The damage that bird fouling causes to historic buildings can be extensive. Apart from the obvious unsightliness, the main problem is acids released from their excrement. These can cause irreversible damage to building surfaces resulting in the scarring of building fabric, damaging appearance and, potentially, causing thousands of pounds worth of damage. Studies have shown that the corrosive effects can continue for a long time after the stone has been contaminated, even if the fouling is removed.
Pigeons are also known to pose a significant health risk to the public.
There have been 176 documented transmissions of illness from feral pigeons to humans (Weber, 1979). Pigeons are known to harbour 60 different diseases, though only seven of these diseases can be transmitted to humans. Research has shown that aerosol transmission accounted for 99.4 per cent of incidences of disease transmission between pigeons and humans. The most commonly transmitted pathogens are Chlamydophila psittati and Cryptococcus neoformans.
Pigeon droppings are known to transmit histoplasmosis, a disease which primarily affects the lungs. Histoplasmosis is caused by Histoplasma capsulatum, a fungus which grows in soil and material contaminated with bat or bird droppings.
About 99 per cent of the bird-control problems on buildings encountered by the authors of this article arise from the activities of just two species: feral pigeons (Columbia livia) and herring gulls (Larus argentatus). These birds fall into the general category of ‘pest bird’ species, all of which are listed on schedule 2 part 2 of the Wildlife & Countryside Act 1981, and they are therefore exempt from the protection offered by the Act.
Buildings constructed of limestone or a calciferous sandstone are most vulnerable to the effects of acids released from bird excrement. Its acidic nature is largely the product of the organisms that live on and in the excrement.
Work done by Bassi and Chiantante (1976) indicated that the fungi which live on pigeon excrement are the actual cause of stonework corrosion rather than the excrement itself. The mycelium (similar to roots) of the fungi enter the stone, transporting the naturally-produced acids, which are strong enough to dissolve stone (especially calciferous stone) to form soluble salts. This process increases the porosity of the stone’s structure, allowing water to penetrate more readily. During winter, if the water in the stone interstices freezes, the expansion of ice crystals can weaken the stone and cause spalling. In addition, the soluble salts themselves cause secondary problems as they are dissolved and absorbed by the masonry, re-crystallising at the point of evaporation. This can appear visually as efflorescence, a bloom of salts on the surface of the stone. Where crystallisation occurs just below the surface, the growth of the crystals exerts pressure on the pores of the masonry causing the fabric to crumble.
The way to reduce the threat of building damage is to identify the problem bird and the main problem areas, treat and clean any fouling that may be in place, and then install deterrents that will prevent the pest bird from fouling the same place again.
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