Dispersal of male ortolan buntings away from areas with low female density and a severely male-biased sex ratio |
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Authors: | Øyvind Steifetten Svein Dale |
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Institution: | 1.Department of Ecology and Natural Resource Management,Norwegian University of Life Sciences,?s,Norway;2.Department of Environmental Sciences,Telemark University College,B? i Telemark,Norway |
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Abstract: | Dispersal is expected to enhance individual fitness, and individuals should thus disperse from areas with poor conditions
to areas with more favourable conditions. Few studies have compared conditions before and after dispersal of the same individuals,
and in birds little is known about the effects of sex ratio and female density on male dispersal decisions. In this study
we examined various fitness-related parameters that adult male ortolan buntings, Emberiza hortulana, might use as cues in their decisions to disperse and settle. The study population has a strongly male-biased sex ratio.
Using pairwise comparisons of pre- and post-dispersal conditions, we found that males moved from areas with low female density
and a severely male-biased sex ratio to areas with higher female density and a less male-biased sex ratio. Male density and
male age structure did not affect male dispersal and settlement. The sex ratio of the pre-dispersal sites was below the population
average, but post-dispersal sites were not significantly better than the population average. This suggests that dispersal
was triggered by poor conditions, whereas settlement may have been unrelated to the conditions at the new site. In the year
of dispersal, males that undertook dispersal were less successful at acquiring a female than males that remained faithful
to the site, but in subsequent years there was no difference. We suggest that dispersal might be adaptive under more natural
conditions with a more balanced sex ratio. These results show that male ortolan buntings abandon areas with low female density
and a severely male-biased sex ratio, and thereby end up in better places, although probably not better than the population
average. |
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Keywords: | |
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