What to Know High above the bustle of cities across the country, the fastest birds on the planet are making nests, hatching young and snagging birds from the sky for meals. Biologists say falcons’ populations have fallen in recent years, particularly among the birds that live and hunt along the shoreline, likely because of bird flu passed from the shorebirds they eat. After rebounding in recent decades due to conservation efforts, the number of once-imperiled peregrine falcons in the U.S. has been dropping again in some places due to the bird flu that has decimated other avian populations in recent years.