Choice between fixed and variable delays with different reward amounts.

Choice between a variable and a fixed food source was studied in pigeons. The variable source yielded different durations of access to food (changed across experimental conditions) uniquely associated with equiprobable delays of either 20 s or 60 s, timed from the choice point. A comparison, elapsing alternative (time-left procedure, J. Gibbon & R.M. Church, 1981), was associated with fixed access to feeder after a 60-s delay, timed from the beginning of the trial. Preference was best characterized as reflecting an average of the 2 local rates of reinforcement in the variable alternative (the average of the ratio of each standard amount and its associated delay). The averaging rule characterized initial choice link data well, but terminal link responding showed that Ss also remembered the 2 delay-amount combinations separately.