Flexible invaders: a study into aggressive behaviour in Öland's range expanding collared flycatchers

Abstract: Behavioural shifts, or changes in behaviour patterns, are an often-overlooked aspect of ecological invasions and range expansions, that can nevertheless be key in determining how and why a species may or may not succeed in their expansion process. We looked at aggressive behaviour in a small insectivore passerine species, the collared flycatcher, on the island of Öland, where it is undergoing a range expansion to the detriment of the native, ecologically similar, pied flycatcher. We found collareds to react more aggressively to simulated intrusions by conspecifics rather than heterospecifics, consistent with previous research on the matter, as well as an increased aggressiveness on later days in the season. We also found a pattern hinting towards an effect of habitat quality on territory defence, with increased aggression on higher quality patches. There was no simple geographic signal in levels of aggression within this population, and we discuss the complex factors interacting to determine aggressive behaviour in this species, including the invasion front itself.