The Effect of Competition Between Two Swan Species: Nesting Site Selection and Reproductive Success

Climate change-driven range expansions are creating novel interspecific interactions that may significantly impact the breeding success of established resident species. This study examines the ecological consequences of competition between Mute Swans (Cygnus olor) and expanding Whooper Swans (Cygnus cygnus) in central and eastern Poland. We monitored 80 Mute Swan breeding pairs across fishpond complexes using UAV-based surveys to assess habitat selection and reproductive output in sites with and without Whooper Swan presence. Mute Swans breeding alongside Whooper Swans selected nest sites deeper within reed vegetation and showed altered habitat preferences compared to pairs breeding without competition from a related species. Significantly, reproductive output was reduced in competitive environments, with pairs breeding in areas without Whooper Swans producing considerably more offspring than those coexisting with the expanding species. These results demonstrate that the expansion of a given species' range can cause immediate costs to local species, both via direct confrontations and through the impact on the quality of the occupied habitat. The substantial reduction in breeding success suggests that interspecific competition may have population-level consequences for established waterbird communities.