Cryptic Prey Research Articles

Search images not proven: A reappraisal of recent evidence

It is commonly agreed that predators hunting for cryptic prey have been demonstrated to adopt search images (involving perceptual specialization) for particular prey types. However, a simple alternative strategy, that of an appropriate reduction in search rate, may in many cases have the same effect as adopting a search image. The search image and search rate hypotheses are examined, and a series of critical predictions are proposed to distinguish them. Briefly, the search rate hypothesis predicts: (1) that adjusting search rate for one cryptic prey type will enhance the ability to detect other equally cryptic types (the search image hypothesis predicts that this will interfere with the ability to detect other equally cryptic types); (2) that adjusting search rate for cryptic prey will be achieved by learning to spend longer looking at each patch of the environment, and that the more cryptic the prey the longer will be the viewing time required to effect accurate detection (the search image hypothesis makes no such predictions). Four recent studies purporting to demonstrate search images are re-examined in the light of these predictions, and it is found that the published data fail to distinguish the two hypotheses. In addition, some evidence that is better explained by the search rate hypothesis is discussed. A brief examination of the ecological consequences of the search rate hypothesis shows that, unlike the search image hypothesis, it does not predict apostatic selection. It is therefore argued that the distinction between the two mechanisms explaining how predators ‘learn to see’ cryptic prey is important, but that the critical tests have not yet been made. The existence of search images remains not proven.

Can Great Tits (Parus major) Acquire Search Images?

A 'search image' is said to be acquired when there is change in the ability of a pred'ator to detect cryptic familiar Young great tits Parus major were presented with 'cryptic' and 'conspicuous' artificial prey on bark backgrounds. The results showed that eleven captive great tits had more difficulty detecting cryptic prey than conspicuous prey. Detailed analysis of video recordings revealed two types of search behaviour in response to cryptic prey: (1) the birds sighted (and successfully flew in to take) the prey from vantage points 0.5 m and further away, or (2) they alighted on the bark and then searched over the bark surface in detail. Search images seem to have been acquired when behaviour (1) was used but there was little evidence for search images when behaviour (2) was used. There was a striking difference in the ability of each bird to locate cryptic prey at first as measured by unsuccessful search, overlooking prey, background-directed pecks, followed by accidental discovery, and its ability to detect them as efficiently as conspicuous prey towards the end of its test.

Searching for cryptic prey: evidence for optimal search rates and the formation of search images in quail

It has previously been proposed that the success of a predator foraging for cryptic prey can be enhanced in two different ways: by the adoption of an optimal search rate and by the formation of a search image. The optimal search rate reflects a trade-off between the rate at which prey are encountered (which increases with search rate) and the probability of detecting an encountered prey item (which decreases with search rate). The formation of a search image increases a predator's ability to detect cryptic prey. To test these hypotheses three series of experiments were performed in which bobwhite quail ( Colinus virginianus) searched for artificial prey in a large aviary. The colour of the prey was varied to achieve different degrees of camouflage. The results provide qualitative support for the hypothesis that the birds adjust their search speed to maximize the rate of prey capture. The use of search images is suggested by the observation that the probability of detection tends to increase with successive captures, and to decrease between captures.

The Evolution of Aposematic Coloration by Individual Selection: A Life-Span Survival Model

Distasteful prey are often conspicuously coloured, i.e. aposematic. As aposematic coloration entails a higher probability that the prey is discovered by a predator, kin selection has been thought to be a prerequisite for the evolution of aposematic coloration. However, by using a life-span survival model we here demonstrate how a gene for aposematic coloration can invade, and spread, in a population of cryptic distasteful prey by means of individual selection. Our model is based on the recent finding that seizing a prey gives a predator information about its distatefulness, thus conferring a higher probability of survival by being released unharmed for distasteful prey, and that predators learn to avoid conspicuously coloured prey more readily than they do cryptic prey. From our model we can predict that the evolution of aposematic coloration will be favoured in prey with long pre-reproductive phases, a high degree of distastefulness, and occupying habitats with small possibilities for hiding from predators.

The detection of cryptic prey by blue jays ( Cyanocitta cristata) I: the effects of travel time

The behaviour of blue jays ( Cyanocitta cristata) hunting for dispersed, cryptic prey was investigated in an operant simulation in which jays were trained to search projected images for noctuid moths. Each image contained either a single moth or no moth. Each trial was structured so as to simulate travelling between patches, searching within patches, and attacking and handling each moth that was detected. In two experiments in which the travel time between patches was manipulated, increases in travel time produced increased persistence within patches. Although this qualitative effect was predicted by the marginal value theorem, quantitative analyses revealed that the blue jays were using a strategy that was more sophisticated and more efficient than the simple time-in-patch rule implied by the marginal value theorem.

Evidence for search image in blackbirds Turdus merula L.: long-term learning

The feeding behaviour of free-flying and captive blackbirds Turdus merula L. was observed over a number of days to investigate their ability to see cryptic food. Sequences of eight tests which contained equal numbers of ‘cryptic’ and ‘conspicuous’ tests were presented to the birds of the first two experiments. The blackbirds in experiment 1 increased their feeding rate for both cryptic and conspicuous food, which suggests that search images were not acquired but that the physical acts of feeding were perfected (or that they altered their acceptance of green food). In contrast, the ‘residual search time’ (the bird's total search time minus handling and movement times) decreased over the series of four tests with cryptic green prey, but remained constant over the four tests with conspicuous prey (experiment 2). The blackbirds exploited the cryptic prey using their previous experience of searching for such food. This suggests that search images can be effective over eight feeding sessions, spanning 4–30 days. Similar results were obtained over a 9-day period with four juvenile blackbirds in captivity (experiment 3). With experience they also became expert in their assessment of backgrounds set up without prey.

Sit-And-Wait Predators and Cryptic Prey: A Field Study with Wild Birds

(1) I recorded the ability of ten colour ringed robins Erithacus rubecula and nine great tits Parus major to detect immobile, cryptic prey in the wild. Artificial prey were presented at high density on horizontal backgrounds at ground level (experiment 1). Another type of artificial prey was presented at low density above ground on bark (experiment 2). The prey were either 'cryptic' or 'conspicuous'. (2) In experiment 1, five robins simply fed faster on the conspicuous food-there was no evidence that the robins acquired 'search images' during repeated feeding bouts on cryptic prey. (3) Only two great tits and a robin out of the seventeen birds that started experiment 2 learnt how to detect cryptic 'Puffa-puffa' rice prey. The juvenile robin used a 'sit-and-wait' style of foraging which allowed it to sight over half the cryptic prey as quickly as it took to scan for conspicuous prey. However, the remaining cryptic prey were preceded by abnormally long 'search times'; the robin also made unsuccessful visits to the background which were followed by pecks to the bark instead of prey. (4) The two great tits also showed an intermittent ability to detect the cryptic prey; if they failed to sight the prey from long distance (which occurred in over three-quarters of their visits), they resorted to an energy and time consuming 'continuous' foraging style on the bark. (5) Thus the ability to detect cryptic prey from vantage points without relying on movement as a cue, was restricted to only a few birds (of those filmed). Without extensive experience these birds could not learn to deal efficiently with cryptic prey.

Evidence for search image in blackbirds ( Turdus merula L.): short-term learning

A ‘search image’ is acquired as a result of a change in the ability of a predator to detect cryptic familiar prey. Blackbirds were presented with artificial prey dyed either to match the colour of the background (cryptic prey) or to contrast with the background (conspicuous prey). The results of experiment 1 provide some evidence that 10 wild blackbirds had more difficulty detecting cryptic prey than conspicuous prey. Detailed analysis of experiments 2 (11 wild blackbirds) and 3 (six captive blackbirds) revealed subtle changes in the reactions of the birds to cryptic prey. At first these birds were unable to detect cryptic prey, but subsequently improved their ability to detect such food. This short-term change is interpreted as evidence for search images.

Lagging Partial Preferences for Cryptic Prey: A Signal Detection Analysis of Great Tit Foraging

Most prey species are cryptic or mimetic, and as a result, they provide variable sensory cues to predators that are only statistically correlated with resource quality. We develop an analogy between foraging for cryptic prey and basic signal detection problems. An optimization model based on this analogy predicts partial preferences for cryptic prey, with the probability of attack varying between 0 and 1 as a function of the relative frequency of prey. Experiments with great tits show that they respond to unpredictable changes in prey relative frequency by adjusting their probabilities of attack in the predicted directions, but the responses lag behind. In our experiments, the birds' behavior is most appropriate to long-term average conditions (over a span of hours, extending into the preceding day. Problems associated with assessing changing resource distributions are discussed.

A Laboratory Simulation of Foraging Behavior: The Effect of Search Rate on the Probability of Detecting Prey

In a series of laboratory simulations using humans as model predators, we examined the effect of changes in search rate and crypticity on the probability of detecting prey. The subjects searched for a target character (the "prey") among an array of background characters displayed on the screen of a small computer. The search rate was controlled by changing the display duration while prey crypticity was varied by changing the background. The results of these experiments provide support for a model we previously developed to elucidate the behavior of predators searching for cryptic prey (Gendron and Staddon 1983).

Searching for cryptic prey: The effect of a search image

Searching for cryptic prey: The effect of a search image

Do blue jays hunting for cryptic prey forage optimally?

Do blue jays hunting for cryptic prey forage optimally?

Optimal Detection of Cryptic Prey May Lead to Predator Switching

On montre dans cet article que la theorie de la detection du signal implique que la dispersion peut reduire le risque courru par les proies cryptiques meme si le predateur ne forme pas des images d'exploration, et qu'un comportement d'approvisionnement optimal permet au predateur de capturer une proie cryptique disproportionnellement plus souvent lorsque sa densite croit (exemples: oiseaux, insectes)

On the Term 'Search Image'

The term 'search image' continues to be used uncritically by many evolutionary and behavioural ecologists. In the sense of Tinbergen (1960) and Dawkins (1971a) a search image is 'a perceptual change in the ability of a predator to detect familiar cryptic prey' . As a result of initial chance encounters with the cryptic prey, the predator 'learns to see', and selectively attends to those cues that enable it to distinguish the prey from the background (Croze 1970, Dawkins 1971a, b, Pietrewicz and Kamil 1979). The adaptive significance of search image formation might be that cryptic prey can be found more effectively when the predator concentrate on one prey type alone. It is assumed (although direct evidence is scarce) that a search image will be adopted only when the encounter rate is high and will be discarded when the encounter rate is either low or very high (Tinbergen 1960, Dawkins 1971a, b, Krebs 1973). Krebs (1973:83) pointed out that 'search image formation results in a preference, and the problem is to distinguish it from all the other reasons for having a preference'. He listed six types of learning that he and Dawkins (1971 a) emphasise must be excluded from any given case before evidence for search image can be accepted. These alternatives are: (a) learning to visit a particular place to find food, (b) learning to look in a particular type of place for food, (c) alteration of the search path to increase the chances of encountering prey, (d) learning to handle prey more effectively, (e) preference or avoidance of a prey over others that is independent of the predator's ability to see the different types and (f) learning of specialized hunting techniques by particular individuals. It would not be difficult to discover whether a predator under particular circumstances is increasing feeding efficiency by using a search image or one of its six alternatives (or any combination of the seven). We feel that it is important to identify the mechanisms) involved, because each can lead to a different expectation not only of what a predator does, but also of what a prey 'does' to avoid being eaten. For example, if the predator is 'learning to see' the evolutionary response of the prey might be quite different from the case where the main effect is of the predator learning 'where to hunt'. Details of the behavioural mechanisms involved can therefore be important even to those interested only in the effects of predation. Some authors still seem unaware of the distinction between search image and the other forms of learning that increase the efficiency of feeding on prey (or if they are aware, they presumably think the distinction trivial). Ricklefs (1980:878), for example, defines a search image as 'a behavioural selection mechanism that enables predators to increase searching efficiency for prey which are abundant and worth capturing'. Quite apart from the lack of reference to cryptic prey in this definition (a common failing), all six alternatives can also be regarded as 'behavioural selection mechanisms'. On page 613 of the same book, Ricklefs describes a search image as 'a preconception of what a given prey looks like and where it is found'; a similar definition is given by Owen (1976:123). In fact 'a preconception of what a given prey looks like' can result from category (e) above. This alternative explanation is frequently overlooked by those who claim that predators use 'search images' when they feed disproportionately more on prey that are common or familiar. Recent examples of authors who appear to have fallen into this trap include Cornell (1976), Jirvinen (1976), Hassell (1978), Rausher (1978), Hughes (1979), Atema et al. (1980) and Andr6n and Nilson (1981). These workers give no clear indication that the prey concerned were cryptic and therefore it is likely that the prey would have been seen immediately by their predators. Common conspicuous prey may be preferred simply because they are common. 'A preconception of where a given prey is found', the remaining part of Ricklefs second definition, is covered by alternatives (a) and (b) above. Pietrewicz and Kamil (1981:313) extend the true meaning of search image to include changes in the ability of predators to detect novel cryptic(?) prey or, in their own words, learning 'what the prey looks like'. These definitions fail to preclude a mechanism that belongs to category (e). To quote from Dawkins (1971a:567): 'we cannot conclude that a change of per-

Searching for Cryptic Prey: The Effect of Search Rate

We have developed a model, based on Holling's (1959b) disc equation, of a predator searching for cryptic prey. In doing so we assumed that the probability of a predator detecting an encountered prey item is inversely related to both prey crypticity and search rate. The optimal search rate will then reflect a trade-off between prey detection and the frequency of encounter. As prey become more cryptic this optimal search rate drops. If the energetic cost of locomotion is taken into account the optimal search rate is somewhat lower, but in general this additional effect is negligible. When a predator is searching for two prey types which differ in crypticity the optimal search rate increases with the relative density of the more conspicuous species. This increase in search rate may result in a dramatic drop in the probability of detecting the more cryptic prey, so that even when it is relatively abundant it may be virtually excluded from the diet. We briefly discuss how this model can be modified to simulate the formation of a search image and generate density-dependent predation.

A REVIEW OF AFRICAN BIRDS FEEDING IN ASSOCIATION WITH MAMMALS

Summary Dean, W. R. J. & MacDonald, I. A. W. 1981. A review of African birds feeding in association with mammals. Ostrich 52:135–155. Feeding associations between African birds and mammals are defined. 96 species of birds show, or possibly show, some form of feeding associations with mammals. Five major categories of feeding association are identified, which differ in relative importance in the various habitat types of Africa. In the most widespread and common feeding association the bird obtains cryptic prey items flushed by the mammal, The opportunistic nature of most bird/mammal feeding associations is stressed. Hypothetical courses In the evolution of the more complex feeding associations are discussed. The possible significance of bird/mammal feeding associations in their biology and conservation is outlined. The lack of quantitative information on these feeding associations limits the conclusions that can be drawn, although their significance for the obligate ectoparasite gleaners, Buphagus species,...

The evolution of conspicuous coloration: Some experiments in bad taste

Unpalatable species are often brightly coloured. Such aposematic coloration may have evolved because predators can learn to avoid conspicuous prey more readily than cryptic prey. Experiments on young male chicks are described and the results are consistent with this hypothesis.

Why are distasteful prey not cryptic?

Bright coloration among unpalatable prey is common1. Turner2 has suggested that, as with Fisher's explanation for the evolution of distastefulness itself3, such aposematic coloration has been favoured by kin selection. Distasteful aposematic prey tend to live in kin groups and predators sampling from such groups will learn to avoid conspicuous prey more readily than cryptic prey and, as a consequence, fewer members of conspicuous kin groups are preyed upon2,4–6. We know of no clear evidence that vertebrate predators take fewer trials when learning to avoid conspicuous rather than cryptic distasteful prey6,7. Here we report results from a series of experiments that provide such evidence.

Optimal Foraging and Cryptic Prey

Optimal Foraging and Cryptic Prey

Effects of aggregation on feeding and survival in a communal wren

Foraging by a social wren, Campylorhynchus nuchalis (Troglodytidae), in a tropical savanna habitat is not enhanced by aggregation. Data for marked individuals show that solitary foraging results in a higher capture rate than foraging near others. We find no evidence of imitative foraging, as individuals actively avoid successful foragers following a capture and successful foragers do not restrict their search to recently productive stations or techniques. Captures are seldom temporally clumped, and clumping is probably not pronounced enough to favor imitation. Juveniles show no greater tendency to respond to captures of others, or to succeed in foraging in a group, than do adults. Aggregation is probably disadvantageous for foraging because of dispersed, scarce, cryptic, and noneruptive prey and because of the searching technique of these foliage-gleaning insectivores. If predator avoidance is enhanced by aggregation, it does not result in either increased survival or increased foraging efficiency in large groups, even by juveniles.