The Theory of Honest Signalling

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Introduction: Part 1

Introduction: Part 2

The Basic Problem

The Basic Solution

Honest signalling in biology

Zahavi's handicap principle

Grafen's model

Attracting mates

Begging for food

Deterring predation

Contesting resources

Autumn color

Honest signalling in economics

Conspicuous consumption


The mathematics of honest signalling

Signalling as a game


Other resources

Carl T. Bergstrom

Using Mathematica

Contact Information

Department of Zoology
University of Washington
Box 351800
Seattle, WA 98195-1800

Signalling among relatives:
Begging baby birds

How can animals use costly signals to solicit food from relatives?

Imagine a mother bird bringing a worm back to her nestlings. When she arrives 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, in slightly more formal language, she is selected to allocate food efficiently among her offspring.) Therefore, she would benefit from knowing precisely how much food each baby bird needs.

But will the nestlings be willing to tell her which of them is the hungriest? Each baby bird might greedily prefer to get the worm for itself. If "asked", each baby would be prone to lie and claim to indeed be the hungriest of the nestlings. Thus parent-offspring and sib-sib conflicts of interest can dissuade each baby from honestly reporting its condition, 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. (I've more often found nests in this way than any other, for example.)

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. At equilibrium, therefore, 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.


Godfray (1991,1995) demonstrated that offspring can use costly signals to honestly convey their hunger level to their parents using basic mathematical models of a scenario much like the one above. 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). Godfray and Johnstone (2000) provide an excellent review and synthesis of this body of work.

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Last modified September 4, 2002
Copyright © 2002 Carl T. Bergstrom