Diet of raccoon Procyon lotor after 20 years of invasion in bird paradise

The raccoon ( Procyon lotor ) is a mesocarnivore native to North and Central America. It was introduced to Europe in the first half of the 20th century via multiple pathways (escapees from fur farms, introductions for fauna enrichment, and releases of pet animals) and is now listed as an invasive alien species of concern in the European Union. The first sightings of raccoons in Poland date back to 1945, and a free-living population has been present since the 1990s. Currently, the population size in the country is increasing exponentially, and approximately 14,000 individuals were shot during the 2024/2025 season. The impact of raccoons on the environment in the introduced range is exerted mainly through predation. Therefore, diet analysis is crucial for assessing this impact on native species. As an omnivorous yet very skilled mesocarnivore, the raccoon may pose a threat to many groups of animals, from invertebrates and fish to birds and small mammals. In 2025, a macroscopic analysis of raccoon diet composition was conducted in the Warta Mouth National Park. This area, located in western Poland, is a highly valuable wetland and an important breeding site and migration stopover for many waterfowl species. At the same time, it is the region where the first free-living raccoon population in Poland was established by individuals arriving from Germany. Right now the population in this area is likely stable and saturated. The Warta Mouth National Park is also the site of the first – and so far only – comprehensive study on raccoon diet in Poland, conducted at the early stage of the invasion (2005-2007) (Bartoszewicz et al. 2008). The diet analysis was based on 635 fecal samples collected throughout the year. Samples collected in the field were placed in paper envelopes, dried, soaked, and rinsed through a mesh sieve. The remaining food items were then identified using identification keys after re-drying. Food composition was expressed as percentage of biomass (%Bio) and percentage of occurrence (%Occ). percentage of biomass (%Bio) and percentage of occurrence (%Occ). The similarity of diet composition between seasons (spring-summer vs. autumn-winter) and study periods (2005-2007 vs. 2025) was calculated using Pianka’s index (α). The most important food category for raccoons were small mammals (%Bio 27.8 and %Occ 47.4) and birds (%Bio 24.6 and %Occ 26.5). Other vertebrate prey included fish (%Bio 10.7 and %Occ 20.8) and amphibians (%Bio 5.8 of biomass and %Occ 21.9). Raccoons also consumed mollusks, mainly snails, which accounted for %Bio 14.5 of the biomass and were present in 57.8% of samples. Insects constituted a food category with a low contribution to biomass (4.4%) but the highest occurrence (74.2%). This group was relatively often represented by the family Vespidae (wasps and hornets), which were not detected during the early stage of the invasion. Plant material was also consumed as an additional food source, accounting for %Bio 10.6 and occurring in %Occ 62.5; this category was dominated by fruits and seeds. The least significant food category was crustaceans, contributing only for %Bio 1.6 and occurring in %Occ 10.8. Despite some differences, the similarity of diet composition between seasons in 2025 was relatively high (α=0.91). Comparison between stages of invasion also showed a high level of similarity (α=0.87). Many of the observed differences may be explained by potential changes in prey populations associated with global warming. In some cases, the impact on prey populations may also have shifted due to climate-driven environmental changes.