The Effects of Car Traffic on Breeding Bird Populations in Woodland. III. Reduction of Density in Relation to the Proximity of Main Roads

It is argued that noise load is probably the most important cause of the reduced densities of birds in deciduous and coniferous woodland, and the importance of noise and visibility of cars as possible factors affecting density is stressed.

Abstract

1. This study investigated the effect of car traffic on the breeding density of birds in deciduous and coniferous woodland, and the importance of noise and visibility of cars as possible factors affecting density. 2. Of the 43 species analysed in both woodland types, 26 species (60%) showed evidence of reduced density adjacent to roads (based on analysis with Wilcoxon signed-ranks test and regression). 3. Regression models with noise load as the only independent variable gave the best overall results. Calculated «effect distances» (the distance from the road up to where a reduced density was present) based on these regressions varied between species from 40-1500 m for a road with 10 000 cars per day to 70-2800 m for a road with 60 000 cars per day (120 km h −1 and 70% amount of woodland along the road). For a zone of 250 m from the road the reduction of the density varied from 20 to 98%. 4. When visibility of cars was controlled for, the number of species showing density reductions was much higher on plots with a high noise load than on ones with a low noise load. When noise conditions were held constant, however, there was no difference in bird densities between plots with high and low visibility of cars. 5. It is argued that noise load is probably the most important cause of the reduced densities. Visibility of cars, direct mortality and pollution are considered unimportant. 6. The results of this study stress the importance of considering the effect of car traffic on the breeding density of birds in planning and constructing main roads