Abstract
<title>Abstract</title> <p>Domestic cats and collisions with buildings are widely recognized as important anthropogenic sources of bird mortality, yet no national-scale estimates have previously been available for Sweden. In this study, we estimated bird mortality caused by domestic cats and window collisions in Sweden during the period 2010–2021 using a capture–recapture approach based on long-term bird ringing data. Population sizes were derived from national breeding-pair estimates, and mortality rates were calculated under the assumption that ringed and unringed birds experience equal risk of predation or collision. Data from the Swedish Bird Ringing Centre were used to identify species- specific causes of death. Our results indicate that approximately 107,000 birds are killed annually by cats and approximately 180,000 birds die each year from collisions with windows in Sweden. The great tit (Parus major) was the species most frequently killed by cats, whereas the bullfinch (Pyrrhula pyrrhula) was the species most commonly involved in window collisions. Species associated with human habitation and winter bird feeding were disproportionately affected. Across all species with known causes of death, window collisions accounted for approximately 70% more fatalities than cat predation. Even when assuming a high national passerine population size representing a seasonal maximum, the estimated mortality attributable to cats and window collisions remains low relative to overall bird population sizes and natural annual mortality rates. Our findings suggest that, in Sweden, large-scale habitat degradation and broader anthropogenic pressures likely pose a greater threat to bird populations than domestic cats or window collisions. This study provides the first nationwide, data-driven assessment of these mortality sources in Sweden and highlights the value of long-term ringing datasets for evaluating anthropogenic impacts on wildlife.</p>