The invasive species rules: competitive exclusion in forest avian mixed-species flocks in a fragmented landscape

Evidence of checkerboard patterns of species' distribution in avian mixed-species flocks suggest that competition is one of the forces shaping the composition and structure of these associations. However, evidence of competition among flock species comes from studies performed in well-preserved regions and no study has reported the interactions between invasive and native flocking species in human-modified landscapes. Such studies are important because evidence show that avian social systems such as mixed-species flocks suffer several negative impacts of habitat fragmentation. In this study, it is shown that an invasive woodcreeper (Lepidocolaptes angustirostris) from open areas of central and western South America is: 1) expanding its range into that of a native Atlantic Forest woodcreeper (L. squamatus); 2) using the same forest fragments in which the native woodcreeper occurs; 3) regularly joining Atlantic Forest mixed-species flocks that contain the native woodcreeper; 4) overlapping in foraging height with the native woodcreeper during flocking; and 5) engaging in aggressive encounters and excluding the native woodcreeper from flocks. We suggest that this aggressive behavior is a consequence of the overlap in foraging height between the invasive and native species in their original habitats and that their contact has so recently been established. This study suggests that competitive interactions mediated by aggressive behaviors of invasive species may have a negative impact on the fitness of native mixed-species flock species in a fragmented landscape.