Summary1. The effects of roads on wildlife populations are widespread and well documented. Many studies have shown that bird abundance, occurrence and species richness are reduced near roads, with the largest reductions where traffic levels are high. Negative correlations have been reported between bird richness ⁄ abundance and traffic noise but the possible causes of road effects are inter-correlated. It is important to disentangle the different effects so that appropriate mitigation measures can be implemented. 2. We tested the hypothesis that traffic noise is a key negative effect by testing three predictions: (i) bird richness ⁄ abundance should reach a maximum at the same distance from roads that traffic noise reaches a minimum; (ii) the effect of traffic noise on bird richness ⁄ abundance should be stronger than the effect of distance from the road on bird richness ⁄ abundance; and (iii) sites with more traffic noise at a given distance from the road should show lower bird richness ⁄ abundance than sites with less traffic noise at the same distance. 3. We collected breeding bird occurrence and traffic noise data along twenty 600-m transects perpendicular to roads at 10 high-traffic road sites. 4. Traffic noise decreased and bird species richness increased with increasing distance from the roads. However, none of the predictions derived from the traffic noise hypothesis was supported. 5. Synthesis and applications. Our results suggest that traffic noise is not the main cause of the negative relationship between bird species richness ⁄ abundance and proximity to roads. Instead, traffic mortality may be the main mechanism causing this relationship. We suggest that mitigation of road impacts on birds should focus mainly on reducing mortality rather than reducing traffic noise. In particular, engineering road surfaces, tyres and vehicle engines to reduce noise would not mitigate road effects; instead, structures to keep birds away from roads or force them to fly above the traffic would be more effective.

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Strategies to reduce wildlife road mortality have become a significant component of many conservation efforts. However, their success depends on knowledge of the temporal and spatial patterns of mortality. We studied these patterns along the 1000 Islands Parkway in Ontario, Canada, a 37 km road that runs adjacent to the St. Lawrence River and bisects the Algonquin-to-Adirondacks international conservation corridor. Characteristics of all vertebrate road kill were recorded during 209 bicycle surveys conducted from 2008 to 2011. We estimate that over 16,700 vertebrates are killed on the road from April to October each year; most are amphibians, but high numbers of birds, mammals, and reptiles were also found, including six reptiles considered at-risk in Canada. Regression tree analysis was used to assess the importance of seasonality, weather, and traffic on road kill magnitude. All taxa except mammals exhibited distinct temporal peaks corresponding to phases in annual life cycles. Variations in weather and traffic were only important outside these peak times. Getis-Ord analysis was used to identify spatial clusters of mortality. Hot spots were found in all years for all taxa, but locations varied annually. A significant spatial association was found between multiyear hot spots and wetlands. The results underscore the notion that multi-species conservation efforts must account for differences in the seasonality of road mortality among species and that multiple years of data are necessary to identify locations where the greatest conservation good can be achieved. This information can be used to inform mitigation strategies with implications for conservation at regional scales.

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We previously found that males of two anuran species – Hyla versicolor and Rana clamitans – alter their mating calls in response to traffic noise. To test whether these alterations compensate for an effect of traffic noise on mate attraction, we (1) recorded a male calling at a quiet site; (2) played traffic noise at the same male and recorded its altered call; (3) used these recordings to attract females to a trap at sites either with or without broadcast traffic noise. The calls produced without traffic noise attracted fewer females when they were played at sites with traffic noise than when they were played at sites without noise. However, the calls of the same individuals produced with traffic noise attracted as many females at sites with noise as at sites without noise, and they attracted as many females as did the call of the same male made without noise and played at sites without noise (the ‘natural’ situation). Therefore, for these species, traffic noise does not affect mate attraction; males change their calls to compensate for a potential effect of traffic noise on mate attraction. A third species – Bufo americanus – does not alter its call in response to traffic noise, and its call made in the absence or presence of traffic noise was equally able to attract females in the absence or presence of traffic noise, indicating that traffic noise does not negatively affect mate attraction. Therefore, it appears that traffic noise does not negatively affect mate attraction in these three species of anurans. We suggest that, if our results apply to anurans in general, the previously documented negative effects of roads on anuran populations are likely caused mainly by road mortality. If this is true, road mitigation for anurans should focus mainly on reducing this mortality.