The reason for their dark plumage is unknown, but it has been suggested that in tropical areas, where food resources are scarce, the less conspicuous colouration makes it harder for other noddies to detect a feeding bird.

[8] Plumage type, especially the head pattern, is linked to the phylogeny of the terns, and the pale-capped, dark-bodied noddies are believed to have diverged earlier than the other genera from an ancestral white-headed gull, followed by the partially black-headed Onychoprion and Sternula groupings.

[9][10] Juvenile terns typically have brown- or yellow-tinged upperparts, and the feathers have dark edges that give the plumage a scaly appearance.

In most species, the subsequent moult does not start until after migration, the plumage then becoming more like the adult, but with some retained juvenile feathers and a white forehead with only a partial dark cap.

For example, the common tern has a distinctive alarm, kee-yah, also used as a warning to intruders, and a shorter kyar, given as an individual takes flight in response to a more serious threat; this quietens the usually noisy colony while its residents assess the danger.

[29][9] One study of part of the cytochrome b gene sequence found a close relationship between terns and a group of waders in the suborder Thinocori.

[32][33] Some authorities consider "tearn" and similar forms to be variants of "stearn",[21] while others derive the English words from Scandinavian equivalents such as Danish and Norwegian terne or Swedish tärna, and ultimately from Old Norse þerna.

[33] The cladogram shows the relationships between the tern genera, and the currently recognised species, based on mitochondrial DNA studies, are listed below:[9] Anous Gygis Onychoprion Sternula Phaetusa Gelochelidon Hydroprogne Larosterna Chlidonias Thalasseus Sterna In addition to extant species, the fossil record includes a Miocene palaeospecies, Sterna milne-edwardsii.

A common tern that hatched in Sweden and was found dead five months later on Stewart Island, New Zealand, must have flown at least 25,000 km (16,000 mi).

Arctic terns from Greenland were shown by radio geolocation to average 70,000 km (43,000 mi) on their annual migrations,[47] while another from the Farne Islands in Northumberland tagged 'G82' covered a staggering 96,000 km in just 10 months from the end of one breeding season to the start of the next, travelling not just the length of the Atlantic Ocean and the width of the Indian Ocean, but also half way across the South Pacific to the boundary between the Ross and Amundsen Seas before returning back west.

[5] The terns are birds of open habitats that typically breed in noisy colonies and lay their eggs on bare ground with little or no nest material.

[5][50] Most terns breed annually and at the same time of year, but some tropical species may nest at intervals shorter than 12 months or asynchronously.

A few species nest in small or dispersed groups, but most breed in colonies of up to a few hundred pairs, often alongside other seabirds such as gulls or skimmers.

Peruvian and Damara terns have small dispersed colonies and rely on the cryptic plumage of the eggs and young for protection.

[55] Sea terns often hunt in association with porpoises or predatory fish, such as bluefish, tuna or bonitos, since these large marine animals drive the prey to the surface.

Sooty terns feed at night as the fish rise to the surface, and are believed to sleep on the wing since they become waterlogged easily.

[18] The gull-billed tern is an opportunist predator, taking a wide variety of prey from marine, freshwater and terrestrial habitats.

[57][58][59] The greater crested tern will also occasionally catch unusual vertebrate species such as agamid lizards and green sea turtle hatchlings, and follows trawlers for discards.

[64] The red colouring reduces ultraviolet sensitivity, which in any case is an adaptation more suited to terrestrial feeders like the gulls,[65] and this protects the eye from UV damage.

[61] The inaccessibility of many tern colonies gave them a measure of protection from mammalian predators, especially on islands, but introduced species brought by humans can seriously affect breeding birds.

[5] Problems arise not only on formerly mammal-free islands, as in New Zealand, but also where an alien carnivore, such as the American mink in Scotland, presents an unfamiliar threat.

[71] Internal parasites include the crustacean Reighardia sternae, and tapeworms such as Ligula intestinalis and members of the genera Diphyllobothrium and Schistocephalus.

Eggs are still illegally harvested in southern Europe, and adults of wintering birds are taken as food in West Africa and South America.

[5][76] Terns have sometimes benefited from human activities, following the plough or fishing boats for easy food supplies, although some birds get trapped in nets or swallow plastic.

[85] In New Zealand, the black-fronted tern is facing a rapid fall in numbers due to predation by introduced mammals and Australian magpies.

[40] The Peruvian tern was initially damaged by the collapse of anchoveta stocks in 1972, but breeding colonies have subsequently been lost due to building, disturbance and pollution in their coastal wetlands.

Disturbance by humans, dogs and vehicles, predation by introduced species and inappropriate water level management in South Australia are the main reasons for its decline.

[5] Most tern species are declining in numbers due to the loss or disturbance of breeding habitat, pollution and increased predation.

Gull populations have increased over the last century because of reduced persecution and the availability of food from human activities, and terns have been forced out of many traditional nesting areas by the larger birds.

The plan is intended to address key issues such as species and habitat conservation, management of human activities, research, education, and implementation.