Abstract
<jats:p>Several authors have identified climate change as a serious threat to bird populations. Numerous studies have shown that bird populations are disrupted by local climatic conditions (e.g. temperature, precipitation, and wind) and large-scale atmospheric indices. These changes may have a significant impact on places where birds congregate in large numbers over a relatively small area, as is the case with waterbirds during wintering. The Pyasachnik reservoir is a wetland classified as Important Bird Area (IBA) for the wintering waterbirds that congregate over 22,000 birds. On the river bearing the same name, in Plovdiv District is established the Pyasachnik reservoir. It is part of the Natura 2000 sites under of Birds Directive – Yazovir Pyasachnik BG0002010. The Pyasachnik reservoir and its surroundings host 146 bird species, 44 of which are listed in the Red Data Book for Bulgaria. The results of an analysis are based on large- scale monitoring data (from 2004 to 2021). Based on data from the mid-winter Waterbird Census, which in mid-January each year is conducted, the trend of a dramatic decrease in the number of wintering waterbirds in Bulgaria can be clearly identified. For Pyasachnik reservoir, within just about 20 years (2004 to 2021), the number of recorded wintering birds decreased by half, and in some years even by approximately 1/3 (2021 compared to 2004). Keywords: climate changes, mid-winter waterbird census, Pyaschnik reservoir</jats:p>