Occupancy and Population Abundance of Two Serpent-Eagles (Spilornis elgini and S. cheela) in the Andaman Islands, India

Abstract

ABSTRACT We estimated occupancy and population abundance of the Andaman Serpent-Eagle (Spilornis elgini) and Crested Serpent-Eagle (Spilornis cheela) in the Andaman Islands, India. We divided the islands that were > 100 km2 into a grid of 84 cells, of which we used 34 cells (5 km × 5 km each) for surveys of two populations in the districts of (1) the North and Middle Andaman Islands, and (2) the South Andaman Islands, which includes Little Andaman Island. We performed single-season, single-species occupancy modeling independently for both regions and single-season, two-species occupancy modeling for co-occupancy in the surveyed landscapes. The estimated abundance for the Andaman Serpent-Eagle and the Crested Serpent-Eagle was 4–5 and 7–8 individuals per 25 km2, respectively. The Andaman Serpent-Eagle is commonly distributed in both regions and has a high detection probability (detection probability, P = 0.8). The Crested Serpent-Eagle is widely distributed in the South Andaman Islands with a low detection probability (P = 0.5). The single-season, two-species occupancy model showed that the endemic Andaman Serpent-Eagle occupied only 22% of the area and did not co-occur with the Crested Serpent-Eagle. It also indicates the possible avoidance or exclusion of the Crested Serpent-Eagle by the Andaman Serpent-Eagle. We encountered both serpent-eagles in all the identified habitat types. The Crested Serpent-Eagle's habitat use was not restricted to the coastal forests, as previously reported, and the Crested Serpent-Eagle is probably dominating the Andaman Serpent-Eagle in the co-occupied areas. We further conclude that the occupancy dynamics and competition for space between these two Spilornis species might significantly affect the distribution and population of the vulnerable and endemic Andaman Serpent-Eagle in the future.