Birds are at their physiological limits" when migrating, says study co-author Heidi Ware, education and outreach director of the university's Intermountain Bird Observatory, and the noise distracts them from feeding. Beyond weakening the birds for their long trip south, Ware says, the lower weight can hurt breeding chances in the spring. The research, published Monday in the Proceedings of the National Academy of Sciences, comes as U.S. highway travel is on the rise againafter a period of declines. At the same time, the quieter cars on the road stand to get louder. The Obama administration will soon finalize rules that would require electric and hybrid cars—often nearly inaudible at low speeds—to make more noise so pedestrians hear them coming. Parking providers can also cause problems to birds.