Habitat segregation in two congeric hawk-eagles (Spizaetus bartelsi abd S. cirrhatus) in Java, Indonesia

On the island of Java, Indonesia, two congeneric hawk-eagles occur, i.e. the endemic Javan hawk-eagle Spizaetus bartelsi and the wide-ranging changeable hawk-eagle S. cirrhatus. Comparisons with similar species-pairs in South East Asia suggest that these species may be competitors both in habitat and diet. The limited area and less-diverse resources of islands should make competition between similar species more acute and should lead to marked niche segregation, with a larger niche breath for the endemic species. Spizaetus bartelsi was recorded significantly more frequently in forest habitats from sea level to 2500 m asl and S. cirrhatus more in open habitats up to 1600 m asl. Contrary to the prediction, the endemic hawk-eagle occurred in a smaller range of habitats (four out of seven) compared to S. cirrhatus that was found in all seven habitat types. Although significant, habitat segregation was not as well marked as in some similar co-occurring species. The relative overlap between the two species may in part result from human influences with the dense continuous rain forest being replaced by more open forest types.