Ecological overlap between two sympatric birds of prey (Milvus milvus and Milvus migrans) during the breeding season

Abstract

The aim of the research was to gather data on two species of birds of prey, the Red Kite Milvus milvus (RK) and the Black Kite Milvus migrans (BK), in the Tolfa Mountains (Central Italy). In 2010-2012 we surveyed 37 RK and 55 BK territorial pairs during the breeding season, and the communal roosts of RK in winter, within an area of 510 km2. The breeding density of both species (mean values: 2.32 and 3.59 territorial pairs per 100 km2 respectively for RK and BK) was almost constant during the study period. Laying dates of BK (9 April-5 May) were shifted of about a month later with respect to RK (20 March-3 April), while a marginal overlap occurred in fledging periods between the species. Most of nests were clumped in mixed loose colonies, while solitary nests were less than 20%. Mean nearest neighbor distances between conspecific pairs were higher in RK than in BK. The lowest value of mean nearest neighbor distance was found between heterospecific pairs. Nest dispersion patterns showed a progressive increase in clustering, from RK to BK, and up to reciprocal interspecific nest distribution. Mean productivity and breeding success were slightly higher for RK. No influence of BK neighbors on RK productivity and breeding success was observed. Prey remains and pellets were collected under 11 RK and 9 BK nests occupied by breeding pairs to compare their food diets. Birds (50%) and other small terrestrial vertebrates represented the largest part of biomass consumed by both the species. The proportion of food items coming from waste dumps (mainly slaughterhouse refuses) was higher in the diet of BK (23%) than in RK (15%). Analyses, performed both on number and biomass of food items, grouped into 11 taxonomic and weight classes, showed similar niche breadth and wide niche overlap between the two species. Habitat features were measured within a circular plot of 0.04 ha (radius 11.29 m) surrounding 15 RK and 21 BK nests occupied by breeding pairs. Moreover GIS analyses were performed on habitat features within circular buffers of 200 m and 500 m ray around each nest by means of Land Use Map. Due to nest clustering usually observed in the study area, the variables measured at nest tree and within the plot showed only marginal differences, while the wide overlap of buffers around the nests didn’t permit to detect significant differences. An evaluation of habitat suitability in the Tolfa Mountains is needed to assess if nest clusters of kite pairs were determined by scarce availability of suitable sites or by other factors, even intrinsic factors, like the heterospecific attraction. More reliable analyses would be needed to estimate food resources availability, but the relevant observed overlap occurred when the amount of preys was probably the highest in the year (mainly large insects, nestlings of other birds and snakes). Moreover, although an evaluation of habitat suitability in the Tolfa Mountains is needed, the extension and distribution of woodlands over the study area didn’t seem to represent a limiting factor, and the clustered interspecific nest dispersion could be due to other factors, even intrinsic, like the heterospecific attraction. The complexity of the interspecific relations observed should be taken into account in planning conservation actions.