No country for small birds: Potential positive association among medium-sized, aggressive species in Australian bird communities

AimAustralian woodlands have been intensively cleared since European settlement and, in parallel, many species of birds inhabiting this habitat type have experienced a marked decline. Conversely, some species such as noisy miner Manorina melanocephala respond positively to habitat disturbance and due to their hyper-aggressiveness can end up driving away specialized and fragmentation-sensitive species. Recent studies have suggested that the negative impact of miners is exacerbated by means of synergistic interactions between these and other aggressive species, including nest predators. However, it is not clear if these positive associations arise through similar habitat requirements or due to potential mutual benefits (‘facilitation effects’), which should predominate in harsh environments as the stress-gradient hypothesis (SGH) predicts.LocationEastern Australia.MethodsWe combined a multi-season community N-mixture model and joint species distribution models within the Hierarchical Modelling of Species Communities framework to examine species co-occurrence patterns after accounting for imperfect detection and the influence of environmental variables. We then explored how the balance between positive and negative associations varied along an abiotic stress gradient and whether species-to-species associations were non-randomly distributed with respect to species traits.ResultsNo significant associations were detected; we only found tentative associations (posterior probability 85%) in a low proportion of species pairs (~4%). Although most non-random (tentative) associations were negative, these were weaker and less consistent across models than the positive ones. Five medium-sized species of swooping birds including the noisy miner monopolized virtually all positive associations. Communities from low-productivity environments tended to show a lower degree of overall (community-level) competitiveness than those located in less stressful (more productive) environments, which supports the SGH. There was no significant relationship between species trait dissimilarity and species-to-species association's strength.Main ConclusionsOur results suggest that aggressive species like miners, kookaburras, lorikeets, magpies and butcherbirds can form potential synergies with each other that may intensify the direct negative influence of each species on small-sized songbirds. Since these species thrive in anthropized landscapes and have drastically increased their numbers