Floater interference reflects territory quality in the Spanish Imperial Eagle Aquila adalberti: a test of a density‐dependent mechanism |
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| Authors: | Miguel Ferrer Virginia Morandini Ian Newton |
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| Affiliation: | 1. Applied Ecology Group, Estación Biológica de Do?ana (EBD‐CSIC), Seville, Spain;2. Centre for Ecology & Hydrology, Benson Lane, Crowmarsh Gifford, Wallingford, UK |
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| Abstract: | We report on an 11‐year study of floater interference in a population of Spanish Imperial Eagles Aquila adalberti. We analysed changes over the years in the productivity of 15 territories to test predictions of two hypotheses of density‐dependent productivity in relation to the presence of floaters (birds without territories). According to the ‘interference' hypothesis, the frequency of intrusion by floaters increases with density, resulting in a decrease in productivity. Thus, in a high‐density population a negative relationship between floater intrusions and productivity of the territory is expected. In contrast, under the ‘habitat heterogeneity' hypothesis, as density increases a higher proportion of individuals is forced to occupy lower quality habitats. Support of this hypothesis requires that floaters detect differences in quality among territories and preferentially visit the better quality territories. Consequently, a positive relationship between floater intrusions and productivity is expected. Results showed that floaters tended to visit their natal area at the beginning of the breeding season. Among floater Eagles, males made significantly more intrusions per day than did females, but females stayed in the natal population for longer each year than males. Floater intrusions and productivity were highly positively correlated, supporting the ‘habitat heterogeneity' hypothesis; individuals were apparently able to assess the quality of a territory and, at the frequencies observed, their interference with the breeding pair had no obvious negative effect on productivity. |
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| Keywords: | density dependence Doñ ana National Park floater habitat heterogeneity interference population dynamics productivity Spanish Imperial Eagle |
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