Influence of traffic and weather on carcass persistence time of small wildlife on roads

The rapidly expanding road network threatens the persistence of many terrestrial species through habitat loss, fragmentation, chemical, light and noise pollution and mortality associated with wildlife-vehicle collisions. Roadkill monitoring under-estimate actual collision numbers as the time during which roadkill carcasses remain visible on the road is often shorter than the frequency of road monitoring. By placing passerines (< 20 g) and amphibian carcasses on stretches of roads we surveyed every 2 hours, we fine-tuned existing persistence estimates for these species. We found median disappearance times (time for half of the carcasses to disappear) of less than 30 minutes for birds and 1-18 hours for amphibians depending on the volume of traffic, which is much shorter than previous estimates. Rainfall did not impact carcass persistence. We show the implications of these results by (1) correcting the results of roadkill surveys performed on common toads in reproductive migration for the number of removed carcasses and (2) estimating the number of passerine-vehicle collisions in the Auvergne-Rhone-Alpes (AuRA) region of France using the citizen-science database Faune-AuRA. We estimate that a road survey conducted 3 hours after amphibian road crossing under-estimates the number of roadkill by half, and that about 6800 passerine collisions were missed in 2022 by contributors because of short carcass persistence on the road. Small-bodied carcasses are hard to detect for drivers, and for a collision-report rate of 1%, total collision numbers for passerines could be as high as 700 000 individuals from 21 reported passerine species in 2022 in AuRA.