Biology 363/563 OrnithologyDr. David Swanson, Office: CL 180OPTIMAL FORAGING THEORY = hypothesis that birds (or other organisms) feed in a manner that is most efficient in terms of their survival and reproductive success. Foraging is viewed in terms of costs and benefits - assumes that natural selection will act to maximize an individual's benefits relative to costs. - Involves "decisions" about:
I. WHAT FOODS TO EAT? (OPTIMAL DIET MODEL) - Optimal foraging theory predicts that birds will select foods that maximize reward intake per time (Reward = energy or a specific combination of nutrients). A) Profitability of Food Items (Assume: P = net energy content) P = E1 / (S1 + H1) where E1 = energy content of food item 1, S1 = search time for item 1, and H1 = handling time for item 1. B) Number of Food Types: Assume 2 food types such that: E1/H1 > E2/H2 Type 1 is more profitable than type 2. C) Deciding which Items to Eat: When a bird encounters a food item, it must eat it or ignore it. According to optimal foraging theory, the bird should eat the item if, while handling and eating it, the bird cannot find a better food item. A general rule from this logic is that the organism should always eat the most profitable food item. Eat if: E2/H2 > E1/S1 + H1 Ignore if: E2/H2 < E1/S1 + H1
E) Empirical Evidence: LABORATORY (Krebs et al. 1977) - Model: Great Tits feeding on whole (Type 1) or half (Type 2) mealworms. - Experimental Procedure: Worms were delivered to the birds on conveyor belts so that the encounter rates could be varied (encounter rates = search times). - Results: (1) Birds switched to taking whole worms when S1 was such that, while eating a Type 2, they missed a Type 1. At longer S1 they took both types. However, the switch was not a sudden as predicted. Perhaps it took some time for the birds to determine relative rewards. (2) When E2/H2 < E1/(S1 + H1), birds took only Type 1 food items, even if the abundance of Type 2 items was increased (decreased S2). (SEE HANDOUTS 1, 2). FIELD EVIDENCE (Davies 1977) - Model: Wagtails feeding on Yellow Dungflies
* But, why were other sizes taken at all? - Maybe there is a trade-off between the degree of selection and the time spent searching Unselective = little search time but relatively low energy intake Selective = high energy intake but longer search times II. OPTIMAL TIME ALLOCATION = When to switch sites? A) Theoretical Predictions = A bird should move when the rate of food intake in a patch falls to a level equal to the average for the habitat as a whole. (SEE HANDOUT) - This decision involves being able to assess the average quality of food patches and the time taken to travel between them. - One corollary of this prediction is that birds should spend longer feeding in good patches than in poor patches. (SEE HANDOUT) B) Laboratory Evidence (Cowie 1977) - Model: Great Tits feeding on mealworms hidden in sawdust-filled cups on the branches of artificial trees. This apparatus assured a declining capture rate with time, as occurs in natural food patches. - Predictions: (1) When the average feeding rate in the habitat is high, birds should spend less time at each cup. (2) If travel time between cups is increased (this decreases the average rate of energy gain for the habitat as a whole), the birds should spend longer in each patch. - Results: (1) Intercatch Interval (inverse of feeding rate) was positively related to time spent at each cup. (SEE HANDOUT) (2) If a top was placed on each cup (increase travel time), time spent at each cup increased (SEE HANDOUT). - Conclusions: Predictions were validated, and the birds behaved as if they were optimizing energy intake. III. INCONSISTENCIES AND PROBLEMS 1) Birds do not always behave as if foraging optimally. Various factors may prevent them from foraging optimally (predation, competition, dominance hierarchies, morphological constraints). 2) It is also very difficult to precisely define the reward around which birds optimize (energy or specific nutrients). 3) Birds will sometimes visit low yield sites repeatedly or feed on inferior food when better "choices" are available. Why? Perhaps to monitor future foraging opportunities in case present conditions change. IV. CENTRAL PLACE FORAGING = Optimization of the load for parents bringing food back to the nest for the young. - Prediction = Birds should carry heavier loads if travel time between patch and nest is increased. (SEE HANDOUT). - Assumption = Energy gain decreases as load size increases (i.e., prey becomes more difficult to handle). - Bryant and Tatner (1982): House Martins - Loads increased as travel time increased, although predicted loads and observed loads were not in particularly close agreement. (SEE HANDOUT). |
