Many species care for their young to considerable effort. Conflicts commonly arise in a family over resources.
Each offspring demands food at the expense of its siblings. Parents decide how to allocate limited food by determining which offspring are in greatest need.
There are numerous begging signals in various species. Therein lies an opportunity for deception: a youngling may signal a greater hunger than it really has. Nature sometimes does not permit such deceit.
Young canaries have a conspicuous and honest begging signal: a mouth gape with a red lining; the red lining from a suffusion of blood near the surface. A parent puts food in the reddest mouth it can find among its young.
When a chick has eaten, its mouth lining pales, as blood is transferred to aid digestion. So, redness of mouth is an honest signal of hunger. Canary parents that allocate food on that signal have the best chance of rearing the most chicks.