Influence of traffic mortality on forest bird abundance

Lower abundance of forest birds near high traffic roads is usually attributed to traffic noise, but the potential role of traffic mortality has not been adequately tested. We tested for the effect of traffic mortality independent of traffic noise, by sampling forest birds at sites with similar traffic volume (and noise levels), that varied in the likelihood of traffic collisions. Collision rates should be higher at forest sites next to roads where there is forest directly across the road, since forest birds are more likely to attempt to cross a small forest gap than a large one. We predicted that if traffic collisions play a significant role in the road effect on birds then in sites where there is a higher risk of traffic collision (small gap sites), there should be a stronger decline through the season in the number of forest birds close to roads, than in sites where there is a lower risk of collision (large gap sites). We compared relative abundance of forest birds, at four distances from high traffic roads, at 10 sites where the birds were more likely to cross the road (small-gap sites, with forest on the other side) versus at 10 sites where they were less likely to cross the road (large-gap sites with open field on the other side). Our prediction was supported; the slope of the relationship between abundance and distance from the road (the negative road effect) became stronger as the season progressed at the small-gap sites but not at the large-gap sites. Our results support the notion that traffic mortality is an important component of the negative road effect, and that mitigation of road effects on birds should include mitigation for traffic mortality.