Nesting American Avocets

From the U.S. Fish and Wildlife Service, Mountain-Prairie Region:

Fun fact: Nesting American avocets fearlessly protect their eggs and their young, aggressively attacking predators as large as ravens and northern harriers. They also have an interesting defense mechanism for confusing would-be predators, issuing a series of alarm call notes that gradually change pitch. These calls simulate the Doppler effect, making their approach seem faster than it actually is, thereby confusing attackers.

Photo Credit: Barbara Wheeler / #USFWS