Peck on red
A dead herring gull was washed up, splayed and sodden on the shingle. Even so, the red spot on its beak still glowed like neon. It's a signal to the chicks: peck here to release dinner - the parent disgorges the food straight down the chick's gullet.
The stimulus is so powerful that Nico Tinbergen the pioneering animal behaviourist of my recent gull's egg blip) was able to demonstrate that gull chicks would not only peck at a red spot on a fake beak or even on stick waved over the nest, but they would became even more excited by three red lines on a stick, or indeed three white lines on a red knitting needle.This is known as a supernormal stimulus, in which an exaggerated version of the normal stimulus can evoke an even stronger reaction. The same process has been suggested as the reason why certain stimuli when manipulated by industry can be so compelling for us humans and the reason why some get hooked on junk food, video games or pornography. The stimulus for a reward is self-reinforcing.
More about that here.