© Allan Drewitt/BTO
1. Epic journey
The common tern can be found breeding across most of Europe and Asia and in parts of North America. After the breeding season, the terns migrate south to spend their winter along the coasts of the tropics and the southern hemisphere in areas including Africa, South America and South-east Asia. They have one of the longest migrations of all birds, with an average round trip of 35,000km each year.
2. Migrating seabirds
Common terns arrive in the UK from their wintering grounds in April, and leave our shores again in August or September. During this time, they’re easiest to see around colonies or when they have young to feed, which forces them to travel further to find food. They’re most commonly seen along stretches of coast with shingle beaches, or inland on gravel pits, reservoirs or lakes.
3. Huge numbers
Common tern colonies usually number around 2,000 birds, but can be as large as 20,000. They are often shared with other tern species such as Arctic and roseate terns. The terns lay, on average, three eggs in May with incubation lasting around 22 days. The chicks have normally fledged by 28 days after hatching.
The characteristic plumage of the common tern © Tom Bickerton/BTO
4. Strange behaviour
One interesting example of tern behaviour is known as ‘dread’. This usually occurs in the early part of the breeding season, and describes the peculiar event in which most or all of the terns will suddenly burst up from the ground and fly low over the colony or the sea, either to deter predators or for apparently no reason at all.
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5. The memory game
You can probably imagine that, with so many nests in one colony, it must be difficult for the terns to remember exactly where their own nest is. However, studies have shown that common terns can find their eggs even if they have been buried and no evidence of the nest remains. This is a necessary adaptation in such an exposed environment where losing a nest is likely.
6. How to identify a common tern
The common tern is a medium-sized tern found in the majority of the UK. These graceful birds are characterised by their silver-grey upperparts, white underparts, black cap and red bill, as well as long tails that have earned them the nickname ‘sea-swallow’. They characteristically hover over the water before diving to catch their prey. They can be separated from arctic terns by a black tip to the bill, darker wing feathers and longer legs.
The British Trust of Ornithology (BTO) works in partnership with over 40,000 volunteer birdwatchers to chart the fortunes of UK birds.
Among the surveys that we coordinate is the Wetland Bird Survey (WeBS) which monitors non-breeding waterbirds in the UK. This is a joint scheme of the BTO, RSPB and JNCC in association with WWT.
For more information about WeBS, including how to take part, please email email@example.com.