"The oldest layer is as old as me — all my life, a bird was nesting here," said Auke-Florian Hiemstra, a biologist at the Naturalis ... down naturally if they end up in the environment.

With the help of amateur bird-watchers like Hurtado, biologists in Colorado and ... into the back of the cavern, climbing up a rockfall for a better look. His headlamp catches a couple of ruffled ...