Nest concealment strategies of the Mute Swan Cygnus olor in response to habitat competition with the Whooper Swan Cygnus cygnus

Interspecific interactions constitute an important factor shaping nesting strategies in birds and are often mediated by competitive behaviour. The expansion of a highly territorial species may intensify negative interactions and increase competitive pressure on native taxa. This study examines how the presence of the Whooper Swan Cygnus cygnus influences the nesting strategies of the Mute Swan Cygnus olor, with particular emphasis on nest-site selection. Fieldwork was conducted at two study sites in central and southern Poland, encompassing both single-species and mixed-species breeding systems. We quantified macrohabitat characteristics and spatial relationships among nests of competing swan pairs. To assess nesting strategies, we developed a novel approach integrating multiple habitat and spatial variables into three composite indices describing nest concealment, nest isolation, and their combined effect within fishpond complexes. Linear mixed-effects models indicated that Mute Swans adjust their nesting behaviour in the presence of Whooper Swans by selecting more concealed nesting sites. In contrast, the index describing spatial isolation of nests contributed less to explaining variation in nesting strategies. The co-occurrence of both species revealed behavioural responses to interspecific competition that were more pronounced than those associated with intraspecific interactions among Mute Swans.