Strong Declines of the White Stork Ciconia ciconia Population in South-Western Poland: A Differentiated Importance of Altitude and Land Use Changes

Abstract

Studies of long-term trends in avian populations require large-scale data not available for most bird species. The White Stork Ciconia ciconia is unique being monitored for over a century and well-reflecting modern environmental changes. Its worldwide and national populations are estimated every 10 years thanks to the International White Stork Censuses (IWSC). We present the yet unpublished data of the last IWSC 2014 from the main species stronghold, Poland. We assessed the population size and compared its changes with the previous IWSC 2004 in 23% of the country area, including mountains. We looked for potential drivers of trends among land use transformations, checked population changes along altitudinal gradient, and compared the long-term trends in the global, national and regional populations. In 2014, 2560 pairs (3.61 pairs/100 km2) bred in south-western Poland, a decline by 35.5% compared to 2004. Decreases were strongly inversely related to the altitude, i.e. declines in lowlands were twice as high as in the mountains. Changes in area of grasslands, croplands, forests, and built-up areas were all weak predictors of the decline. Stork decrease in south-western Poland contradicts a stable country-wide trend in 2004–2014 (although the latest countrywide data also suggest a decline), which is inconsistent with the increase of the global population. Heterogeneity in trends indicates that Poland could be currently viewed as a transitional area, lying between large breeding areas inhabited by increasing stork populations. Notably, the western border between areas of increases and decreases coincides with the division into two migratory populations suggesting that the heterogeneity of trends may be related to migration paths and wintering grounds. Overall, our data confirm recent range shift of the species, and show difficulties in drawing general conclusions on stork demography based on regional data.