Collisions with vehicles can be a significant source of mortality for butterflies and other insects living near roads. The level of effect can be estimated by counting the local butterfly population and estimating the number of butterflies killed in collisions. To estimate the collisions, it is important to know how long the butterflies remain after the collision. Piotr Skórka studied* the detectability and persistence of dead butterflies placed on a road.
Larger butterflies typically drop to the road or the verge after a collision. Over 95% of the remains are removed in less than 48 hours. Thus, dead butterflies on a road are almost all killed in the previous 2 day period. Dead butterflies are more likely to be detected on the road (97%) than on the verge (75%), especially if the vegetation was tall. Carcasses are removed in less time from roads with more traffic. About 15% of the carcasses were removed by cars and 10% by birds. A substantial percentage of the carcasses were removed at night. Removal could have been by rodents, insects or vertebrate scavengers.
The conclusion is that:
1) A census of dead butterflies on a road will substantially underestimate the number of collisions
2) A more accurate estimate requires a correction based on persistence and detectability.
*Piotr Skórka. The detectability and persistence of road-killed butterflies: An experimental study. Biological Conservation 200 (2016) 36–43