They're the jobless futon crashers of the animal kingdom.
Scientists concluded that the fairy wrens evolved this system of password voice recognition to tell their real offspring from cuckoo interlopers, even if the latter are massive, gray monstrosities that look absolutely nothing like wrens. Yeah, for all their complex nest security, birds are kind of stupid like that.
Apparently "chirp" is bird for "Duh!"
Naturally, the cuckoo chick will try to copy the wren's "feedin' time!" call, but in most cases it fails to hack into the avian motherboard and access the sweet database of regurgitated bird food. No wonder, though, considering that the complex two-second trill password consists of 19 separate melodic elements and differs from nest to nest ... which means that if you're the type of person who uses "password1" on all of your online accounts, you're literally worse at security than a bird that needs to password protect its own children.