Releasers and Heerman's Gull

By Dave Hanks

A releaser is an action or visual stimulus that triggers a response. The feeding of chicks is based on a series of releasers. Parent birds gather food, and hearing the feeding cries of the chicks (who are always hungry) fly to the nest. A bunch of wide open mouths, the insides of which are colored, stimulates the parent to stuff food into the widest gape. This is a survival adaptation that ensures that the largest, healthiest chick survives in case there is a food shortage that particular year. Experiments using a funnel, with painted insides, will always receive the food, if it is bigger than the chick’s mouths.

Color patterns function as releasers. Robins have orange-red breasts. Place a stuffed juvenile, with its spotted breast, in an adult’s territory and you get no response. However, a “stuffed” adult is immediately attacked. Also, by placing a red-orange ball of feathers in the same area will result in the same amount of aggression. Baby flickers also react to color. They will respond and peck at a male flicker’s red mustache.

Heerman’s Gull, which is a true sea gull, is rarely found inland. It is 19 inches long, gray-bodied, white-headed, with black legs, and a reddish beak. The beak has a black tip which its chicks peck at to get the parent to regurgitate food – small fish or marine invertebrates. Much food is the result of scavenging or stealing from other birds – especially from Brown Pelicans. Food is also picked from the ocean’s surface or, after hovering, plunging into the waves. (Heerman's Gull – A true sea gull)