BOZEMAN, Mont. — FALL is the season of apples, frost, turning leaves and roadkill. A 2008 congressional study found that one in 20 reported motor vehicle collisions is animal-related, and the numbers peak in autumn. Annually, these incidents result in about 26,000 injuries and 200 human deaths.

Across the country, collisions with deer — the most common type of animal-related incident — cost more than $8.3 billion per year, including vehicle repair, medical services, towing, law enforcement time and carcass disposal. The damages increase when larger animals like moose or elk are hit.

Spring and autumn, when animals are migrating, searching for mates or evading hunters, are the riskiest periods. Many animals, including deer, are active at dusk and dawn, when twilight reduces their visibility to predators — and to drivers, resulting in more collisions. The transitions to and from daylight saving time are especially hazardous because the timing of our commutes shifts overnight before animals can adjust to avoid the rush hour. This also contributes to the seasonal spike in animals killed.

It’s hard to quantify the full impact of vehicle collisions on wildlife populations because most reported incidents involve larger animals like deer (and even those are underreported). The toll on smaller creatures like squirrels, salamanders and birds goes largely uncounted, but a recent study estimated that as many as 340 million birds are killed by vehicles annually. For 21 species listed by federal authorities as threatened or endangered — including the Canada lynx, the red wolf, the Florida panther, the crested caracara and Florida scrub-jay — road death is a major threat to survival.