One important goal of community ecology is to disentangle how communities are assembled, because the factors determining the species composition are many (Ştefănescu et al. 2013). Orians and Soulé (2001) suggested that communities are recognised as the level of ecological structure on which conservation should devote most research efforts. Hence, although our study did not concentrate on assemblages, our analyses of six-raptor species from the same ecological regions are of relevance for elucidating their respective resource exploitation and affinities, and a tool for future studies in this rapidly developing region. Our data showed that C. gallicus and S. cheela had the greatest similarity of diet composition and to a lesser degree also A. fasciata and N. cirrhatus. Differences in prey taken were evident in that the largest composition of reptiles was characteristic for C. gallicus, S. cheela and E. caeruleus, while A. fasciata, F. chicquera and N. cirrhatus show a higher proportion of large and small birds. However, owing to the habitat separation wherein S. cheela was found only in the hilly terrain while C. gallicus and A. fasciata on the Deccan Plateau, although we can see in the analyses diet and habitat affinities between the species, there is no real interaction between them.
Restani (1991) found that the buteo hawks partitioned habitat resources, specifically nest sites and that the species with the overlap in nesting chronology and prey use, had the least similar nest sites. Further, they found that the two species with the greatest nest substrate overlap had the lowest dietary overlap. They concluded that the diets appeared to reflect prey availability. Restani (1991) showed that the overlap in one characteristic did not mean also an overlap of other biological characteristics.Similarly, in our study, we found that although C. gallicus, S. cheela (predominantly large snakes) and E. caeruleus (mostly small snakes, lizards, skinks, etc.) showed greatest food niche overlap, but they built their nests in different habitats. However, one must take into account that reptiles are a diverse taxonomic group with many ecological divisions and can be exploited by a wide range of predators in the same habitat. Further, N. cirrhatus and S. cheela were related to the presence of water bodies, dry deciduous forest and evergreen forests, while E. caeruleus and C. gallicus were related to more open habitats. On a larger scale, this was also translated to geographical isolation of the nest sites.
Elucidating affinities between coexisting raptor species is of great importance for conservation in view of the constant change wrought by human expansion into wild habitats. Sanchez-Zapata et al. (2003) found that in Kazakhstan there were fewer species in agricultural habitats than in natural grassland and steppe habitats. Ground-nesting raptors were negatively affected by land use changes and four species were never detected in agricultural zones. Further, raptor abundance patterns differed between natural steppe habitats and human-transformed habitats, where a patchy distribution was detected. Butet et al. (2010) studied the response of Common Buzzard (Buteo buteo) and Eurasian Kestrel (F. tinnunculus) in agricultural landscapes of Western France. They found that the two raptor species showed negative but differentiated response patterns to human land-use intensification and small mammal abundance. The abundance of the common buzzard decreased significantly with the reduction of hedgerows, woodlots and grasslands areas, as well as with the decrease in prey abundance at the landscape scale. They found that the abundance of the kestrel showed the same trend, but fall of abundance was less marked and resulted in a reverse ratio of species density according to agricultural intensification and landscape openness. Authors pointed out that the specific feeding habits, nesting habitats and spatial partitioning in foraging areas could explain the difference in the numerical response observed for these two raptor species. They reasoned that this was a result of the fact that buzzards forage mainly on highly profitable prey areas and avoid highly fragmented woodland habitats for nesting, in contrast to kestrels, who are able to nest in more fragmented landscapes and are better adapted to exploit less abundant but more widespread small mammal prey species within the cultivated matrix. Baladron et al. (2017) studied raptor assemblages in the flooding pampas of Argentina and concluded that several raptor species were threatened by the expansion of urban areas and agriculture. Future studies must concentrate on the influence of the expanding human population on these assemblages in order to further understand if this leads to homogenization of the populations, or to changes in their relative abundance in the region.
Until now, similar studies on raptors have not been conducted in India, a country with the second largest human population on earth. Hence, the main importance of our study is in demonstrating the degrees of nesting and diet affinities amongst six relatively common raptor species of varying sizes that are to be found in the same geographical area. This lack of knowledge is further underwritten by the fact that we have also documented a hitherto unknown phenomenon of temporal separation wherein during the breeding season, F. chicquera and A. fasciata were absent in the coastal regions (unpubl. data). This suggests a seasonal migration of the population to higher breeding grounds, suggesting that further studies are required in order to ascertain the habitat connectivity of the two ranges, breeding and non-breeding, of the individuals that comprise the populations.
Jaksic and Braker (1983) found that prey taken was mainly determined by prey availability. They further found that food overlap was frequently very high, did not narrow in larger assemblages, nor was it correlated with body size. However, they found that the mean weight of prey taken was positively correlated with raptor weight within assemblages, but varied widely across assemblages, with any given species showing manifold differences. The researchers concluded that they found little support for predictions based on competition but thought that the opportunistic feeding behaviour of the raptors, and also because the food might not be a limiting resource, resulted in assemblages whose size is larger where less prey are available to per raptor species. It will be of interest to check whether in the study area, with fast human-encroachment and development, the raptor assemblage will change in the near future and conform to the conclusions of Jaksic and Braker (1983).
Divisek et al. (2014) reasoned that it is not well understood how the distribution of natural habitats affects broad-scale patterns in the distribution of animal species. They concluded that most spatial variation in the composition of assemblages of almost all animal groups probably arises from biological processes operating within a spatially structured environment and suggest that natural habitats are important to explain observed patterns because they often perform better than habitat descriptions based on remote sensing. They further state that it underlines the value of using appropriate habitat data, for which high-resolution and large-area field mapping projects are necessary. Most of the species in our study included a large proportion of agricultural land in their 5-km radius home ranges. It remains unclear whether this is due to most pristine habitats being limited in availability (i.e., most have been converted to agriculture, hence it is almost a necessity that these species nest in agriculture-dominated landscapes), or alternatively, could it be that they are adaptable to anthropogenic change and may be resistant to moderate agricultural land use. This needs to be elucidated in future studies. Sadly, the fact that no such studies were undertaken on any of the study species in the past prevents a comparison with the present situation wherein continued habitat modification for human-purposes occurs in the study area.
Although our study encompassed a large area of Maharashtra State, the sample sizes for each of the species are limited. However, the study contributes greatly to the understanding of these raptor species and an effort needs to be made to increase the sample sizes of the assemblages greatly, and to identify a wider range of parameters within the breeding territories. These can include potential prey diversity as compared to prey hunted, or camera trapping at the nest to identify the number of prey species brought to the nest as compared to those identified in the pellets, higher resolution and relative proportions of the habitat mosaic, etc. Another aspect that needs to be studied is whether interference competition occurs within the guild (cf. Oro et al. 2009) and inter-specific resource kleptoparasitism (e.g., Hakkarainen et al. 2004; Rebollo et al. 2017).
