imagination. birds do sound louder. a phenomenon that some experts say comes in part from birds being less stressed by human sounds, causing them to congregate in larger numbers and more easily communicate with each other. >> translator: the birds are more relaxed. they are not trying to get away from cars, from the crowds of people. even the heart of the city itself. >> reporter: and in thailand, researchers say there's been a baby boom as sea turtle nests are at a 20-year high, thanks to the absence of people walking in the sand where these endangered species lay their eggs. but some animals are noticing the void left by their human counterparts. monkeys like these used to tourists feeding them on a daily basis swarm over a little bit of food left behind.