Changes in bird populations can be described as simple changes in abundance or density of individuals or, in larger areas, as changes of distribution or population indices. Although these measures describe different aspects of population change, they are often used interchangeably when discussing the relationships between various predictors of bird population changes found in different studies. This hampers a meaningful comparison of results, because it is unclear which differences between studies are biologically relevant and which are just methodological artifacts, caused by the different nature of measures used to quantify population changes. We compared predictors of the three measures of population changes: (i) change in abundance, (ii) change in magnitude of population trend and (iii) change in distribution, using data collected in a single region, the Czech Republic, and over similar time periods. We also aimed to identify significant drivers of population changes of Czech birds, using a large set of predictor variables and virtually all species regularly breeding in the region. No significant predictors were common across the three measures of population change tested. Abundance increased mostly in forest birds, while the change in magnitude of population trend was positive in species with a higher level of legal protection and in r-selected species. Species extending their distribution include wetland birds, above-ground nesters and north European species. Although individual measures of population changes are positively correlated, their predictors are different. This limits possibilities of simple comparisons, but also offers a better insight into forces shaping bird population changes in time and space.
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Vol. 46 • No. 2