THE EFFECT OF CHANGES IN AVAILABILITY ON FOOD SELECTION AT DIFFERENT LEVELS OF DEPRiVATION. TERRY W. BELKE Mount Allison University

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1 The Psychological Record, 2000, 50, THE EFFECT OF CHANGES IN AVAILABILITY ON FOOD SELECTION AT DIFFERENT LEVELS OF DEPRiVATION TERRY W. BELKE Mount Allison University TAMMI Y. KWAN Dalhousie University In the present study, two experiments investigated the effect of a change in food availability on selection between two food items at different deprivation levels. In the first study, 6 pigeons chose between a nonpreferred food item (Le., checkers) after a fixed delay and a preferred food item (Le., corn) after an adjusting delay. The delay to the preferred food item adjusted depending upon the pigeon's choices. The dependent measure was the amount of delay added to corn to produce indifference at 70% and 950/0 of body weight. Results showed that less delay had to be added when pigeons were more deprived. In the second study, the availability of both corn and checkers was varied by manipulating the amount of each food item presented to the pigeons at 70% and 95% of body weight. The dependent measure was the amount of each item taken. For the nonpreferred food item, consumption depended on an interaction between deprivation and availability. A decrease in availability only affected checker consumption when body weight was at 70%. Together these two studies showed that the effect of a change in food availability on selectivity was greater when pigeons were more deprived. Optimal foraging theory uses the principle of optimization to predict diet choice. According to optimal foraging theory, if all food items in the environment are ranked according to some measure of profit for the feeder, such as net energy per unit feeding time, then the optimal diet model predicts that an animal should always choose the food item that maximizes the net rate of energy intake per unit time, all other things being equal (Krebs, 1978; Pyke, Pulliam, & Charnov, 1977). However this rule of thumb is qualified by considerations of food availability and deprivation. This research was conducted in the Harvard Psychological Laboratory, William James Hall, Cambridge, MA while Terry Belke was in the graduate program in the Department of Psychology, Harvard University and while Tammi Kwan was in the graduate program in the Department of Psychology, University of Alberta, Edmonton, Canada. Correspondence regarding this article can be sent by mail to Terry W. Belke, Department of Psychology, Mount Allison University, Sackville, New Brunswick, Canada, E4L 1 C7. ( to MTA.CA)

2 530 BELKE AND KWAN In the natural environment, both seasonal and nonseasonal factors (e.g., diseases, fire) contribute variance in the availability of different food items. As food in the environment becomes scarce or difficult to obtain, optimal foraging theory predicts that at some point the animal should give. up on the better alternative in favor of a worse alternative as the better alternative becomes more scarce or difficult to obtain. Thus, food selection or diet choice is a function of -the availability of better or preferred food items while the availability of worse food items is irrelevant. As long as both worse and better food items are readily available, the animal should choose the latter, but as the availability of the latter decreases, the probability that the animal will take the worse food item should increase (Walker, 1987). Previous research on the effects of changes in food availability on food selection is consistent with the predictions of optimal foraging theory. Goss-Custard (1977) showed that selection of a less preferred prey item (a worm) by a shorebird (redshank) depended on the absolute rather than the relative density of the preferred prey item (a shrimp). Lacher, Willig, and Mares (1982) studied the food preference of a florivorous rodent (kerodon rupestris) for different species of leaves at different levels of availability. At the higher level of availability, consumption of preferred leaf types increased while consumption of less preferred leaf types decreased. Schoener (1971) reported that decreased selectivity as a function of decreasing food availability had been observed in fish, weaver birds, blackbirds, swifts, and mollusc. Laboratory studies of selection among food items varying in size or amount rather than type have also found that selection varies ~ith availability. For example, Krebs, Erichsen, Webber, and Charnov (1977), using a conveyor belt to present food items of different sizes to birds, varied the probability of encountering large and small food items. Results showed that dietary specialization occurred when the probability of encountering a better food type was high while dietary generalization occurred when the probability of encountering the better food type was low. Thus, the availability of preferred food items controls selectivity or diet choice. I n contrast, the effect of deprivation on selectivity is less clear. Optimal foraging theory predicts no effect of deprivation on diet selection in the absence of changes in food availability. If an animal is selecting among different food items so as to maximize net energy per unit time when relatively nondeprived, then an increase in deprivation level should not affect food choice. Instead, other aspects of consumption should change, such as an increase in the rate of consumption. Studies of the effects of deprivation on selection between qualitatively different food items presented simultaneously to subjects have found no systematic effects in the absence of changes in food availability. For example, Moon and Zeigler (1979) found no systematic effects of body weight deprivation on selection among different seed types by pigeons.. However, studies of the effects of deprivation on selection of food items where food items are presented successively rather than simultaneously

3 DEPRIVATION, AVAILABILITY, AND FOOD SELECTION 531 have found that the probability of accepting a less profitable food item increases with deprivation. For example, Rechten, Avery, and Stevens (1983) found that deprived or hungry tits were less likely to turn down profitable items and were more likely to accept unprofitable items than were birds that were prefed prior to the session. Thus, the appearance of an effect of deprivation on selectivity seems to depend, in part, on the manner of presentation of the food items to be selected from. Research has documented effects of changes in food availability and deprivation on selection of food items; however, these two variables are not necessarily independent Deprivation and food availability may interact. As food availability decreases, deprivation should increase. According to Schoener (19, 71), the role of deprivation is to create an internal state of "hunger" which functions as a monitor of changes in food availability in the environment. As food availability decreases, the animal becomes hungry and this hunger ''tells the animal what intake is needed by what future time to forestall a given degree of weakness or starvation" (p. 383). Thus, deprivation is related to sensitivity to changes in food availability. The nature of this interaction between deprivation and food availability was investigated in the present study. Of specific interest was whether changes in food availability have similar effects at different deprivation levels. Is a deprived animal more sensitive to changes in food availability than a less deprived animal? Does a unit of change in food availability have different effects at different deprivation levels? The present study investigated the effect of changes in food availability on food preference at different deprivation levels using two different procedures. In the first experiment, the availability of the better or more preferred food item was varied by titrating the delay to the preferred food item. In an adjusting delay procedure (Mazur, 1987, 1988; Mazur, Snyderman, & Coe, 1985), the delay to a preferred food item is increased when the preferred food item is chosen and decreased when the nonpreferred food item is chosen. The delay added to the preferred food item to produce indifference provides a measure of the value of the preferred food item relative to the nonpreferred item. Delays required to produce indifference were assessed at two different deprivation levels. The second experiment varied availability by varying the amount of food that was immediately available. In this procedure, equal amounts of the food items used in Experiment 1 were presented in food cups and the amount was varied at two different levels of deprivation. The amounts of each item taken were measured. Although these procedures are not foraging simulations, generalizing from the tenets of optimal foraging theory one might predict that (a) changes in availability through either delay or amount should increase the probability of selecting the worse food item and (b) that the probability that the animal selects the worse food item should increase under conditions of greater deprivation. That is, in Experiment 1, the indifference delay value should be shorter under conditions of more severe deprivation; and in Experiment 2, the likelihood of the worse food item being taken with a change in availability should be greater under more severe deprivation.

4 532 BELKE AND KWAN Experiment 1 Method Subject Selection Pigeons were selected for the study based on demonstration of a stable moderate to strong preference for corn over Purina pigeon checkers. Prior to the selection procedure, the pigeons were on ad-lib feeding conditions. Pigeons were fed 30 g of a mixture of assorted seeds and Purina pigeon checkers once a day. During the selection phase, 10 pigeons were presented with 30 g of corn and 30 g of checkers in a two-compartment feeding tray attached to the home cages. Each pigeon was given access to the corn and checkers for 2 min. After 2 min, the tray was removed and the remaining amount of each food type was weighed. To eliminate the possibility of side preferences, the side on which each food item was presented was reversed across days. In order for a pigeon to be selected into the present study, the pigeon nad to consume more corn than checkers each day over a period of 10 consecutive days. Table 1 shows the total amounts of each food item taken by each selected pigeon over the 10 days. Table 1 Total Amounts (g) Consumed by Pigeons Over 1 O-Day Testing Period Pigeon Corn Checkers P P P P P P Based on this procedure, 6 pigeons were selected as subjects. All pigeons had prior experience with operant conditioning procedures. The pigeons had free access to water and grit in their home cages, but were food deprived to, and maintained at, body weights of 70% and 95% of their ad-lib weights. Pigeons were not run on days when their body weights deviated more than 10 g from their target weight. Supplemental food, which contained various seeds and grains, including corn and checkers, was given when needed following an experimental session. Miller (1976) reported that the composition of a postsession food supplement produced no differential effects on preference. Apparatus The experimental chamber was 32 cm long, 29 cm wide, and 30 cm high. Three response keys were mounted in the front wall of the chamber. Each key was 20.5 cm above the floor. Response keys were 2 cm in diameter and ~eparated by 7 cm, center to center. The force required for

5 DEPRIVATION, AVAILABILITY, AND FOOD SELECTION 533 microswitch closure was approximately 1.5 N. A food hopper was located below each response key. Each hopper was illuminated by two 6-W white lights when in operation. The chamber was illuminated by two 6-W white lights located in a box mounted on the top of the chamber. The chamber was located in a sound-attenuating box with a fan for ventilation and to mask extraneous noise. A PDP-8A computer in another room was used to control experimental events and record responses. Procedure Informal observations of pigeons in the colony revealed that Purina pigeon checkers tend to accumulate in the food cups of birds that are at or near ad-lib weight. During feeding, the seeds and grains were selected out and the pigeon checkers remained in the food cup. New birds introduced to the loft also showed this behavior. In contrast, experimental birds at less than ad-lib body weights do not leave pigeon checkers in their food cups, but do tend to pick out seeds and grains first. These casual observations suggest that pigeons prefer other food items over pigeon checkers. Unpopped popcorn, readily available at a local supermarket, was chosen as an alternative to pigeon checkers. The subject selection procedure revealed that corn was preferred over checkers. Information on the constituents and nutritional value of corn and checkers is provided in Appendix A. Pigeons were divided into two groups. Each group received a different order of deprivation conditions. For pigeons in Group 1, the order of body weights was 70%, 95%, and 70%. For pigeons in Group 2, the order was 95%, 70%, and 95%. The experiment employed an adjusting-delay procedure to measure changes in the relative value of the two food reinforcers as a function of deprivation. In this procedure, each session consisted of 64 trials. The trials were grouped into blocks of four trials. Each block consisted of two forced-choice trials followed by two free-choice trials. Each trial began with the center key illuminated red. The purpose of the center key was to ensure that the pigeon's head was approximately equidistant from both side keys on the free-choice trials. On free-choice trials, a response on the center key illuminated the two side keys and extinguished illumination of the center key. For Group 1 birds, the left key was illuminated blue and the right key was illuminated white. Key illumination colors were reversed for birds in Group 2. Despite differences in illumination color, the left hopper always produced pigeon checkers and the right hopper always presented corn. An intertrial interval (ITI) of 30 s was programmed between trials. Continuing with free-choice trials, a response on the keylight associated with checkers initiated a standard delay of 5 s. During the passage of this delay, the keylight flashed on and off twice a second. After 5 s elapsed, the keylight stopped flashing and the checker hopper was activated for a 3-s duration. If the pigeon pecked the keylight associated with corn, the adjusting delay was initiated. Again, the keylight flashed

6 534 BELKE AND KWAN and following the elapse of the adjusting delay, the corn hopper was activated for 3 s. The next trial began after the ITI elapsed. On forced-choice trials, a single peck on the center key illuminated either the keylight associated with checkers or the keylight associated with corn. A response on whichever key was illuminated initiated the associated delay and provided the appropriate reinforcement following passage of the delay. Responses on the nonilluminated keys had no effect. Following the termination of the hopper presentation, the ITI began. Of two forced-choice trials in each block of four trials, one involved the standard delay to checkers and the other involved the adjusting delay to corn. The order of presentation of these two forced-choice trials within each block of four trials was randomly varied. The delay associated with corn was adjusted following completion of the free-choice trials. If corn was chosen on both free-choice trials, then the delay associated with corn was increased by 1 s. If checkers were chosen on both trials, the delay to corn was decreased by 1 s. Finally, if corn was chosen on one trial and checkers on the other, then the delay remained unchanged. The delay remained in effect for the subsequent block of trials. At the start of the first session, the delay associated with corn was set at 1 s. For subsequent sessions, the adjusting delay always began at the last value from the previous session. The delay on the adjusting alternative could not be decreased to a value less than 1 s. Stability criteria. For the first condition, a minimum of 25 sessions had to occur before performance could be judged stable. For subsequent conditions, the minimum was 15 sessions. After the minimum number of sessions had been met, a condition was terminated when several stability criteria were met. To assess stability, each session was divided into two blocks of 32 trials. A mean delay was calculated from the eight delays obtained from the free-choice trials in each 32-trial block. Performance was judged stable when the following criteria were met: (1) Neither the highest nor the lowest single block means of a condition could occur in the last six blocks, (2) the mean adjusting delay across the last six blocks could not be the highest nor the lowest six-block mean of the condition, and (3) the mean delay of the last six blocks could not differ from the mean delay of the preceding six blocks by more than 1 s. The stable value was the average of the two six-block means used to judge stability. In this experiment, a condition was not terminated until these criteria were met three times and the indifference delay reported for a condition was the average of the three stable values. Results Table 2 presents the mean and standard deviations for the indifference delay values for each pigeon by body weight level. The number of sessions to stability are also presented. For Group 1, the mean delays across the 70%, 95%, and 70% body weight conditions were 8.96, 14.01, and s, respectively. For Group 2, the mean delays across the 95%, 70%, and 95%

7 DEPRIVATION, AVAILABILITY, AND FOOD SELECTION 535 body weight conditions were 8.38, 6.21, and s, respectively. Both groups displayed the same pattern of shorter delays at 70% than at 95% and changes in delays covaried with changes in body weight. Table 2 Group 1 P57 P306 P453 Mean Group 2 P48 P423 P496 Mean Mean I ndifference Delays for Corn (s), Standard Deviations ( ), and Sessions to 8tability (8) for Each Pigeon by Body Weight Level Body Weight Level 70% S 950/0 S 70% 6.87(1.14) (0.92) (0.43) (0.82) (0.99) (0.47) 8.53 (1.16) (0.45) (4.55) /0 8 70% 8 95% 9.51 (1.39) (0.32) (0.48) 8.88 (0.30) (0.83) (0.11) 6.75 (0.46) (0.14) (1.68) For the purpose of statistical analysis, values for the same body weight conditions in Table 2 were averaged and the resultant values were tested using a paired t test. The results of this analysis showed that indifference delays were significantly shorter (t(5) = 3.62, P =.015} at 70% (M = 7.87 s) than at 95% (M = s) body weight. On average, indifference values for corn were approximately 5 s less in the 70% body weight condition. This result implies that the difference in the value of corn relative to checkers was less under conditions of greater deprivation. Finally, Pig'eons 48 and 496 both produced longer indifference delays upon return to 950/0 body weight than when initially put into this condition. This may reflect an order effect. Previous experience with more severe deprivation may enhance the value of a preferred food item when the pigeon is returned to a less deprived state. Discussion In general, the results showed that the difference in relative value between the preferred and nonpreferred food items decreased as deprivation increased. This interpretation follows the theoretical basis of the adjusting delay procedure in which value is a function of the magnitude and delay of reinforcement (Mazur, 1987). An alternative interpretation is that pigeons were willing to wait longer for the preferred food item when they were less deprived. This interpretation suggests that impulsivity increased with deprivation. However, Logue and Pena Correal (1985) failed to find an effect of deprivation on self-control in pigeons. Regardless of the interpretation, this finding is consistent with the hypothesis that selectivity decreases rather than increases with deprivation.

8 536 BELKE AND KWAN Experiment 2 Experiment 1 used delay as a measure of value in order to investigate the effects of a change in the availability of a preferred food item on the value of the preferred food item relative to the nonpreferred item at different deprivation levels. Availability was varied by adjusting the delay to the preferred food item. However, availability can also be altered by decreasing amount rather than by increasing delay. A change in availability based on amount can be accomplished by simply decreasing the amount of food given to pigeons in a food cup. An advantage of changing availability in this way is that it allows separate assessment of the effects of availability, deprivation, and an availability by deprivation interaction. Method Subjects With the exception of Pigeon 453, the same pigeons were used in Experiment 2. Pigeon 453 died in the 12-month interval between the first and second experiments. Apparatus The experiment was carried out in the pigeon colony room with the birds in their home cages. Water and grit were continuously available. Food preferences were tested using a 14-cm x 6.5-cm x 6.5-cm metal food container designed to attach to an opening on the home cage. The portion of the container where the food was placed was partitioned into two equal areas (6.5 cm x 3 cm x 4 cm). The pigeon was able to access both areas through a 6.2-cm x 4.7-cm opening at the rear of the container. A stopwatch was used to measure time and a scale was used to measure food amount and pigeon weights. Procedure The pigeons were kept in the same groups as in Experiment 1. Birds in Group 1 were tested at 70% and then 95% ad-lib body weight whereas birds in Group 2 were tested in the opposite order. When the birds reached their target weights, food preference tests began. At each deprivation level, high and low food availability tests were conducted. In the high availability test, pigeons were presented with 30 g of corn and 30 g of checkers. After 2 min of access, the food container was removed and the amount of each item remaining was measured. Eight test sessions were conducted. On alternate days, the sides on which the corn and checkers were placed were reversed to control for side preferences. In the low food availability test, an amount equivalent to 750/0 of the mean total amount of both items taken during the high availability test was calculated. Subjects were then presented with half that amount in checkers and half in corn. Eight test sessions were again conducted with the sides reversed between test sessions. After 2 min of access to both food items, the remainder of each item was weighed and recorded.

9 DEPRIVATION, AVAILABILITY, AND FOOD SELECTION 537 \~ Results Table 3 presents the amounts of corn and checkers taken by each pigeon ~nder each condition. Because availability was manipulated as a percentage of the amounts consumed in the high availability condition and the amounts consumed in the high availability conditions differed as a function of deprivation level, this raises the possibility of spurious effects if the data are analyzed solely in terms of amounts consumed. Wherever appropriate, the data are analyzed in terms of both grams consumed and percentage of the amount offered that was consumed. Pigeon P57 P306 P48 P423 P496 Mean Table 3 Mean Amounts (g) of Corn (Co) and Checkers (Ch) Consumed Under Different Levels of Deprivation and Availability Deprivation Level 70% 95% Availability Availability High Low High Low Co Ch Co Ch Co Ch Co (9) (9) (10) (10) (10) (9.6) Ch 0.50 (4) 2:63 (6) 0.13 (5) 0.00 (5) 1.00 (5) 0.85 (5) Note. The amounts (g) of each food type offered under conditions of low food availability are given in parentheses. Low availability amounts were 75% of the amounts consumed under conditions of high availability divided equally between the two food types. In high availability conditions, 30 g of each food type was offered. First, with availability held constant at a high level, the effect of deprivation on the rate of eating can be seen in the amount of food consumed per min. At 95% body weight, pigeons consumed on average 6.59 g/min compared to g/min at 70% body weight. A paired t test showed that the rate of eating significantly increased with deprivation, «4) = 11.87, P < In terms of an effect of availability, less food was consumed in the low (M = g) than the high availability condition (M = g); however, because less food was available in the low availability condition, this effect is best interpreted in terms of percentage of amount offered that was consumed. In the high and low availability conditions, pigeons were presented with 120 g and 29.2 g of food, respectively. When amounts offered are taken into consideration, pigeons consumed 32.7% of the food offered in the high availability condition and 81.7% in the low availability condition. Thus, the effect of a change in availability was to increase the percentage of available food that was consumed. However, even if the pigeons had consumed the same absolute amount of food in both conditions, this effect of availability on the percentage of food consumed would be expected to occur.

10 538 BELKE AND KWAN A repeated measures analysis of variance with deprivation level (70%, 95%) and food availability (high, low) as within-subject factors was performed on the amounts of checkers consumed. Although this analysis showed significant main effects of deprivation, F(1, 4) = 13.67, P <.05, and availability, F(1, 4) = 21.96, P <.05, these main effects were generated as a function of a significant deprivation x availability interaction, F(1, 4) = 67.00, P <.01. Figure 1 shows the mean amounts of checkers consumed as a function of deprivation and availability. In parentheses beside each data point are the mean number of grams offered and the percentage of the grams offered that the grams consumed represent. Analyses of these data as either grams consumed or percentage consumed produce the same pattern of results and yield the same interpretation. First, from Figure 1, it is apparent that deprivation had no effect on checkers consumed when' availability of both food items was high, t(4) = -c 0) E ::::J Ul c::: 0 u (f) ~ CJ.) ~ u Q) L:. U "Po 4 Ul E co ~ t!) 2 (30, 5.6%) 0 0 High availability Low availability 92'.6 % ) 70% 95% Percent ad-lib body weight (30, 3.40/0) (5, 15.80/0) Figure 1. Mean amounts (g) of pigeon checkers consumed under different levels of availability and deprivation. Mean amounts offered (g) and the percentage of the amount offered that the amount consumed represents are shown in the parentheses.

11 DEPRIVATION, AVAILABILITY, AND FOOD SELECTION , P =.64 for both grams consumed and percentages. Nor did a change in availability of both items affect consumption of checkers when deprivation was less severe (Le., 95%), ~4) = 0.44, P =.69 for grams, ~4) =. 2.14, P =.10 for percent checkers consumed. However, when the availability of both items was low, deprivation strongly affected checker consumption, ~4) = 6.5, P <.01 for grams consumed and ~4) = 5.34, P <.01 for percent checkers consumed. When deprivation was more severe (Le., 70%), a decrease in availability strongly increased both grams consumed, ~4) = 6.15, P <.01, and percent checkers consumed, ~4) = 11.70, P <.001. Consumption of the preferred food item (i.e., corn) was not analyzed in a similar manner because when availability was low, birds typically ate all the corn that was offered, but differed in whether or not checkers were also consumed. In this case, the amounts of corn consumed in terms of grams consumed differed only as a function of the procedure for manipulating availability and as such would produce spurious effects. Similarly, when availability was high, birds almost exclusively ate corn regardless 9f the deprivation level. Consequently, the difference in amounts consumed reflect the effect of deprivation on the rate of eating. Discussion Experiment 2 further explicated the roles played by deprivation, availability, and food type. Corn was preferred to checkers under all conditions; however, the conditions differed with respect to whether or not checkers were consumed. Checkers were consumed when availability was low and deprivation was severe, but not when availability was high regardless of deprivation level, or when availability was low and deprivation was mild. In other words, selectivity diminished only under conditions of decreased availability and increased deprivation. The results from Experiment 2 can be reconciled with those of Experiment 1 by conceptualizing the amounts of the preferred item given in Experiment 2 as equivalent to delays of the preferred food item that were either less than or greater than the indifference delay values that varied as a function of deprivation. Under conditions of mild deprivation, the amounts given in both the high and low availability conditions would be equivalent to delay values less than the indifference point. At both levels of availability, corn was chosen, checkers were not. Under conditions of more severe deprivation, the indifference delay would be shorter and the change in availability would cross the indifference delay value. In the high availability condition, corn was chosen, checkers were not; in the low availability condition, both corn and checkers were chosen. General Discussion In general, the results from the present experiments demonstrate, as suggested by Schoener (1971), an interaction between availability and

12 540 BELKE AND KWAN deprivation on food selectivity. Changes in food availability had different effects on the probability of choosing the less preferred food item at different deprivation levels. In Experiment 1, less delay had to be added to the preferred food item to produce indifference. in choice between the two items in the condition of greater deprivation. In Experiment 2, the decline in availability of both food items markedly increased the likelihood of eating the less preferred food item, but only under conditions of greater deprivation. Similar results occurred whether availability of the food items was decreased by increasing the delay to the preferred food item or decreasing the amounts of both items. ' It could be argued that the increased consumption of checkers in the low availability condition at the greater deprivation level may reflect an attempt to maintain a constant overall level of food intake. However, the fact that pigeons did not increase their consumption of checkers in the less severe deprivation condition contradicts this interpretation. Finally, although neither procedure is a foraging simulation, the effects of deprivation and availability on food selection in these experimental procedures appear consistent with those that optimal foraging theory would predict for more ecologically valid contexts. The direct effect of deprivation was to increase rate of food consumption. In Experiment 2, rate of food consumption doubled when body weight was decreased from 95% to 70%. However, as observed by Moon and Zeigler (1979), although rate of consumption changed, preference did not. At both deprivation levels corn was strongly preferred over checkers. - References GOSS-CUSTARD, J. D. (1977). Responses of the redshank, Tringa Totanus, to the absolute and relative prey densities of two prey items. Journal of Animal Ecology, 46, KREBS, J. R. (1978). Optimal foraging: Decision rules for predators. In J. R. Krebs & N. B. Davies (Eds.), Behavioural ecology (pp ). Oxford: Blackwell. KREBS, J. R., ERICHSEN, J. T., WEBBER, J. I., & CHARNOV, E. L. (1977). Optimal prey selction in the great tit (Parus majot). Animal Behavior, 25, LOGUE, A. W., & PENA-CORREAL, T. E. (1985). The effect of food deprivation on self control. Behavioural Processes, 10, LACHER, T. E., Jr., WILLIG, M. R., & MARES, M. A. (1982). Food preferences as a function of resource abundance with multiple prey types: An experimental analysis of optimal foraging theory. American Naturalist, 120, MAZUR, J. E. (1987). An adjusting procedure for studying delayed reinforcement. In M. L. Commons, J. E. Mazur, A. Nevin, & H. Rachlin (Eds.), Quantitative analyses of behavior: Vol. 5. The effect of delay and intervening events on reinforcement value (pp ). Hillsdale, NJ: Erlbaum. MAZUR, J. E. (1988). Choice between small certain and large uncertain reinforcers. Animal Learning & Behavior, 16,

13 DEPRIVATION, AVAILABILITY, AND FOOD SELECTION 541 MAZUR, J. E., SNYDERMAN, M., & COE, D. (1985). Influences of delay and rate of reinforcement on discrete trial choice. Journal of Experimental Psychology: Animal Behavior Processes, 11, MILLER, H. L., Jr. (1976). Matching-based hedonic scaling in the pigeon. Journal of the Experimental Analysis of Behavior, 26, MOON, R. D., & ZEIGLER, H. P. (1979). Food preferences in the pigeon (Columbia livia). Physiology and Behavior, 22, PYKE, G. H., PULLIAM, H. R., & CHARNOV, E. L. (1977). Optimal foraging: a selective review of theory and tests. Quarterly Review of Biology, 52, RECHTEN, C., AVERY, M., & STEVENS, A. (1983). Optimal prey selection - why do great tits show partial preferences. Animal Behavior, 31, SCHOENER, T. W. (1971). Theories of feeding strategies. Annual Review of Ecology and Systematics, 11, WALKER, S. (1987). Animal learning: An introduction. New York: Routledge & Kegan Paul Inc. Appendix A Nutritional Analysis and Iingredients for Purina Pigeon Checkers and Corn Purina Pigeon Chow Checkers Analysis: (0/0 weight by ounce) Crude Protein 15.0% Crude Fat 2.50/0 Crude Fiber 5.5% Calories/gram = 2.78 Ingredients: Grain products, plant protein products, forage products, animal protein products, process grain by-products, vitamin A supplement, D activated animal sterol (source of vitamin D), ethoxyquin (a preservative), vitamin E supplement, riboflavin supplement, DL-methionine, vitamin 812 supplement, menadione sodium bisulfate (source of vitamin K activity), sodium selenite, choline chloride, calcium pantothenate, niacin supplement, calcium carbonate, dicalcium phosphate, manganous oxide, calcium iodate, copper oxide, zinc oxide Corn Analysis: Protein Carbohydrate Fat (% weight by ounce) 60/0 40% 15% Calories/gram = 3.62

14 542 BELKE AND KWAN