The effects of car traffic on breeding bird populations in woodland. IV. Influence of population size on the reduction of density close to a highway

1. This study tested the assumption that in years with a low overall population size the density-depressing effect of roads on breeding birds will be more important than in years with a high overall population size. 2. The effect on the density was investigated in 1984, 1986 and 1988. Of the 23 species that could be analysed each year, 17 species had a lowered density close to the road in at least 1 year. In 1984 and 1986 the number of affected species was much higher (10 and 14, respectively) than in 1988 (4). 3. The number of affected species, as well as the effect size for all species combined and for most individual species, was negatively correlated with the overall population size. 4. It is concluded that in years with a high-overall population size, the use of density as a response variable will cause a considerable underestimation of the reduction of the habitat quality close to roads. This has not been recognized well in impact studies concerning road traffic, nor in other impact studies.