Effects of roads on the abundance of birds in Swedish forest and farmland

We studied the abundance of birds in forest and farmland adjacent to trafficated roads, primarily to test a model to determine the width of the zone along a road in which breeding bird density is reduced because of traffic noise (the "effect zone"; M. Reijnen et al. 1995. Predicting the effects of motorway traffic on breeding bird populations. DLO-Institute for Forestry and Nature Research, The Netherlands). Birds were counted in spring, along transects perpendicular to larger roads in mid-Sweden in 1998-1999. In farmland, a lower abundance of birds within the effect zone was observed, both for some selected focal species (for which a negative effect has been found in previous studies) and for all species combined. Data were however not consistent in this respect. In forest, no general difference in bird abundance with distance to road could be established. We found a tendency for lower abundance proximate to roads in six species, but the opposite tendency in four. This pattern could be explained by the different habitat requirements of the species. Accordingly, the predictions derived from the model were only partly correct, and we conclude that the model may not be directly applicable under the present conditions. We further suggest that habitat changes as a consequence of road construction under some circumstances can override the negative effects of traffic noise on the surroundings, and argue that also the potential positive effects of roads should be considered in road management