Cliff swallows are social birds that build their nests on the sides of cliffs and steep bluffs. Large numbers of cliff swallows build nests under highway bridges. In a three-year study of a population of cliff swallows, it was found that birds with shorter wingspans and bodies survived automobile interactions more frequently than birds with longer wingspans and bodies. As the study proceeded, the proportion of birds with shorter wingspans and bodies in the population increased. Which of the following identifies a selection pressure and describes its effect on the fitness of birds in the population?
