The black-naped tern (Sterna sumatrana) is an oceanic tern mostly found in tropical and subtropical areas of the West-Pacific and Indian Oceans. They are rarely found inland.[2]
The Black-naped tern is named for the black band beginning at the eyes connecting behind the head and descending the back of the neck. The rest of their plumage is a grey-white colour while their beaks and legs are black. They have a body length of 33-35cm and a wingspan of 66cm. Adults weigh between 97-120g. Hatchlings are a motled brown colour.[3][4]
Lady Elliot Island, Qld, AustraliaSterna sumatrana - MHNT
Distribution & Habitat
The Black-naped tern inhabits rocky shores throughout the Western-Pacific and Indian oceans, migrating yearly to islets and islands serving as breeding grounds.[5]
Behaviour
Hunting & Diet
Like most terns, this species hunts by diving; they partially breach the surface of the water and directly capture their prey carrying it horizontally in their beak. Most hunt alone at sea, but hunt closer to shore in the surf when feeding nestlings and mates.[2]
Diet
The Black-naped tern diet consists of small fish up to 10cm long including those in families:[2]
The Black-naped tern breeds on oceanic islands in small colonies of 5-20 pairs, occasionally up to 200. Nests are made past high tide in depressions in sand or rock, sometimes delineated with shells or coral fragments, otherwise with little preparation. The rocky outcrops may be returned to yearly during the breeding season.[6]
Laying period
Laying time varies from May to December depending on the location of the colony. Egg laying begins in India and Polynesia in May, followed by colonies in the Philippines in July, North-East Australia in September, oceanic Indian colonies from September-November, and the Great Barrier Reef from November-December. Leading up to egg laying, the male feeds the female her daily requirement of fish.[6]
Clutch Size & Brooding
Clutch size varies from island to island, between 1-3 eggs per pair. Both mates take shifts incubating the eggs for 21-23 days; shifts last anywhere between 4 minutes and 7 hours. Once the chicks hatch, the mates alternate in feeding and brooding for the first 7 days. As the chicks reach later stages of development, the mates hunt simultaneously.[6]
Predation
Until the chicks reach a weight of roughly 65g, they are vulnerable to predation by gulls; at this point, they are too large for the gulls to carry. Development to this weight takes 10.5 days; since the parents protect their nest for the first 7 days, chicks spend 3.5 days more vulnerable to predation. To defend against this, parents camouflage the nest by removing eggshell remnants and defecating several meters away to avoid bright eye-catching material near the chicks. When aerial predators are present, chicks hide themselves among vegetation and debris.[6]
On one occasion, a colony of Black-naped terns were observed using mobbing, shrill cries, and defecation as defence mechanisms against an encroaching Grey Heron during the breeding season.[4]
Fledging
Chicks fledge after 21-23 days; however, fledglings rely on their parents for at least 2 months afterwards. Parents and fledglings therefore leave breeding grounds together.[6]
Vocalizations
This tern calls with short, high-pitched, repeated sharp notes; "chit", "chip", "chrrut", "tsip"[3].
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