Gouldian finches (Erythrura gouldiae) are colorful Australian seed-eating birds that nest in tree holes in the wild or in nest boxes when bred in captivity. Their nests are always dark inside so the nestlings have unusual mouth markings to attract their parents’ attention. Each nestling has:

• Four opalescent tubercles at the corners of the gape that reflect a bluish light.
• Patterns inside the mouth that guide toward the gullet.

Words cannot describe Gouldian finch chicks! Thanks @SaveTheGouldian, gorgeous pic of a wild Gouldian nestling pic.twitter.com/MaFQUEqTkM

— Aus Bird Study Assoc (@ABSAbirds) June 30, 2015

To further attract attention, the nestlings hold up their mouths and rotate their heads as shown in this video of 3-day-old chicks.

The nestlings’ elaborate show may have evolved because …

Gouldians are among the most difficult finches to breed successfully because they are not wonderful parents and have a tendency to abandon both eggs and babies, or even refuse to nest at all. People who raise Gouldians usually keep society finches as well to serve as foster parents for eggs and babies. Societies are marvelous parents and will be happy to foster other species. 

— Gouldian finch description at Lafeber website

The babies that survive are the ones with beckoning beaks.

(photos from Wikimedia Commons and embedded from Twitter; click on the captions to see the originals)

p.s. See more examples of weird baby bird mouths at Audubon Magazine’s What’s Up With the Weird Mouths of These Finch Chicks?