between different goals may involve involve complex
computations and comparisons.|
Abstract. Mallards distribute themselves between two patches of food in a close approximation to the distribution predicted by the ideal free model. However an important assumption of this model is violated since the despotic behaviour of some individuals results in different birds receiving unequal payoffs.... Evidence is presented to suggest that the ducks initially use the frequency of supply of food items at a patch to assess its profitability, but they can, over a longer time scale, use other cues.
Introduction. The 'ideal free' model assumes that competing individuals distribute themselves between resource patches in such a way that each individual receives the same payoff. This can be achieved by individuals distributing themselves between patches in the ratio of the patch profitabilities. ........ In contrast, the 'despotic' model assumes that some individuals are able to monopolize an unfair share of the available resources through dominance and/or territorial behaviour.
Methods. Expts on a flock of 33 wild ducks on a lake in Cambridge, in winter of 79-80. Only 24 of these could be individually identified rapidly. Resource patches were two sites, A & B, 20 m apart, at which two observes threw in pre-cut and pre-weighed pieces of white bread, at varying rates. At most 3 trials per day were run: Ss were never satiated. Best site was randomly varied between A & B.
Expt 1. 53 trials with 2 gm pieces, either thrown in every 5 sec (12 per minute) at both sites, or 12 per min at one site and 6 per min at the other. Very good immediate matching of no of ducks, either 1:1 (16 each) or 1:2 (11:22). But the Ss did not in fact eat equal amounts: 6 Ss took a large proportion of the available food and did so consistently.
Expt 2. Using a single site it was confirmed that these 6 Ss ate 60% of available food, and this measure correlated with a measure of dominance taken from aggressive interactions outside the feeding context.
Expt 3. Like expt 1, but studied changes over time, and found that a group of 4 dominant birds tended to stay together, and that the other birds tended to avoid this group.
Expt 4. Varied size of lumps thrown in at same frequency (2g vs 4g). When same size and frequency the flock formed two equal groups within 80 sec. When sizes were different, it took 380 sec for the distribution of ducks to change to the 2:1 ratio.
Gallistel's (1990) discussion: the choices of individual ducks must involve representations of the rate at which food is being thrown, and also the size or desirability of the thrown items. He believes an internal respresentational variable "computes net quantity per unit time" by processes analogous to multiplication and division. Note that for these expts the internal representations must be at least partly observational and social, since individual ducks (especially the non-dominant ones) do not have time to compute on the basis of amount of food eaten, and choices between which sub-flock to join must be influenced by the numbers already present at the two sites.(In lab expts on single Ss relative frequency of reward influences choices between two responses more strongly than relative quality or quantity of reward.)
Harper (1982) concluded "It is clear that the behaviour of ducks being fed bread on a park pond is more complicated than might be thought."