site.btaStudy Reveals Long-term Changes in Pygmy Cormorant Population in Southern Bulgaria

A research article published recently in the Acta Ornithologica scientific journal looks into the changes in the wintering population of the pygmy cormorant (Microcarbo pygmaeus) in Southern Bulgaria across 23 years. The study shows that the population grew between 1996 and 2019, with the highest number of birds being observed in December (over 2,000 individuals) and the lowest, in March (more than 1,000 individuals). The area of pygmy cormorants’ wintering foraging habitat increased as well (from 2,675.97 ha to 2,2564.35 ha) across all roosting sites during the study period. 

Waterbirds are declining globally due to different threats that affect their abundance and shift both their breeding and wintering ranges. Cormorants disperse over vast distances during winter and are suitable indicators of the impacts of human-wildlife interactions and abiotic factors. During wintering periods, these birds use regular roosts where they overnight on suitable perches, a refuge from adverse weather conditions and disturbances.

The researchers collected population dynamics data from seven roosting sites in the Maritsa River valley, a critical site for the conservation of the pygmy cormorant where 3,065 to 8,180 birds winter annually. 

According to the study, the number of pygmy cormorants at the roost increased with the drop in the daily mean air temperatures and the decrease of the day length. The closer to the onset of breeding, the fewer pygmy cormorants remained at the wintering sites. 

Most roosting sites in the study persisted only for a short time after their formation. They were exposed to more disturbances and lacked enough suitable perches and/or food resources. Human disturbances, including illegal shooting of cormorants, might have led to the abandonment of some of the roosting sites. 

Despite the global population increase, the pygmy cormorant faces a high risk of human-induced mortality during wintering and is thus regionally classified as endangered, the research article reads.

The study was conducted by Dobromir Dobrev, Vladimir Dobrev, and Polina Hristova of the Bulgarian Society for the Protection of Birds and by Georgi Popgeorgiev, Dimitar Demerdzhiev, and Dimitar Plachyiski of the National Museum of Natural History with the Bulgarian Academy of Sciences.