Three papers confirm the gut feeling that many people have that birds and roads do not mix. Not so much because of the sad bundles of feathers on the carriageways but because of the traffic noise which has now been proved to reduce the bird's breeding density. The paper about a range of species is by four Dutch ornithologists (Rien Reijnen, Ruud Foppen, Cajo ter Braak and Johan Thissen) took paired sites close to and distant from busy roads and analyzed the densities of 43 different species of breeding birds in woodland. Of these 26 species (60%) showed evidence of reduced density.
For roads with 10,000 cars per day the reduced density was apparent up to 1.5 kms from the road and for very busy roads (up to 60,000 cars per day) the effect was felt up to 2.8 kms away. The analysis clearly showed that it was the noise and not the sight of the traffic that was affecting the birds. Various different analyses were performed on different data sets with significant results (P<0.05) found for the following species: Buzzard, Pheasant, Woodcock, Cuckoo, Woodpigeon, Great and Lesser Spotted Woodpecker, Tree Pipit, Icterine Warbler, Garden Warbler, Wood Warbler, Goldcrest, Golden Oriole, Magpie, Hawfinch and Chaffinch. These cover a whole range of species, some very rare or unknown in Britain, but also include very common and widespread ones.
The effect could be detected for five of these species over a kilometre from roads with 10,000 cars per day and over two kilometres (almost three with some) from roads with 60,000 cars per day. These results have particular implications for planning as they prove a harmful effect may be present even if an area is not itself directly invaded by a new road. Perhaps the idea of a buffer area round SSSI's, notified for their breeding birds, should now be declared. Three species, scarce or rare in lowland SE England, come out as particularly vulnerable. They are Wood Warbler, Golden Oriole and Hawfinch which are calculated to be diminished by 73%, 85% and 81% by the presence of a road carrying 10,000 cars per day some 250 metres away.
The other two papers were concerned with just one species, the Willow Warbler. This is the most common summer migrant to breed in Britain and the effects were dire. Close to roads Willow Warblers bred less densely, there were fewer experienced males, they were less successful at breeding and the birds that survived to breed positively shunned the road area. All three papers appear in the Journal of Applied Ecology - one of the scientific journals published by the British Ecological Society. Refs are 1994 31, 95-101; 31, 85-94 & 32, 187-202.
A handbook has been produced by the team 'Predicting the effects of Motorway Traffic on Breeding Bird Populations' ISBN 903693707 B. For copies contact Road & Hydraulic Engineers Division, PO BOX 5044, 2600 GA DELFT, The Netherlands. Ph 3115 269 9111 1111. I have published a paper about birds and roads concentrating on RTA in the journal British Wildlife in 1997.
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