30 C O N T E X T 1 2 8 : M A R C H 2 0 1 3 JAKE ALLSOP Conserving swifts in historic buildings Some say that an old building is not complete unless it is host to a vibrant colony of breeding swifts providing exuberant summer displays set off against the architecture. Among the creatures that make their homes on or in our buildings, one species of bird in particular needs all the help we can give it: the common swift. Unlike other bird species that make use of human habitation, swifts are totally dependent on buildings for their nest sites, except for a tiny proportion in natural holes. Swifts, declining at an annual rate of about three per cent, are on the amber list of birds of conservation concern. So it is important to try to conserve their nest sites when buildings are maintained and repaired. Fortunately this is often not difficult or costly to do, and to do without compromising the integrity or the appearance of the building. Swifts are special. The only time they land is at the nest, with the whole of the rest of their lives spent on the wing.They migrate to southernAfrica after breeding and come back to exactly the same spot each year. Once swifts are established in a building they will breed in the same nest site every year thereafter. It is important to distinguish swifts from superficially similar species, especially house martins. House martins are the birds that make cup-shaped nests out of mud under eaves, whereas swifts typically nest in crevices in stonework, under roof tiles or on the wall plate within the roof. Swifts could also be confused with swallows, but swallows typically build their nests on ledges in barns and other outbuildings. Swifts are easy to live with.They are summer migrants that are with us for a relatively short time, normally between early May and mid-August, and are single brooded.They are not messy birds.They use very little nesting material, and they leave few droppings and minimal splashing.They are beneficial, in that they are voracious insect eaters, feeding their young throughout the hours of daylight on food boles of up to 500 insects at a time. They are not noisy at the nest beyond a soft ‘duetting’ of the adults and weak begging calls from the young (usually two or three in number). Outside the nest, they are more vocal: most people are familiar with the ‘screaming’ parties of swifts that whirl around rooftops and church spires on a summer’s evening. Swifts require only a modest nest cavity.The dimensions of a typical nestbox, for example, are: floor area between 350 and 700 sq cm; minimum dimension 12cm (10cm in extremis) and minimum height 10cm (7.5cm in extremis).These dimensions are sufficient to allow adult birds to turn around and to permit the young birds to exercise their long wings before fledging. The reasons why historic buildings are often so attractive to swifts are not hard to find. Old buildings often used lime mortar, which is soft and therefore deteriorates over time. Stones fall out and timbers How to seal off a nest cavity from the roof void A louvre nest cabinet in a church belfry rot, giving the birds the chance to get in through the gaps. A favourite site is in holes made or left under corbelled brickwork. The architectural features of many old buildings favour the birds, and provide easy solutions to keeping their nesting accommodationwhen renovation or maintenance work is undertaken. Perhaps the most famous swift colony is in the muchstudied tower of the Natural History Museum in Oxford. Originally 80 nestboxes were built behind the ventilation flutes on all four sides of the tower. In some buildings the style of the eaves is helpful: rafters often protrude outside the building, with open soffits and, usually, no soffit boards, or the soffit is often on top of the rafter.When the mortar crumbles, the birds gain access to a nesting ledge.To maintain the integrity of the roof void, it is a simple matter to seal the cavity
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