Country diary: A sea-view of house martins in their ancestral home | Claire Stares

We jumped off the boat and swam through a narrow arch in the outcrop known as No Man’s Land, then struck out for Old Harry, the towering sea stack that gives the iconic chalk formation its name.

Though primarily known as a site of geological interest, Old Harry Rocks is also significant for its biodiversity both above and below the waves. The water was crystal clear, and as I glided above the luxuriant seagrass meadow, I scanned the vegetation, hoping to glimpse a seahorse. This stretch of coast is a stronghold for short‑snouted and long-snouted (spiny) seahorses, and the largest specimen recorded worldwide – a 34cm-long spiny – was found here in 2015. Alas, the elusive creatures remained hidden among the eelgrass, but there were plenty of brown crabs scuttling about and a fist-sized, spindly-limbed spiny spider crab picking a path through ribbons of kelp.

Related: Country diary: A giant of the sea, reduced to a fragment | Sara Hudston

As the current swooshed me through another archway, I rolled on to my back and gazed up at the looming cliffs. Spotting the distinctive spitfire-like silhouette of a house martin against the sheer rock face, I watched it swoop up, its white rump disappearing under a 30ft-high ledge. In Macbeth, Banquo speaks of “the temple-haunting martlet”, and this charismatic little hirundine’s common name reflects the species’s long association with human dwellings. But while they now nest almost exclusively in the eaves of buildings, a few colonies can still be found nesting wild, like their ancestors, on coastal cliffs.

Sculling closer, I peered under the overhang and located the well-camouflaged dried mud nest. Tucked neatly into a crevice, it was protected from the elements and its lofty position made it impervious to even the most tenacious predators. Nevertheless, the departing adult appeared hyper-vigilant, jinking as a cormorant flew overhead.

A grizzly discovery at the foot of the cliff explained why. The fresh remains of a juvenile herring gull lay on the exposed pebble beach, its carcass dismembered and stripped clean, leaving only the head and a fan of tail feathers connected by the spine – the work of a peregrine falcon, one of the few avian predators possessing the speed and agility to pluck a house martin from the sky.

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