Glorious mud
Only a handful of species use mud to build their nests – find out which ones do and why.
MAY is a busy month for House Martins. Most return from Africa in April and May, feeding up before starting the process of nest building and refurbishment, a process which takes a lot of mud, the main ingredient of their nests. This is far from a common place construction material in the nests of other birds, however. Fewer than 5% of the world’s birds incorporate mud into their nests, but its use is geographically widespread, being found among birds from six continents.
Few species make nests from just mud and most of these are swallows and martins. House Martin collects mud pellets and carefully sticks them to a wall, allowing the first line to dry before adding more. The cup-shaped nest is built up gradually and small amounts of saliva are mixed with the mud to bind it all together. Many other swallow species make nests similarly, but some are shaped like flasks with tunnel entrances, as in Red-rumped Swallow.
Nesting holes are sometimes complemented with mud, especially by nuthatches. European Nuthatch plasters the edges of an existing hole to get the size right, while rock nuthatches take this a step further, building a substantial front onto a cavity in a rock face. The nest holes of most hornbills are plastered over with mud, leaving the incubating female trapped inside, with only a small slit through which the male can pass food.
Several other British birds incorporate mud as a part of their nest building. Blackbird and Song and Mistle Thrushes start with a strong foundation of twigs and grasses then apply a layer of mud before adding a softer lining. Fieldfare and Redwing also build nests like this, as does American Robin and Austral Thrush in the Americas. When Magpies build a nest the male creates the main structure complete with a roof, while the female lines the interior with mud and grasses.
In Africa, palm thrushes plaster a mud cup onto a palm branch or ledge, lining it with grass, while the massive nest of Hamerkop is made of twigs, reeds and grass cemented together with mud. The much-sought-after rockfowl (Picathartes) build a large mud nest on the roof of a cave or on a rock wall, often where mud-daubing wasps have made nests, using this as a firm foundation for their larger nest. Small amounts of grass give further strength and sometimes mud is swallowed and regurgitated to mix with saliva as extra glue.
In Australia, Magpie Lark, Apostlebird and White-winged Chough all build a mud bowl strengthened with grass. Starting with a strong base of mud and grass, the side of the bowl is built up using mud, and as each layer is added, the builder vibrates its beak to bind the fresh mud with the already dry mud of the layer below. In South America, the master mud-nest builders are the horneros or ovenbirds which may take two months to build a carefully crafted nest of mud and grass, twice the size of a football, with the inside smoothly polished using the beak.