While a small number of Sandwich terns winter along the south coast, the appearance of migrants returning from Ghana and Senegal tends to coincide with the reopening of the marked swim zone at the beginning of May. The sea is noticeably warmer today, so for the first time this year, we strike out for the buoys. The council is in the process of replacing the faded buff-coloured balls with vibrant yellow cans, and the terns have taken a shine to these new flat-topped perches.
Approaching the first marker, I can appreciate the two birds atop at close quarters. The largest of the five tern species that nest in the UK, they are similar in size to the black-headed gulls wheeling overhead. Though they lack the elegance of common and little terns, which also frequent the harbour, their crested black caps give them a raffish air.
As another tern comes into land, its wings flared just enough to slow its descent, one of the pair takes off. Unperturbed by my presence, it gives a sharp kerrickk – a call often likened to a squeaky gate – and plunge-dives within a fingertip’s reach. The three clockwise rotations it executes as it plummets through the air are Olympic-medal-worthy, but it wouldn’t win any points for its entry – a large splash sending spray flying into my face.
The bird remains underwater for several seconds, but emerges triumphant, a glittering sand eel writhing in its yellow-tipped bill. Despite their name, sand eels aren’t true eels but burrowing fish, also known as sand lances, which spend most of their lives hidden in the sandy seabed. When they rise to feed on plankton, they become targets for a host of predators. They’re a staple of the Sandwich tern’s diet and provide vital nourishment for the chicks that will soon start hatching on the islands within Langstone Harbour and the Hayling oyster beds.
The terns track the shoals with uncanny accuracy. From above, they follow the subtle changes in the sea’s surface, signs I can’t spot at water-eye level: shifting shadows, a shimmer out of rhythm, the flicker of silver scales catching the sunlight.
• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 15% discount
