Friends volunteer, popular guide and bird expert Trevor Gunton introduces the next in this series of informative articles.
Image by Natural England
It used to be said that all you had to do was dig a hole, fill it with water, and down come the ducks! To some extent, that really is true, but they still need somewhere to breed of course, often as far away as the Arctic. Other wetland birds have benefited from the creation of reservoirs, such as Grafham and Rutland Water, and the proliferation along many river valleys of flooded quarries and pits, such as Little Paxton’s own justifiably famous pools.
Yet not everything is so simple. An island really is pretty much essential for many breeding birds, including black-headed gulls. Gulls tend to be numerous, aggressive and domineering – not always good neighbours. Many sites that would be good for Britain’s only inland-breeding tern, the common tern, are taken early in the spring by gulls, so the terns, for all their sharp beaks and feisty behaviour, are simply pushed out and prevented from settling to nest, although the presence of the gulls nearby can help them as a defence against predation.
So, the tern raft comes to the rescue! Rafts designed for nesting terns, put out on the water after the gulls have more or less settled elsewhere, give them a great chance of rearing their chicks, away from land-based predators if not from avian ones. I’m not sure where the first ones were used, or when, but they go back a few decades and are used almost worldwide.
Common terns are widespread in Europe. On the coast, they nest on shingle and sand beaches. Since every beach, however remote, now seems liable to constant disturbance from people, let alone the risks of high tides and storms, inland nesters are increasingly important. Coastal terns conform to the text-book norm, diving headlong for fish, but I wonder how many Paxton birds dive? It would be interesting to test this: but on rivers, at least, and most gravel pits and reservoirs, they seem to feed by dipping to the surface, just like black terns and little gulls.
How far away do Paxton birds go to feed? To Grafham, presumably, via the river; also to other pits and reservoirs further afield? Probably. Studies elsewhere have shown that they may travel five or six miles (more on the coast) to feed, but interaction between isolated inland waters is not so easy to define, and a river valley, at least, is probably needed as a connecting route. Freshwater breeders will certainly take small fish – you can see them carrying little silver slivers in their bills – but are likely to eat more insects and various aquatic invertebrates than coastal ones. Terns’ eyes have special red oil droplets in the retina, which serve to enhance contrast and improve vision through the water (rather like using Polaroid spectacles).
In 2018, some terns fledged early, some very late, on the Paxton rafts. They are not normally double-brooded, but it has been recorded on occasion, at least in North America, and they can replace lost clutches with later eggs, and possibly this is what happened. The usual clutch is of two or three eggs and success varies, but about 90% of eggs will hatch and an average of between one and two chicks will be reared per pair on rafts inland, but only one or so on the coast.
The survival of young birds then depends on what happens elsewhere: they will migrate as far as West Africa, perhaps farther south towards South Africa, and need good feeding, somewhere ashore to rest, and safety from predation and human persecution if they are to return as breeding birds.
Little Paxton chicks are likely to return to the same site but may not breed until four years old. It is very difficult to tell the age of a common tern after the first autumn/winter. Some return when a year old, but telling these from adults is hard unless they retain some old, worn (and very dark) juvenile flight feathers. Many immatures are likely to remain in the southern hemisphere for the first year or two.
Fortunately, perhaps, for visitors to Paxton Pits, the common tern is not nearly so aggressive towards intruders as the Arctic tern, which is well-known for being quite happy to dive at people’s heads and regularly draw blood with their spike-like bills! You might, though, get a tern or two screaming aggressively at you as you walk nearby. Just accept this as part of the fun of being host to such wonderful birds and wish them well. And thank those willing helpers who manage the tern raft scheme every year, for the benefit of the birds and for visitors too.
The tern rafts on Heronry North could not be launched in 2020 due to a number of restrictions. The terns moved to the northern Pits but no young were raised.
In 2021, the rafts were were put in place in April, complete with an electronic device to protect the breeding terns from predation by mink and maybe otters. We hope you’ve been keeping an eye on their progress this spring and summer.
Rob Hume is a well-known wildlife artist and author of many bird-related publications. He worked at the RSPB in Sandy before moving south, having been a regular visitor to the reserve. Rob is a nationally respected authority on gulls and terns.
An abridged version of this article appeared in the members’ newsletter, Between Friends.