Cliff swallows traditionally build their mud nests on vertical cliff faces. In recent decades, many populations have begun nesting under highway bridges, bringing them into close contact with busy roads.
Average wing length in these populations has been studied over several decades.
Having longer wings is an advantage in open country areas where birds must fly long distances to catch insects.
However, near busy highway bridges, having longer wings is a disadvantage. Long-winged swallows require a longer distance to take off and are less maneuverable than short-winged swallows.
Explain these two observations.
Over a 30-year period, scientists measured a significant decrease in the average wing length of cliff swallows living near highway bridges.
Explain how this change in the average wing length of the population occurred. Use ideas about natural selection.