Environmental factors affecting patterns of distribution and co‐occurrence of two competing raptor species

Abstract

Habitat selection is a complex process, that is affected by several factors, including habitat characteristics, environmental conditions, and both intra‐ and interspecific interactions. We analysed habitat preferences of two top avian predators, Peregrine Falcon Falco peregrinus, a medium‐sized diurnal raptor, and Eagle Owl Bubo bubo, a large nocturnal raptor. These two species are known to compete for preferred nest‐sites, and proximity to cliffs with Eagle Owls may reduce Peregrine breeding output through predation of young Falcons. We investigated the environmental factors affecting occurrence and coexistence of the two species and the potential role of habitat suitability in favouring co‐occurrence in 3519 km2 of the central pre‐Alps of Italy, where the two species breed on cliffs and sometimes co‐occur on the same cliff. Peregrines settled on long, steep and favourably orientated cliffs in woodland landscapes close to urban areas. Eagle Owls settled on topographically similar cliffs, but in lower rainfall areas compared with cliffs occupied by Peregrines and cliffs unoccupied by either species. Sites where the two species co‐occurred were characterized by more horizontally extended cliffs compared with sites of exclusive occurrence of each species. An analysis of relative habitat suitability revealed that sites where the two species co‐occurred had the highest predicted probability of occupancy for both species, suggesting that those sites should be regarded as high‐quality sites. Breeding productivity of Eagle Owls was negatively affected by the co‐occurrence of Peregrines, whereas the effect of Eagle Owl proximity on Peregrine productivity varied according to cliff suitability for the Peregrines. Habitat selection had fitness consequences for Eagle Owls because breeding productivity increased with cliff length. Environmental conditions, particularly climatic factors, could allow the widespread coexistence of these competing raptors at the landscape scale, whereas at the local scale co‐occurrence could take place only on larger cliffs. These were preferred sites for both species, presumably because breeding at such sites offsets the costs of settling close to the competitor species.

Rainfall and landscape features affect productivity in an alpine population of Eagle Owl Bubo bubo

“…Habitat quality studies have either analysed the effect of several factors together (e.g., Krüger 2004) or have focused on some specific elements: climatic variables (e.g., Lehikoinen et al 2009), habitat structure (e.g., Sergio et al 2004), land-use cover and specific pattern of resource distribution (e.g., Penteriani et al 2004), intra-and interspecific interactions (e.g., Brambilla et al 2006), impact of human activity (Marchesi et al 2002;Brambilla et al 2010), and food availability (e.g., Lehikoinen et al 2009) are among the ones which can affect habitat quality for animal species.…”

“…Our model species, the Eagle Owl Bubo bubo, has been the subject of different studies on habitat quality, which focused almost exclusively on the effect of habitat traits (Sergio et al 2004;Ortego 2007;Brambilla et al 2010), or on spatially varying habitat traits coupled with density (Penteriani et al 2004). Recently, interspecific competition effects and coarse measures of climatic traits have also been considered, but on a limited dataset (Brambilla et al 2010).…”

“…Our model species, the Eagle Owl Bubo bubo, has been the subject of different studies on habitat quality, which focused almost exclusively on the effect of habitat traits (Sergio et al 2004;Ortego 2007;Brambilla et al 2010), or on spatially varying habitat traits coupled with density (Penteriani et al 2004). Recently, interspecific competition effects and coarse measures of climatic traits have also been considered, but on a limited dataset (Brambilla et al 2010). Moreover, some females abandon their nest after prolonged rainfall (V. Penteriani, personal communications), and the species has been recently reported to avoid high-rainfall sectors within otherwise rather uniform areas (Brambilla et al 2010), but no study has investigated in detail the effect of rainfall on the owl breeding performance.…”

The effects of habitat and spatial features of wetland fragments on the abundance of two rallid species with different degrees of habitat specialization

“…The distribution and abundance of biological species may be affected by several factors, including climate, habitat type, spatial configuration of habitats and inter-and intra-specific interactions (McCoy & Mushinsky 1994, Katzner et al 2003, Brambilla, Bassi et al 2010.…”

“…The results of the GLM selection and averaging procedures were cross-checked by a hierarchical partitioning analysis (Mac Nally 1996, Brambilla et al 2006, Brambilla, Bassi et al 2010, which provides estimates of the independent and joint explanatory power of each predictor, considering all possible models in a multivariate regression setting (Chevan & Sutherland 1991, Mac Nally 1996, 2000. Variables with large independent values are most likely to be important for habitat selection (Mac Nally 2000), and consequently relevant for management when models are used as practical tools (Brambilla & Rubolini 2005, Brambilla, Casale et al 2010.…”