Road Noise Affects Breeding

Bird On! News
19th April 1996

Three papers published in 1995 confirmed the gut feeling of many people 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 it has now been proved will reduce the birds' breeding density.

One of the papers (by Dutch ornithologists Rien Reijne, Ruud Foppen, Cajo ter Braak and Johan Thissen) concerned a study of a range of species. They took paired sites close to and distant from busy roads and analyzed the densities of 43 different species of breeding birds in woodland. 26 of the 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. 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.

At the end of 1995 these findings were brought together with the results on grassland birds which were about to be published. These showed that of 12 species present in sufficient numbers to show whether an effect existed or not, eight had their populations depressed by road noise. The Coot was the least affected and Black-tailed Godwit the worst. Lapwing, Snipe and Skylark were among the others where depressed populations were shown - all three species are severely declining in Britain.

These results all clearly show that it is the noise of the traffic that is disturbing the birds by drowning out their song. In years when populations are high, the territories near roads may be occupied but the breeding success of the birds is much reduced. The effected species are able to cope with different levels of noise - about 60 decibels for the Woodcock and Coot and about 40 decibels for Cuckoo and Black-tailed Godwit. With a busy motorway having 75,000 vehicle movements a day, Cuckoos are affected out to 990m and Black-tailed Godwits to 1130m.

These results have important applications in planning. The potential depletion of breeding birds in some nature reserves, whose status is recognised by European legislation, must be taken into account. Alteration of the proposed route or screening to protect these areas would normally be expected. The research has been used to provide a handbook (see below) which can be used by planners or any other interested parties to predict the effects of different routes and road treatments - screening banks, cuttings, reducing the speed of the traffic. Tables show that the effect on grassland birds of 40,000 to 50,000 cars per day on a road would be severe to 335m at 80 kph but out to 730m at 130 kph. For patchy woodland the affected area is much further from the road than in dense woodland - 780m for an area with up to 30% of woodland but only 183m for 90% woodland (traffic travelling at 100 kph and 40,000 - 50,000 per day).

The handbook Predicting the Effects of Motorway Traffic on Breeding Bird Populations is available free (and in English) from the Road and Hydraulic Engineering Division, PO Box 5044, 2600 GA DELFT, The Netherlands. Please quote "P-DWW-92-709" and mention Bird On!.

Chris Mead