COLONIALITY (Concluded)

Colonies may be "neutral assemblages", i.e., there may be no net benefit of coloniality. For example, if colonies are in areas that are inaccessible to predators, many pairs may settle there and not necessarily benefit because others are close by.

On the other hand, once coloniality has developed, behaviors may evolve to enhance the benefits and decrease the costs. For example, the ìselfish herdî effect will be most effective for individuals starting their breeding attempt later than some neighbors but completing it earlier than others and would favor those who lay when most others are laying. Unfortunately, the advantage of synchronous breeding has been documented in relatively few studies and evidence for it has often been overstated. For example, Feare's study of sooty terns had been cited repeatedly as an example to indicate that synchronous breeding is more successful than early or late breeding (see handout, Figure 8-5). However, the very first nest attempt of the season failed and other early nests had high success. Higher success of earlier attempts is typical for birds (generally experienced breeders in the best condition breed earliest). My studies of common murres at Bluff, Alaska generally show an advantage of breeding synchronously with neighbors rather than earlier or later (see handout, Fig. 5). This is because ravens prey on eggs laid early in the season and only breeding neighbors usually hold their ground and make it difficult for a raven to land and attack an adult on its egg.

To summarize, both territoriality and coloniality have many potential costs and benefits. Strategies adopted by individuals are likely those that maximize their own benefit:cost ratio. In most species all individuals adopt a particular strategy (e.g.., either territoriality or coloniality) but in some species, e.g., the fieldfare in Europe, some individuals are territorial and others are colonial. It should be recognized that colonial species defend nesting territories but generally not feeding territories. Therefore, coloniality and territoriality are extremes in terms of spacing patterns but opposite ends of a continuim of territory size, because individuals in even the most colonial species defend the nest site and generally a small area around the nest site.

FORAGING STRATEGIES

The underlying assumption in discussions of optimality in foraging is that organisms will maximize their efficiency in finding food, i.e., will maximize the benefit:cost ratio of foraging. This is certainly a real concern for a chickadee which must catch an insect every several seconds while it's foraging on a winter day to stay alive. Foraging competes with other activities that are also necessary for maintenance and reproductive success; consequently, efficiency will be advantageous, i.e., favored by natural selection. The work on foraging strategies is aimed at finding how animals maximize their net rate of food intake while foraging. The three major decisions to be considered for actively foraging animals are what foods to eat, where to hunt, and how to search for food.

Predictions about how many food types should be included in the diet are usually based on the net benefit in energetic terms of including particular prey. Once one or more prey types are already incorporated in the diet, an additional prey type should not be included if its inclusion would reduce the overall rate of food intake by the animal (see handout). This could occur if the time spent searching for that prey type and/or time spent handling it reduces the time the animal could be devoting to securing more profitable prey types. Lab experiments on several species (e.g., Bluegill Sunfish and Great Tits -- see handout) have shown that the predictions of the optimal diet are generally upheld, but unprofitable times typically arenít completely eliminated from the diet.

There are several possible reasons why less profitable prey are included in the diet when predictive models and controlled laboratory experiments show that they should not. The animal may be monitoring its environment and sampling new prey types which may be increasing in abundance -- this would be to the animalís long-term advantage in a variable environment. Alternatively, perhaps less profitable items in energetic terms provide some nutrients that the more profitable item does not. In the field an animal may not be able to anticipate the likelihood of encountering a preferred prey in the near future if it passes up an available suboptimal prey type; high temporal variation in encounter rates with preferred prey may result in a more varied diet (see below).

Goss-Custard (1977. Anim. Behav. 25:10-29) studied selection polychaete worms by size class on mudflats by redshanks (Tringa totanus). The redshanks selected prey sizes of 7mm more than any other size class even though prey of 8mm are more common. The worms were eaten in direct proportion to their net energy benefit --not simply in relation to their abundance (see handout). Goss-Custard quantified prey choice, prey density, and redshank walking speed and peck time. He then built a simulation model and found that the redshanks optimized prey choice -- no other model of prey choice increased the rate of food intake.

Such studies have essentially verified the optimality predictions even without consideration of variability in prey types in digestibility, nutrient quality, secondary-chemistry, and other aspects of prey quality. Yet, in another study, Goss-Custard found that redshanks preferred Corophium, an amphipod, to all types of worms. This amphipod is smaller than any of the worms and yields less food per unit handing time than do the worms. The amphipod apparently contains some important nutrient that the worms don't. Such a choice does not negate models of optimal foraging - it just shows that models based on energetic considerations alone may be oversimplified.

Gary Belovskyís (1978. Theor. Pop. Biol. 14:105-134) study of moose diets shows that moose balance the intake of aquatic and terrestrial plants to optimize the balance between energy and sodium intake. Terrestrial plants have high energy and low sodium content relative to aquatic plants. The size of the rumen and the fiber content of the forage determine the rate of food processing and set an upper limit to the amount of food eaten. Only diets above both the sodium and energy constraints are suitable, and moose maximize their daily energy intake subject to the constraints of sodium need.

Other factors may also influence foraging behavior, causing the animal to compromise between competing demands. For example, animals often are at greater risk to predators while foraging, and the risk of predation may (1) reduce the amount of time spent foraging or (2) alter the areas and food types used (e.g., see handout).

Optimal foraging models are reductionist in approach. First, a simple model (e.g., time, energy) is constructed, and the fit between observations and prediction is tested. If the fit is poor additional factors (e.g., nutrients, predators, competitors) are added one-by-one until the model adequately mirrors what the animal does. Reto Zach's study of crows dropping whelks showed that 4 components were necessary (see handout, Alcock). Furthermore, the requirements may be dynamic, particularly for herbivorous animals who must have a mixed diet to maintain secondary compounds below critical levels.

Exploiting patchy foods

Food may be clumped (patchy) in distribution. Optimal foraging in this context means that predators should concentrate their efforts in the most profitable patches. There are numerous examples showing that predators do tend to concentrate in the most profitable patches, e.g., redshank density is tightly correlated with amphipod density in the most profitable patches.

The degree to which the quality of a patch changes with time will affect how long an optimal predator should forage in each patch. The predator may change the quality of a patch by its own activities, or the quality may change independently of the predator's activities. These possibilities have different implications for optimal choice. A third possibility - that patch quality is constant - is unlikely in nature, and there is an obvious optimal solution: predators should confine their activities to the most profitable alternative. This is found in laboratory studies.

Yellow Wagtails forage for dung flies on cow dung, and the flies disperse into the surrounding grass once a bird starts to hunt for them. Thus profitability of that patch rapidly declines with time. We can consider that a foraging predator either spends its time foraging within a patch or traveling between patches. Its overall intake of food is then the average food intake/patch divided by the average time in a patch plus travel time. The graph on the handout shows the optimal solution. In order to maximize the rate of food intake, the predator should eat in the patch just long enough to make the slope of a line AB, the net rate of food intake, as steep as possible (it must pass through the origin). If patches differ in quality, the model is more complex, but the optimal predator should stay in each patch until its rate of intake drops to a level equal to the average rate of the intake for the habitat. As soon as the cumulative net gain falls below the slope AB, the predator should switch patches (see handout), because it would do better by traveling to another patch. These graphic results provide the same insights as common sense; you'd expect animals to spend more time in more profitable patches.

If travel time between patches is long, a predator should spend more of its time in each patch. In his experiments with Great Tits on artificial trees, Cowie altered "travel time" by placing loose or tight fitting cardboard tops on plastic food cups. He found the expected relationship of increased time in the patch when time between utilization of patches was increased. When he took the energetics of traveling time into account, he found a tighter fit of observed data to predicted data (see handout).

Search paths

Other aspects of optimal foraging include consideration of search paths. Considerations of effective search paths depend on the rate at which prey are replenished in an area once that area has been exploited by a predator. If prey are essentially non-renewing, a random path or any retracing of the route would be inefficient. However, a random path would minimize the chance of predation and might be expected if the risk of predation is high. The rate at which food is replenished determines how soon a predator should return to a particular place. These considerations only apply if a predator has exclusive use of an area; otherwise, there may be interference with recovery of prey after depletion. As noted earlier, territorial Golden-winged Sunbirds and Pied Wagtails optimize their return rate to flowers and areas of shoreline, respectively, so that nectar levels and insect abundance, respectively, are replenished.

Risk-sensitive foraging

Increasingly, current research on foraging behavior of animals is focused on the foraging preference of animals in relation to the variance in probability of rewards. "Risk" in this context concerns an organism's preferences when reward frequency is variable. The general idea is that the environment of a forager is stochastic (unpredictable), at least to some extent, and that the foraging strategy may be adjusted to variability, as well as mean levels in reward frequency.

Several experiments have been conducted in which the mean probability of rewards is fixed but the variance is different. For example, Leslie Real provided bumble bees with an artificial environment of yellow flowers and blue flowers. Flowers of one color always provided 2 microliters of nectar while flowers of the other color were either empty (67%) or contained 6 microliters of nectar; thus the means were the same, but the variances were different. Bumblebees concentrated their visits (85%) on the flower type providing the constant reward, demonstrating risk-aversion. Similar results have been found for other nectar-feeding insects and birds that have been studied.

Tom Caraco studied risk-sensitive foraging in caged seed-eating birds. Birds were confined to large aviaries in which two feeding stations were separated by a partition. When Dark-eyed Juncos and White-crowned Sparrows were maintained on a positive energy budget, they preferred the constant reward station (risk-aversion). When their energy budget was negative, however, they preferred the variable-reward station (risk-prone) . Similar results have been found for shrews. All of these study species must forage intensively each day to maintain a positive energy budget, and continuing to specialize on a predictable alternative that does not meet the daily requirement would endanger the animal's chances of survival.

In a study of Yellow-rumped Warblers, pre-migratory individuals showed risk-prone behavior and quiescent birds showed risk-averse behavior. Once pre-migratory birds had accumulated maximum levels of reserves, they switched to a risk-averse strategy. Such studies demonstrate that variances, as well as means, play a role in the foraging decisions of animals and that this role may vary throughout the life cycle; perhaps risk aversion is more prevalent at stages where the animals are time-minimizers (foraging just long enough each day to meet metabolic requirements) and risk sensitivity is predominant when they are energy maximizers (foraging long enough to maximize energy intake/unit time).

Time and energy can be interchanged as critical currencies and constraints. For time-minimizers, time is the currency, but energy is the constraint -- enough time must be spent foraging to meet minimal energy needs. For energy maximizers, energy is the currency and time is the constraint -- thereís only a limited amount of time available to find, capture and process food.

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