This study describes the climatic factors determining the distribution of two sympatric steppe birds, the little and the great bustards, on a Western Palearctic scale. We used the European Bird Census Council breeding bird atlas, recording presence/absence for 50×50 km UTM squares. Climate data were extracted from a 10′ latitude × longitude global compilation. Linear and quadratic terms were included to account for non-linear responses. A model-averaging approach was performed to determine the weight and effect of those variables describing the climatic preferences for each species. Although both showed a preference for drier areas in Europe, our results suggest that the great bustard's climate niche is mainly defined by variables related to primary production (e.g. rainfall, soil moisture), with temperature-related variables being less influential. These results contrast with those for the little bustard, whose climate niche seems to be defined by both temperature and precipitation-related variables (e.g. annual rainfall, temperature range). Our analyses detected significant non-linear responses of presence/absence of both species to some climate variables: great bustard with precipitation and spring temperature; little bustard with temperature range, precipitation and soil moisture index. This study shows the reliability of model-averaging methodology for describing the climatic niches of species.
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Vol. 58 • No. 2