Caught by the River

Little Miners

Mat Bingham | 13th September 2018

Behold the sand martin – the latest species to be captured by Mat Bingham‘s lens.

There is surprisingly little information available about sand martins given that they are not a particularly rare bird. Plenty of books appear to have been written about their more famous cousins, the house martin, the swallow and the swift, perhaps because they are much more visible. After all, house martins nest in the eaves of houses and swallows are frequently seen hawking over fields, hedgerows and gardens. Look up to the sky on a summer evening and if you are lucky, you might see the anvil shape of a swift soaring high above, the stratospheric global traveller of the passerine world.

But sand martins have to be actively sought out; they prefer communal nest sites tunnelled into the vertical faces of quarries or the outside bends of meandering rivers.

I spent the summer this year visiting a sand martin colony on a river in the Peak District. The location was idyllic, the only visitors being cattle that sometimes come down to the water’s edge for a drink, the odd fly fisherman heading upriver, and me.

There are about fifty nest holes in the sandbank but not all of them are occupied. When the sand martins start to dig into the bank, if they uncover a large pebble that can’t be moved, they will abandon the excavation and start another. Their small beaks and round heads hardly seem ideal for excavating into the sand, and yet this is what they do. I have an old bird guide at home, written in 1910, which calls sand martins “little miners”.

The sand martin’s food comprises of airborne insects, such as mayflies and caddisflies, which they hawk in groups feeding on the wing. Watching them hunt is like an aerial dog fight, with birds turning here and there, climbing, diving and barrel rolling, snatching insects in mid-air. Bristly feathers near the sand martin’s beak help trap insects – most likely the sharp feathers impale the thin delicate insect wing membranes, making it easier for the sand martin to manipulate the prey into their beak.

Occasionally an individual will turn away, leaving the melee to skim across the surface of the river and take a sip of water mid-flight with all the skill of the most accomplished aerial acrobat.

The adult birds will sometimes fly past and “buzz” the nests, calling to encourage their offspring to take flight and follow them. These training flights don’t normally last too long before the juveniles tire and return. The first time a fledgling takes flight can be risky, as the nests’ chambers are too small for the juveniles to practise flapping their wings much before their first flight.  It’s a do or die moment; if the juveniles can’t fly when they launch themselves away from the nest, they will either land in the river and drown, or will be left stranded below the nest bank, unable to fly to the safety of the colony above, and will be picked off by predators.

The nest site is really rather chaotic. Juveniles poke their heads out of the excavated tunnels looking for their parents. Once fledged, they regularly switch nests in the hope that other adult birds will mistake them for their own offspring and feed them. The returning parents regularly peel away from landing at the last minute, confused as to why they have too many chicks in their nest. They then turn and loop around for another flyby to check they are at the right nest hole before landing.  The cheeky juveniles are very successful at this deception and often obtain a free meal!

At the start of the breeding season when the chicks have hatched, the adult birds can be seen carrying faecal sacks away from the nests in an attempt to keep them clean and make them less obvious to predators, but after a few weeks they seem to give up on this. The white stains around the tunnel entrances clearly identify which nests are in use and which are vacant.

The chaos of the sand martin colony continues through to the end of August, until one day, without warning, the colony is empty. The sand martins have left, heading to Africa for the winter.  But the little miners will be back next spring.