Alike but still different: coexistence of four raptor species explained by breeding niche overlap

Kai-Philipp Gladow, Jonas Beck, Patrick Benjamin Langthaler, Nayden Chakarov, Oliver Krüger

Understanding how species competing for similar resources coexist and influence each other has been, and still is, one of the big questions of community ecology. This question has often been tackled by comparing ecological niches of species pairs, although usually more than just two species compete for the same resources. We analysed the niche overlap of the breeding niche of four raptorial bird species (red kites, common buzzards, northern goshawks and eagle owls) living in sympatry in very high densities using a large sample size of n = 3439 breeding attempts from over 12 years. Using a new method to measure multiple species niche overlap, we show that each species overlaps a lot with the rest of the community but still shows key differences in its respective breeding niche. Afterwards, we take a closer look at the species pairs with the highest niche overlap. Red kites and buzzards overlap to a large degree in all measured niche dimensions except nearest neighbour distance to buzzard breeding pairs. Apparently, buzzards seem to be more territorial against their conspecifics than against red kites, meaning intraspecific competition is higher than interspecific competition. This might have enabled red kites to increase in breeding pair numbers during the last years despite sharing their habitat with many buzzard pairs. Goshawks lack such a key difference to eagle owls, being very similar in their breeding niche in all measured dimensions. As a consequence, goshawks fail to avoid competition and presumably predation by the dominant eagle owl, constraining the possibility for stable coexistence and explaining their reduction in breeding pairs as eagle owls increase, and vice versa. Experimental designs to test for direct behavioural reactions to intra- and interspecific competitors and territory intruders could help to understand these mechanisms better.