[ Tiffany Peacock. Image: Oliver Hammond ]

Begging for food

A mother bird brings a worm back to her nestlings. Arriving at the nest, she finds herself faced with an array of gaping beaks. She would like to give the worm to the hungriest of her nestlings (or, more precisely, she is selected to allocate food efficiently among her offspring). Therefore, she would benefit from knowing precisely how much food each baby bird needs.

[ Photo: Mr. Greenjeans at flickr.com ]

But will the nestlings be willing to tell her the truth? Each greedy baby may prefer to get the worm for itself. If "asked", each baby might lie and claim to be the hungriest. Here we have an example of parent-offspring conflict: the parent would like to feed the hungriest baby, but each offspring would like to receive the food itself. As a result, nestlings may exagerate their level of hunger, unless some mechanism prevents deception.

Fortunately for the mother, costly signals can provide a way out of this dilemma. Suppose that baby birds must signal their hunger by squawking loudly - the louder a baby squawks , the hungrier the mother infers it to be. And suppose that squawking in this way is not without its risks. Among other things, the begging calls may attract predators to the nest.

Under these conditions, the nestlings may end up honestly revealing their hunger levels. If a nestling's hunger is satiated, the risk of predation will outweigh any potential gain from begging. By contrast, if a nestling is starving, then the predation risk is overshadowed by the need for food. As a result, the hungry babies will beg, the satiated ones will stay silent, and the mother will receive honest information about each offspring's condition. Because the begging signal is costly (in terms of predation), it ends up being honest as well.

Primary literature

Charles Godfray (1991,1995) developed basic mathematical models of a scenario much like the one above; he found that at least in theory, offspring can use costly signals to honestly convey their hunger level to their parents. Maynard Smith (1991) presented a simple and elegant model of signalling among relatives, which he called the Sir Philip Sidney Game. The properties and dynamics of this game have been explored in great detail (by Michael Lachmann and myself, among numerous other authors). A number of researchers have tried to experimentally and observationally test the costly signalling model of nestling begging; Godfray and Rufus Johnstone (2000) provide an excellent review and synthesis of this body of work.