Total Foraging Time Research Articles

On the Optimal Searching Strategy for Refuging Predators

Refuging predators are under selective pressure to minimize not only the time needed to find a prey patch but also the distance from this patch back to the refuge. A simple geometrical model of a search path reveals that minimizing search time and minimizing commuting distance require conflicting search-path characteristics. Search time is minimized by paths with relatively low rates of turning, as these reduce the chances of searching over areas previously searched. In contrast, commuting distance is minimized by paths with relatively high turning rates, as these improve the chances of encountering a prey patch nearer the refuge. The relative importance of searching versus commuting depends on the number of return visits a predator can expect to make to a known patch. The model predicts that even for predators which return to a known patch many times, total foraging time is minimized by a search path expected to find not the nearest patch but one a few times more distant. Therefore, many of the prey patc...

Prey selection and foraging period of the predaceous rocky intertidal snail, Acanthina punctulata.

The diet and foraging period of the neogastropod Acanthina punctulata were investigated in order to test various aspects of recent optimal foraging strategy models. This intertidal snail is an actively searching predator which preys on snails and barnacles by boring a hole in the shell and rasping out the flesh. Unlike many gastropod predators, Acanthina drill its gastropod prey at a very specific location on the columella, the thickest portion of the shell. Acanthina's foraging period can be interpreted as a compromise between maximizing the energy obtained by feeding and minimizing risk of mortality from exposure to wave action. That foraging period minimizing risk of being dislodged by waves appears to be during low tide when the predators can be in shallow pools. However, prey cannot be captured and consumed during one low tide. Thus Acanthina must be exposed during some high tides, and its strategy appears to be to restrict movement while exposed. Thus search is not initiated during high tide, but drilling and prey consumption are continued during that time. A snail not drilling or consuming prey seeks the protection of crevices or large anemones during high tide. A model is presented to indicate the relative amounts of risk and net energy for Acanthina at successive low and high tides. Predictions from the model, e.g., minimizing search time to avoid being exposed for an additional high tide and no movement during high tide are supported by field data. Acanthina commences foraging at the beginning of low tide, searches initially for preferred prey, but if unsuccessful, settles for a less preferred prey and begins drilling this prey before the end of low tide. Drilling and ingestion of prey occur during the following high and sometimes low tides. These "handling times" take 95% of the total foraging time in the field, while search time takes only 5% (pursuit time is negligible). Drilling alone accounts for 48-70% of the total drilling and eating time. In the laboratory, drilling and eating time for littorine food ranged from 15-60 hrs per item. The time to drill and eat a littorine increases exponentially with prey length. Since handling and processing prey items represents such a large investment of time, Acanthina would be expected to be very selective with respect to choice of prey items. Electivity coefficients from field data suggest that littorines are preferred over barnacles. Acanthina in the laboratory optimizes the amount of biomass ingested per time by choosing larger littorines over smaller ones and by preferring the more readily drilled species.It is suggested that Acanthina obtains information about the range of prey available initially by encountering and evaluating quite a few prey before making a selection, but usually by comparing an item of prey encountered to the prey it recently ingested. This latter method should provide a basis for evaluating prey encountered and has the advantage of reducing search time, the total amount of time spent feeding and thus the high-tide time exposed to wave action.In a similar manner, the decrease in the level of acceptability of prey as search time increases represents a compromise between maximizing energy obtained and minimizing risk from mortality.

The Food Searching Behaviour of Two European Thrushes

Abstract1. The movement path of a predator will clearly be an important determinant of its ability to encounter and subsequently attack suitable prey items. Previous work on this aspect of searching behaviour has been mainly carried out on invertebrates, while this study describes the movement paths of two bird predators, the european blackbird and the song thrush (collectively referred to as 'thrushes') on a grass meadow in central Oxford. 2. The paths of foraging thrushes are divided up into natural units consisting of a series of alternating moves and pauses. A method of mapping these moves is presented (Figs 2, 3). 3. The large scale movements of the thrushes on the meadow was not uniform and this was probably related to non-uniformity in the environment, possibly including factors influencing feeding success (Figs 4, 6). 4. The durations of the moves and pauses were measured from cine film and video tape records. Only between one sixth and one tenth of the total foraging time was spent in actual movement, the remainder probably being spent largely in scanning for, or attacking, potential prey objects (Figs 7, 8). 5. Measures of the individual move lengths and angles. turned between successive moves were taken from the maps (Figs 3, 9), and the average speeds of movement were calculated for each track. Song thrushes and female blackbirds made longer moves than male blackbirds, and female blackbirds moved across the meadow at a higher overall speed than male blackbirds or song thrushes (Tables 1-4). 6. Methods are presented for describing the 'rules of movement' of the thrushes across the study meadow. When feeding mainly on earthworms, thrushes tended to make sequences of three, or possibly more, moves that were similar in length, and to make pairs, triplets, and possibly bigger groupings of alternating left and right turns. The size of each move was independent of the sign of the preceding turn and vice versa. The tendency of successive turns to alternate in sign, combined with the relatively restricted size of turns between moves, produced an ongoing search path, thus avoiding the pitfall of searching the same ground twice in a short time.