on 14 passerine species collected during regular ringing activities at 2 locations were analyzed. Long-term decreases in body weight were observed for some species, an observation that is most parsimoniously explained by global warming, as predicted by Bergmann's rule. Although wing length increased, greater site- and species-specific effects were observed, indicating a less straightforward conclusion regarding Allen's rule. Changes in residual body weights and wing lengths frequently varied between species and were sometimes nonlinear, possibly reflecting short-term modifications in selection pressures.; We tested the prediction that global warming has caused recent decreases in body weight (Bergmann's rule) and increases in wing length (Allen's rule) in 14 species of passerine birds at two localities in England: Wicken Fen (1968-2003) and Treswell Wood (1973-2003). Predicted long-term linear decreases in residual body weight occurred in four species: dunnocks (Wicken Fen), and great tits, blue tits and bullfinches (Treswell Wood). Non-linear decreases also occurred in reed warblers and blackcaps at Wicken Fen, which also had a surprising linear increase in residual body weight in blackbirds. Residual wing lengths increased linearly, as predicted, in six of seven species at Wicken Fen. Whereas there were non-linear long-term increases in wrens, dunnocks and blackbirds in Treswell Wood. Unexpected linear decreases also occurred in residual wing lengths in willow warblers (Wicken Fen), and blue tits, great tits and chaffinches (Treswell Wood). The most parsimonious explanation for such long-term changes in body weight is global warming, as predicted by Bergmann's rule. Greater site and species-specific effects on wing length (e.g. non-linear changes plus shorter wings in the woodland habitat) suggest a less straightforward conclusion concerning Allen's rule, probably because wing length involved variation in both bone and feather growth. Changes in residual body weights and wing lengths often differed between species and were sometimes non-linear, perhaps reflecting short-term modifications in selection pressures. Human-induced influences are discussed, such as avian predator population densities and land-use change. Short-term variation in temperature had little effect, but rainfall did explain the unusual increase in blackbird body weight, possibly as a result of improving food (earthworm) availability.

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Authors

  • Jonathan WrightNorwegian University of Science and Technology (NTNU)

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  • Yoram Yom-Tov

  • Shlomith Yom-Tov

  • Chris J.R. Thorne

  • Richard Du Feu

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