Their life revolves around signals

Herring gull nestlings solicit food as soon as they are born. For several hours they stare at the world with their yellow eyes, apparently heedless of anything around them in their search for a red spot, says Nutan Shukla

RELEASERS are signals that elicit instinctive reactions in animals. There are a great number of releasers. A classic subject of laboratory studies is the stickleback fish. When the breeding season begins, the male puts on his colourful attire. The most important thing about his nuptial colours is his bright red belly.

The fish also reveals a new behaviour: he selects an area at the bottom of his habitat — his personal territory — from which to attack any male conspecific passerby (occasionally that of another species). Scientists looked into what acts as a releaser evoking aggression in this case: the shape of the intruder or, perhaps, the red patch.

Experiments revealed that the male is excited by any oblong object that is coloured red on the bottom. This object may not even be a detailed copy of a fish as long as the bottom is red. During the breeding season this red colour in the stickleback functions as a releaser.

In June brown coloured nymphalid butterflies emerge with two eye-shaped patterns on the front wings. They flutter among the flowers to sip the nectar, but when a male becomes satiated, he settles down on some hillock and waits. He is waiting for a female to court her. He may wait for a long time until at last he loses his patience and, blind in his heat, he sets off in pursuit of birds flying by and even falling leaves. He may even dash after his own shadow!

At this moment it is easy to attract the butterfly with a paper model of a female. In an experiment it was found that the most desirable models were darker and four times as large as a live female. In this case the dark colour and size are releasers for a nymphalid male.

Herring gull nestlings solicit food as soon as they are born. For several hours they stare at the world with their yellow eyes, apparently heedless of anything around them in their search for a red spot. For them this spot is now the focus of the entire universe. It plays an important role in the signal code of the herring gull: it is a nestling releaser.

An adult herring gull has a yellow beak, but at the tip of the lower beak one can see a bright red spot that looks like a ripe berry. To a newly hatched nestling, this "berry" represents the outside world: it is an intermediary between the nest and the environment. The nestlings’ response to the red spot is instinctual; the spot is saying: "As soon as you are out of the shell, look for the red spot! It will bring you food and water, it will warm and protect you. Look for it, run after it."

The nestling keeps searching. It pokes its little beak into its parent’s beak with the red spot at the tip. This behaviour is also a releaser for the parent. A normal bird cannot disobey this order — it immediately opens its mouth to feed its young.

During the first days of their lives, featherless and blind songbird nestlings cannot identify their parents by any visible trait. Hence, for their yet undeveloped brain, a slight shake of the nest is a signal to gape for food. It means that the mother has flown in! One can see this for oneself by pushing the nest slightly. The blind nestlings will raise their heads, and as if on order, open their yellow bills. (The yellow bill of the nestlings is also a releaser: it urges the parents to bring food to their young.)

Relatives of the sparrows, waxbills are found in Africa, South Asia and Australia. The nestlings of these birds do not have a plain yellow bill: on the contrary, it is coloured as brightly as the body of these birds. There are yellow, white or light-blue ridges, occasionally edged with a black ring in the corners of the mouth, and black dots and stripes are found on the palate, tongue and corners of the beak.

When this colourful mouth opens, the parents are eager to put more and more food in it. It is conspicuous in the semi-darkness of the nest, and the multi-coloured ridges, at least in some waxbill species, gleam in the darkness, reflecting light! The nest of waxbills is a well-made, closed basket with a narrow entrance. It is partially dark inside, even on a bright day.