Damage to Historic Buildings
Abouzeid, David Channon and Phil Sever
perching on Robert Adam's Pultney Bridge, Bath (Photo: Jonathan
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.
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.
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.
a number of high-profile locations that have been affected by
pigeon fouling. In London, damage caused by pigeon droppings to
monuments in Trafalgar Square is well documented and there have
been a number of reports and scientific papers on the problem.
EC Harris (1996) suggested that there is a definite health risk
in there being such a large population of pigeons concentrated
in this location, and the damage to paving and statuary is considerable
both in terms of the problem it causes and in the cost of its
removal. The Historic Buildings and Monuments Commission in 1986
calculated that the annual cost for cleaning statuary was £14,000
and the annual cost for cleaning paving was £91,000. Harris found
that proofing of buildings by netting or spikes was the best way
to protect buildings from bird damage.
netting over dormer windows
However, proofing carried
out in isolation has not solved the pigeon problem in Trafalgar
Square; it has simply moved it on to other buildings. Pigeons
in this area exhibit what is known as an 'ideal free distribution'
(Fretwell and Lucas 1970). This is an ecological term that describes
the way in which animals distribute themselves between several
patches of resources. The theory
states that individual animals will aggregate in various patches
proportionately to the amount of resources available in each patch.
Pigeons will travel around Trafalgar Square and the surrounding
area searching for resources: particularly feeding, nesting and
roosting sites. Harris believed that it is likely that resources
such as nesting or perching sites available to feral pigeons elsewhere
in London are broadly similar to those in Trafalgar Square, and
that the only major difference is the availability of a super-abundant
and reliable source of food. Therefore, food availability is the
main limiting factor of feral pigeon populations in London. Culling
and providing nests so that eggs can be collected and destroyed
have been proposed for controlling pigeon populations but these
are controversial and more importantly not effective or appropriate
in every location.
Since it is
clear that feral pigeon populations in London are almost entirely
controlled by food availability, if pigeons in Trafalgar Square
were culled or removed, the niche left vacant by the culled pigeons
would be immediately filled by other pigeons from the surrounding
areas. These forms of control would not achieve the desired results.
However, a combination of removing superfluous food and proofing
the buildings would both reduce the population to an acceptable
level and reduce damage to the buildings.
In some unique cases
culling may work when there is a small population to be removed,
but in terms of the protection of buildings, culling or nest removal
is not an entirely effective measure.
devices: top, sprung wires on vertical posts fixed to a cornice
(top) and spikes glued to a parapet and its guard rail.
fall into a number of groups: anti-perching devices and anti-entry
devices. Anti-perching devices are the most commonly used method
for protecting building surfaces from damage caused by the acidic
components of bird fouling. These devices work by preventing birds from landing and roosting, and therefore from
fouling certain areas. There are three main forms; anti-perching
wire (sprung wire), antiperching spikes and anti-perching gel.
wire (sprung wire) is a commonly used device which is proven to
be most effective in many situations. The wire consists of nylon-coated
stainless steel with a diameter that is too small for either pigeons
or gulls to grip. The wires are attached by tension springs to
either horizontal or vertical posts. The springs cause the wire
to 'bounce' when birds try to land on them, therefore disorienting
the bird and subsequently putting them off landing in that particular
site. The wires are relatively low in visibility and are therefore
more suitable for protecting historic buildings given their ability
to conform to the contours of a building. As well as being very
discreet, all the components of the system are rustproof and therefore
do not create rust marks on the building surface, and the plugs
into which the posts are fitted ensure that rainwater does not
penetrate and later damage the masonry.
spikes are designed to prevent birds from landing in certain areas.
Spikes are most commonly used on surfaces such as the leading
edges of sills and ledges. Birds tend to sit on the leading edges
of these surfaces in order to look for food and at the same time
foul the face of the building. However, if they are unable to
do so they will only perch in this area for a number of seconds,
realise they cannot get a comfortable view for seeking out food
and therefore fly away. Spikes are glued to the surface using
a silicone-based adhesive. The glue does not cause damage to the
building surface, which it does not penetrate, and it can be completely
removed at a later date if required, leaving the surface completely
gel consists of a polybutylene gel sealed with a skinning solution.
The gel never sets hard. When a bird lands on the gel its feet break
through the skin and onto the sticky polybutylene gel underneath.
The pigeon or gull then flies off with some of the gel attached
to its feet. As the birds do not have the means to remove it, the
gel will remain stuck to the bird's feet for a while and create
an uncomfortable feeling. The unpleasant experience deters pigeons
from subsequently landing on any surface that looks like gel.
As the gel is very difficult to see once installed, it has little impact on the aesthetics of a building. However, this
particular form of proofing is unsuitable for use on historic
buildings because the gel tends to bleed into the stonework over
a period of time, trapping moisture, possibly resulting in spalling.
It also has a very short lifespan because dust and debris soon get stuck
to it and absorbed into the surface. In order to maintain its
effectiveness it should be removed and re-applied annually.
netting is best suited for keeping pest birds out of central courtyard
areas, and is also deployed as a screening device for features
such as balconies, windows, pipework and airconditioning units.
The net's lifespan may be expected to be ten years or more, depending
on the degree of exposure, as it is treated with a UV-resistant
coating. In a sheltered location, some nets have lasted as long
as 20 years. Mesh sizes vary with 19mm-mesh nets for sparrows,
28mm-mesh nets for starlings, 50mm-mesh nets for pigeons and 75mm-mesh
nets for gulls.
Once the netting has been installed it is relatively
inconspicuous and does not noticeably reduce the light reaching any windows behind it. It must be maintained regularly
to keep it clear of windblown leaves and rubbish, which can create
a real eyesore. An annual inspection is usually sufficient. The
netting excludes the birds from the premises' voids and they have
to look elsewhere for nesting and roosting opportunities.
gulls do not damage heritage buildings in the same way as pigeons.
They are not vectors of disease, although they have been associated
with Salmonella transmission. Generally in the urban environment
they do not live in large, closely grouped flocks, although they
do flock together to feed on playing fields. Gulls make a considerable
amount of noise, which can be a nuisance. Gulls leave some fouling
on the buildings where they nest and in the surrounding areas.
This fouling can stain and damage masonry. Because they
are territorial when nesting, gulls tend not to congregate close
together in such large numbers or with such regularity to either
nest or roost. Gull droppings don't accumulate in quite the same
way as with feral pigeons. Nesting gulls can be particularly aggressive
and will drive off anything, including people, they perceive to
be a risk to their nest. Naturally, herring gulls nest in colonies
on the coast where they compete aggressively for space. In the
urban environment they generally nest in single pairs, spread
over a wider area. Nest defence is carried out by males (McVey
et al, 1993). Gulls form lifelong pair bonds and tend to return
to the same nest year after year, and site fidelity increases
with age (Elrich et al, 1988). This means that problems left unresolved
become worse each year. Occasionally in urban areas, colonies
of gulls have become a major nuisance where expanding nesting
groups have spread over several rooftops, causing fouling, and
the additional problems of noise and mobbing people.
issue for residents is the gulls' habit of calling to affirm territory
and the location of other gulls. This can take place from the
early hours to late at night disturbing anyone in the vicinity.
Pigeon and gull control may be undertaken for many reasons, including:
prevention of the spread of disease; preserving public health;
safety; or, most frequently, for the protection of buildings,
building surfaces and the prevention of nuisance. When dealing
with historic buildings, the installation of proofing devices is
better left to professional companies who can provide a high-quality
product that both protects the building from bird damage and does
not damage the building during the installation process or
subsequent operation. While there are products, such as spikes,
that can be bought and applied on a DIY basis, these are not
normally approved for placement on the facades of listed buildings.
- D Channon, ‘Feral Pigeon Excrement On Heritage Stonework’, International Pest Control, 46:1, January/February 2004
- M Bassi and D Chiantante, ‘The role of pigeon excrement in stone biodeterioration’, International Biodeterioration Bulletin, 12:3, 1976
- P Ehrlich et al, The Birder’s Handbook: A Field Guide to the Natural History of Northern American Birds, Simon & Schuster, New York and London, 1988
- S D Fretwell and H L Lucas, ‘On territorial behavior and other factors influencing habitat distribution in birds’, Acta Biotheoretica, 19, 1970
- M McVey et al, Wildlife Exposure Factors Handbook Volume 1, US Environmental Protection Agency (EPA/COO/ R-931187a), 1993
- WJ Weber, Health Hazards from Pigeons, Starlings and English Sparrows, Thomson, Fresno, California, 1979
article is reproduced from The Building Conservation Directory, 2007
is the Bird Team Manager at Microbee Bird Control Ltd, which
provides various protection measures against all birds that
are considered to be pest species.
CHANNON is a director of the company and a zoologist
with a 20-year research background into pest-related problems.
SEVER is a zoologist working on various aspects of pest
research with the company.
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