I generally think of birds as nesting in high places, like on cliffs or in trees. Of course, there are lots of birds that nest in other places, like on the ground (especially if they can’t fly!) Today’s bird, the sand martin, is a burrowing bird that builds its nests in riverbanks.
Sand martins are found widely throughout the world, including in South and North America, Europe, Africa, and Asia. In the Americas these birds are known as bank swallows, but the rest of the world calls them sand martins. Sand martins are migratory, breeding in the northern parts of their ranges and heading south for winter. They prefer to be close to bodies of water, and are often found in river valleys or coastal areas.
These are small birds, reaching only twelve centimetres in length. They are quite plain, with brown markings on their backs, and white on their undersides. Young birds are slightly more colourful, with a slight pink tinge to their throats.
Sand martins feed on insects, and almost nothing else. Most of their feeding is done on the wing, since they are quite awkward on the ground. They even drink while flying, skimming low enough across the water so their jaw hit the surface. Sand martins like each other’s company, usually foraging in large groups, of up to 2100 pairs.
Feeding isn’t the only thing sand martins do together – they also nest in large colonies, often nesting very close together. Upon arrival at their breeding grounds, male martins are responsible for building the nesting site. They dig tunnels into soft sand, usually on river banks. The tunnels are about 60 cm long when finished, with a nest chamber at the end. When the tunnel is about halfway done, males sit at the entrance and sing, hoping passing females will like their tunnels and their songs. If a female does find a male martin attractive, she will sing in response and start to perch near his nest.
Once the pair bond is formed, the birds will guard their nest, and each other. If they don’t keep an eye on one another, both sexes will try and mate with other birds. The female will then help excavate any remaining sand, and build the nest chamber, which is lined with feathers and grass. The birds sleep, copulate and lay their eggs in this chamber. Four to five eggs are usually laid, which hatch in about two weeks. The young fledge twenty days after hatching, and are independent less than a week later.
Nesting in sandbanks provides some additional protection against predators, though animals like badgers and snakes are known to raid sand martin colonies. The nests are also hotbeds for parasites, such as fleas, blowflies, and mites. I’m not sure I’d want to grow up in a burrow infested with insects, but it seems to work alright for the sand martins, because the species as a whole is doing quite well. Let’s hope they keep it up!