Bird-window collisions and carcass removal in southern Finland: patterns and implications

Collisions with human-made structures such as the windows of buildings and houses are a significant source of bird mortality worldwide and a leading cause of avian fatalities. However, research across many countries and continents, including Europe, remains limited, particularly in northern regions like Finland. This study provides an exploratory assessment of bird-window collisions in southern Finland, focusing on the cities of Lahti and Helsinki. We conducted surveys across 180 buildings, documenting 29 collisions (both lethal and non-lethal) involving 14 species (in addition to three unidentified corvids and one unidentified larid), with the goldcrest Regulus regulus being the most frequent victim, both in lethal and non-lethal collisions. Our carcass removal experiment revealed that all carcasses were removed during the day, most within the first 6 hours and the remainder within 13 hours and 45 minutes, mainly by scavengers such as hooded crows Corvus cornix. Notably, no nocturnal removal activity was recorded. These findings highlight the importance of carcass persistence studies in accurately estimating collision mortality. Despite potentially high collision rates, standardized data in Finland remains scarce, underscoring the need for comprehensive research to evaluate the real magnitude of bird-window collisions and their conservation impact. Thus, we recommend prioritizing bird-window collision monitoring, including multi-city surveys in different types of habitats surrounding the studied buildings and include carcass removal assessments. Furthermore, evidence-based mitigation strategies derived from local or regional data are urgently needed to address this historically overlooked, though recently recognized, conservation challenge.