Crested serpent eagle

Description

This medium-large, dark brown eagle is stocky, with rounded wings and a short tail. Its short black and white fan-shaped nuchal crest gives it a thick-necked appearance. The bare facial skin and feet are yellow. The underside is spotted with white and yellowish-brown. When perched the wing tips do not reach until the tail tip. In soaring flight, the broad and paddle-shaped wings are held in a shallow V. The tail and underside of the flight feathers are black with broad white bars. Young birds show a lot of white on the head. The tarsus is unfeathered and covered by hexagonal scales. The upper mandible does not have an overhanging festoon to the tip.

Distribution, taxonomy and status

Within its widespread range across tropical Asia, 21 populations have been named as subspecies. The most widespread sub-species are the nominate from along the sub-Himalayan range in India and Nepal,

• S. c. batu from southern Sumatra and Batu,
• S. c. bido from Java and Bali,
• S. c. burmanicus in most of Indochina,
• S. c. hoya from Taiwan,
• S. c. malayensis of the Thai-Malay Peninsula and northern Sumatra,
• S. c. melanotis in Peninsular India,
• S. c. palawanensis from Palawan,
• S. c. pallidus from northern Borneo,
• S. c. richmondi from southern Borneo,
• S. c. ricketti in northern Vietnam and southern China,
• S. c. rutherfordi from Hainan, and
• S. c. spilogaster of Sri Lanka.

The remaining sub-species are all restricted to smaller islands:

• S. c. abbotti (Simeulue serpent eagle) from Simeulue,
• S. c. asturinus (Nias serpent eagle) from Nias,
• S. c. baweanus (Bawean serpent eagle) of the Bawean,
• S. c. davisoni in the Andamans,
• S. c. minimus (Central Nicobar serpent eagle) from the central Nicobars,
• S. c. natunensis (Natuna serpent eagle) from Natuna,
• S. c. perplexus (Ryukyu serpent eagle) from Ryukyu, and
• S. c. sipora (Mentawai serpent eagle) from Mentawai.

The last seven (with English names in brackets) are sometimes treated as separate species. Although the crested serpent eagle remains widespread and fairly common overall, some of the taxa that are restricted to small islands are believed to have relatively small populations that likely are in the hundreds. The rarest is probably the Bawean serpent eagle with a declining population of about 26–37 pairs, which makes it critically endangered.

The nominate subspecies has a black throat while the peninsular Indian form has a brownish throat. There are clinal latitudinal variations, with size decreasing southward. The small islands taxa are generally smaller in size than the taxa from the Asian mainland/larger islands in a phenomenon termed as insular dwarfism.

The specific name cheela is derived from the Hindi name for kites.

One syntype specimen of Spilornis rutherfordi Swinhoe (Ibis, 1870, p.85.), the Hainan subspecies, is held in the collections of National Museums Liverpool at World Museum, with accession number T2819 (male adult). The specimen was collected in Hainan, China in February 1868 by Robert Swinhoe and came to the Liverpool national collection via Canon Henry Baker Tristram’s collection which was purchased in 1896.

Behaviour and ecology

The crested serpent eagle, as its English name suggests, is a reptile eater which hunts over forests, often close to wet grassland, for snakes and lizards. It has also been observed to prey on birds, amphibians, mammals and fishes. It is placed along with the snake eagles of the genus Circaetus in the subfamily Circaetinae. It is found mainly over areas with thick vegetation both on the low hills and the plains. This species is a resident species, but in some parts of their range they are found only in summer.

The call is a distinctive Kluee-wip-wip with the first note being high and rising. They call a lot in the late mornings from their perches where they spend a lot of time and they rise on thermals in the mornings. In southern Taiwan, males have a larger home range than females. Males on average had a home range of 16.7 km² while females used about 7 km². When alarmed, they erect the crest and the head appears large and framed by the ruff. They will sometimes follow snakes on the ground. They roost in the interiors of trees with dense foliage. A radio-telemetric study of the species in Taiwan found that the birds spend 98% of the day perched and usually finding food in the morning hours. They appear to use a sit and wait foraging strategy.

The breeding season begins in late winter when they start courting and establishing territories. The eggs are laid in early summer. Old nests are often refurbished and reused in India but a study in Penang found them to build fresh nests each year. A study in India found that most nests were built along riverine trees. The nest is a large platform built high on a tree. Both birds in a pair build the nest but the female alone incubates. The male guards when the female forages. In central India, the Terminalia tomentosa is often used while Terminalia bellirica and Dalbergia latifolia was often used in southern India. In Penang, the nest trees were typically large and isolated from other trees with lot of room for the birds to fly in and out. The nests are lined with green leaves collected from nearby and are placed facing down on the nest floor. The usual clutch is one egg but two are sometimes laid and only a single chick is successfully raised in a season. When eggs are lost, a replacement is laid two to seven weeks later. The eggs hatch after about 41 days and the young fledge after about two months. Nests are defended by the parents.

Several species of endoparasitic nematodes have been recovered from the intestines of crested serpent eagles, including Madelinema angelae. Avian pox virus infections which cause warts on the face have been observed in a wild bird living in Taiwan. A number of ectoparasitic bird lice have been described from the species including Kurodaia cheelae. In Penang, it was found that cinereous tits (Parus cinereus ambiguus) tended to nest close to nesting crested serpent eagles, presumably due to safety from predators like crows that may be driven off by the eagles. They have also been found to visit the nests of the eagles to collect fur from the remains of dead mammal prey.