Black-naped tern, August 2020, Sai Kung.
Description and Distribution
Black-naped terns are medium sized seabirds in the tern subfamily of the larger family, Laridae. Their faces and breasts are snowy white with grayish wings, though the grey is more pronounced in some individuals than in others. The primary of the wing is also black.
Their legs and bills are black, and the bill is tipped with a bit of yellow. They get their name from the black stripe that extends from its eyes to its nape. The species is not sexually dimorphic, with breeding plumage being signalled only by a bit of pink on the breast during breeding season.
They can be found throughout the Indian and Pacific oceans, preferring tropical and subtropical waters on shores ranging from Africa to French Polynesia and from Australia to eastern China.
Internationally, black-naped terns are not classified as threatened, though in certain locales, such as in Malaysia and Singapore, there is evidence of declining populations based on studies of certain breeding grounds.
Ecology and Behavior
Black-naped terns breed in colonies between May and August, including in Hong Kong. They lay their eggs on bare rocks and on rocky shores without nesting material, though they do occasionally encircle their eggs with small pebbles.
These terns primarily eat fish, making shallow plunges into water in order to grab their prey. It’s been claimed that they are solitary feeders, though I’ve witnessed them feeding in groups of three or four before.
Phylogenetic analyses suggest that terns and gulls split off from a common ancestor about 13 million years ago, though some experts have suggested a divergence point of 24.4 million years ago instead.
Black-naped terns are unique in so far as they are one of the only tern species without a black head during their breeding plumage, the other being the snowy-crowned tern (sterna trudeaui). All other terns have a pronounced black cap, and sometimes a crest, during breeding season that is reduced for the rest of the year.
For the black-naped tern though, it’s pre-breeding molt either stops at the nape, foregoing the cap, or the process is foregone entirely, leaving it to breed without a tern’s signature black cap. Not enough research has been done to confirm either possibility, nor is it known why black-naped terns evolved this way.
One possibility has to do with the purpose of the black cap itself on terns, which may be to signal to non-breeding resident birds and in tern (sorry) mitigate conflict, as breeding terns would know not to attack or pursue non-breeding ones. The white crown of the black-naped tern may be the result of evolutionary pressures that preferred a more juvenile, non-breeding look in these terns until eventually they lost their black caps completely.