The Sandwich tern (Thallasseus sandvicensis) is the largest of the five species of terns that breed in the UK and the Republic of Ireland, and they get their english name from Sandwich in Kent, which once held a large colony. Powerful plunge-diving predators, they have long, silvery grey-backed wings, black legs and a yellow-tipped black bill. Their shaggy crest is evident when they are on the ground, particularly when they display to their mates in extravagant courtship ‘dances’ at the start of the breeding season.
Cemlyn is the only Welsh site for this species, and the largest in the west of Britain, holding up to 20% of the entire UK population. Sandwich terns from Cemlyn may travel as far as 30km around the Anglesey coastline in search of protein-rich fish for their chicks, and birds ringed as chicks at Cemlyn have been recently re-sighted at wintering grounds in Gambia, Mauritania, South Africa and Namibia. They will return to Wales as two or three-year olds to try and breed themselves, usually arriving in late March and April.
The roseate tern (Sterna dougallii) is the rarest breeding seabird in the UK & Republic of Ireland. Historically, because of their beautiful plumage, these rare birds were hunted almost to extinction in Britain during the hat-making craze. Today, there are only three small colonies supporting just over 1850 pairs (2016).
Adult roseate terns have a black cap and extremely pale body plumage, with light grey back 'mantle' and creamy-white underparts with a faint rosy tinge to the breast (not always easy to discern). An important distinction is that their bill is all black in May, then usually becomes red at the base and is half red by August. This, along with their very long white flexible tail-streamers and characteristic voice, helps in distinguishing these terns from the others.
Roseate terns have not nested at Cemlyn since the mid-1990s, but sometimes call in at the colony during the course of the season. The site is maintained to be as attractive to roseate terns as possible however, in the event of the species re-colonising.
A designated viewing area is marked out on Cemlyn’s shingle ridge during the tern breeding season, opposite the lagoon islands, providing one of the best viewpoints in the UK from which to see a tern colony close-up. The spectacle and soundscape of a busy chick nursery is impressive, and the exhilaration of parent terns flashing past with fish – sometimes at head height – is a unique experience.
Two summer wardens are employed by North Wales Wildlife Trust every year to protect the tern colony and other wildlife, and to liaise with the many visitors to the ridge. They are assisted by dedicated volunteers, without whose help the work of the Trust would be impossible. A warden is usually in place at the viewing area during daylight hours, monitoring the terns and providing information to the public. Wardens also conduct research such as feeding studies from the ridge, recording the size and kind of fish being brought in by parent terns to their chicks.
Esgair Gemlyn, the crescent-shaped spit of shingle dividing Cemlyn Bay from the lagoon, was dramatically built up by a violent storm in the 19th century, and is a dynamic and fascinating strip of habitat. Inching inland every year, and changing profile according to the direction and severity of winter gales, the ridge provides a home for some hardy specialist plants like yellow horned poppy and sea campion. The sweet-flowering sea kale is the toughest species of all, pushing deep roots down through the shingle, right up to the strandline.
Ground-nesting birds like ringed plover and oystercatcher sometimes attempt to nest along the ridge, laying their beautifully camouflaged eggs in scrapes in the shingle. For this reason, as well as to prevent disturbing the terns, The Wildlife Trust ropes off the top of the ridge every spring and summer, asking visitors to stay on the seaward side, and keep all dogs on a lead.
Common & Arctic terns
Smaller than Sandwich terns, with darker grey plumage, these two species are very similar to each other in appearance, and it can take practice to distinguish them in the field. Our id guide will help you pick out their key features. They return from Africa later than Sandwich terns – usually from mid-April onwards, and tend to make their nests around the periphery of the islands – often on bare shingle or short grass. Arctic terns make one of the longest migrations of any bird, and may see more daylight in their lives than any other species.
Both these terns are fiercely defensive parents and will actively attack anything (including humans sometimes!) that they see as a threat to their young. In a mixed colony, this behaviour can benefit other species like roseate terns, that are less aggressive towards aerial predators, and the presence of common terns in particular is thought to be an important factor in attracting roseate terns to a site.
[The Roseate Tern LIFE Recovery Project aims to restore historical sites where roseate terns used to nest. Through improving site conditions for common tern colonies, roseate terns might be more likely to recolonize these sites in the future.]
To attract more common terns (that may in time attract roseate terns), the Wildlife Trust is trying to create additional potential nesting habitat at Cemlyn. One way of doing this is by placing rafts around the lagoon islands. These anchored floating platforms are covered with shingle, and have steep Perspex sides that exclude predators approaching from the water. Rafts have been shown to be successful nesting options for common terns, and they have taken readily to them at other sites. It is hoped the rafts may alleviate competition for space with Sandwich terns that may be an issue on the islands themselves.
To maximise available nesting habitat on the islands, North Wales Wildlife Trust carry out regular management work, often with the help of volunteers. This has included clearing larger stands of sea beet, which may build up in some places, particularly after mild winters, and filling in muddy, lower-lying areas with shingle, to create well-drained, open spaces that terns can nest in.
Managing water level
An important part of getting the habitat right for nesting terns at Cemlyn is managing the water level in the lagoon during spring and summer. If levels get too low, it could be an invitation for ground predators, but if they get too high the islands could be inundated. Stop-logs are therefore put into the Bryn Aber weir at the start of every breeding season, reducing tidal throughflow and keeping the lagoon water level constant. They are removed after the season to allow the islands to be flooded by high tides, and for salt water to replenish the lagoon. This is important for some of the specialised lagoon wildlife that is adapted to brackish conditions.
Roseate tern nest boxes
Unlike the other four tern species (Sandwich, common, little and Arctic) which breed in the UK and Republic of Ireland, roseate terns prefer to nest in sheltered locations like crevices between rocks, amongst taller vegetation, or behind beach debris on shingle. All terns make a slight scrape (shallow depression) in the ground, and their eggs are camouflaged which helps to hide them from predators.
Every season since the mid-1990s (when roseate terns last nested at Cemlyn), North Wales Wildlife Trust has put out special roseate nest-boxes on the lagoon islands. This is to make Cemlyn as attractive to roseate terns as possible, in the event of them recolonising the site. These boxes are used extensively at the main European site for the species – Rockabill in the Republic of Ireland. In the absence of roseate terns however, the boxes also provide shelter for tern chicks of other species – both from aerial predators and adverse weather conditions.
All terns need as much help as they can get, and conservation work for these species often involves protecting nesting areas from predators, as well as from human disturbance. Predation is part of any natural ecosystem, but sometimes a particular issue threatens the viability of a whole colony. On their lagoon islands, Cemlyn’s terns have benefitted over the years from being relatively inaccessible to ground predators, but in 2017, there was a wholesale failure of the breeding season, thought to be caused by nocturnal visits from otters.
In response to this, the Wildlife Trust installed a solar-powered electric fence around the perimeter of the two islands, to deter access via the lagoon. This was to try and prevent complete desertion of the colony, which can sometimes follow from repeated problems with mammal predation. So far, the fence seems to have been successful, allowing tern numbers to start building up again, without causing any harm to the otters.
Otters were absent from Anglesey for some time but have returned in recent decades and are now found on most of the island’s catchments. At Cemlyn, they are shy and elusive; the lagoon and shores form part of a large range that may extend inland along rivers and streams. Carefully searching reveals their spraints and from these we know that when they are at Cemlyn they feed regularly on shellfish, fish including eels and occasionally birds.
Otters are just one of several mammal species that can be easily seen around Cemlyn. Hares are a common sight, even on the shingle ridge. Stoats and weasels can be seen running in and out of the earth banks around the nature reserve. And off-shore the sound of seals can be heard on nearby rocks; dolphins and porpoises may also be seen from the adjacent headland.