Bank swallows are birds that make their nests on the edges of rivers. Bank swallows can’t swim or breathe underwater. They feed on flying insects and, in the spring, lay four or five eggs. Which of the following observations would provide evidence that the population of bank swallows was affected by a change in the environment?

A The largest swallows catch and eat more insects than the smallest ones.
B Swallows that lay four eggs one year may lay five eggs the following year.
C Fewer swallows build nests along the river when the water level rises.
D Swallows share a food source with other animals that also eat insects.