Singing for your supper: sentinel calling by kleptoparasites can mitigate the cost to victims

Parasitism generally imposes costs on victims, yet many victims appear to tolerate their parasites. We suggest that in some cases this may be because parasites provide victims with mitigating benefits, paradoxically giving rise to selection for advertisement rather than concealment by parasites. We investigate this possibility using the interaction between an avian kleptoparasite, the fork-tailed drongo (Dicrurus adsimilis), and one of its victims, the pied babbler (Turdoides bicolor). Combining field observations and a playback experiment, we demonstrate that a conspicuous vocal signal broadcast by drongos perched waiting to steal food from foraging babblers allows the latter to improve their own foraging efficiency, although not to the same extent as that experienced in response to conspecific sentinel calling. We argue that "sentinel" calling by drongos may originally have arisen as a means of manipulating babblers: because babblers find more food items and venture into the open more in response to these vocalizations, drongos are presented with more kleptoparasitism opportunities. However, the resulting benefit to babblers could be sufficient to reduce selection for the evolution of defenses against drongos, and the current situation may represent a rare example of an interspecific relationship in transition from a parasitism to a mutualism.     

A strategy to increase cooperation in the volunteer's dilemma: reducing vigilance improves alarm calls

One of the most common examples of cooperation in animal societies is giving the alarm in the presence of a predator. A reduction in individual vigilance against predators when group size increases (the "group size effect") is one of the most frequently reported relationships in the study of animal behavior, and is thought to be due to relaxed selection, either because more individuals can detect the predator more easily (the "many eyes" effect) or because the risk of predator attack is diluted on more individuals (the "selfish herd" effect). I show that these hypotheses are not theoretically grounded: because everybody relies on someone else to raise the alarm, the probability that at least one raises the alarm declines with group size; therefore increasing group size does not lead to relaxed selection. Game theory shows, instead, that increasing the risk that the predator is not reported (by reducing vigilance) induces everybody to give the alarm more often. The group size effect, therefore, can be due to strategic behavior to improve the production of a public good. This shows how a selfish behavior can lead to a benefit for the group, and suggests a way to solve social dilemmas in the absence of relatedness and repeated interactions.     

Non‐Redundant Social Information Use in Avian Flocks with Multisensory Stimuli

Animals generally live in multisensory worlds; however, our understanding of multisensory perception is rather limited, despite its relevance for explaining the mechanisms behind social interactions, such as collective detection while foraging in groups. We tested how multisensory stimuli affected the antipredator behavior of dark‐eyed juncos (Junco hyemalis) using alarm calls as an auditory signal and flushing behavior as a visual cue. We varied the degree of risk within the group by manipulating the number of group mates alarm calling and/or flushing using robotic birds. We assumed that alarm calling and flushing were redundant stimuli and predicted that they could generate one of three types of responses (enhancement, equivalence, or antagonism) depending on the mechanism of multisensory perception. We set up an artificial flock with three robotic juncos surrounding a live junco and controlled for multiple confounding factors (e.g., identity of the focal, body mass, food deprivation time). We found that the degree of alarm of live juncos increased when at least one robot flushed. However, the time it took the live individuals to react to the robots' behavior increased, rather than decreased, with at least one alarm call. This could be the result of an orienting response or sensory overload, as live juncos increased scanning behavior after being exposed solely to alarm calls. Contrary to some theoretical assumptions, alarm calling and flushing behavior elicited independent unimodal responses, suggesting that they are non‐redundant stimuli and that together they could reduce the occurrence of false alarms and facilitate flock cohesion.     

Are the Costs of Site Unfamiliarity Compensated With Vigilance? A Field Test in Eurasian Siskins

Determination of fitness differentials between individuals adopting different migratory and dispersal strategies is basic to understand the evolution of migration. In the Eurasian siskin Carduelis spinus, both resident and transient birds forage within the same wintering area, providing the rare opportunity to compare their foraging behaviour in the same area and habitat. The aim of this study was to test the predictions associated to the different hypothesized costs of transience by studying the vigilance and foraging behaviour of wild wintering siskins foraging at three bird tables with different predation risk and interference competition levels. Transient siskins showed longer scan durations than residents, either because of site unfamiliarity or subordination (i.e. prior‐occupancy effect). However, residents and transients did not differ in aggression rates, contrary to the dear‐enemy effect. Transient siskins did not show a higher allocation of time to vigilance, contrary to the hypothesis of compensation vigilance to reduce predation risk by dispersing animals. Moreover, transients increased pecking rate with increasing predation risk, showed lower scan rates, longer foraging bouts and, in males, presented marginally higher proportions far from cover. Altogether these results strongly support the hypothesis that transients incur a predation cost due to a less efficient vigilance and foraging system.     

Vigilance and antipredator responses of Caribbean reef squid

Antipredator responses, especially those of open-ocean squid, have been seldom studied in the natural environment. Sepioteuthis sepioidea, observed by snorkellers near the shore in early morning/late afternoon, produced an average of eight moves of over 1?m per hour, apparently mostly antipredator behaviours. Close approaches by herbivorous parrotfish elicited no response in 74% of encounters; otherwise, squid produced agonistic zebra stripes or startle-mantle-dots skin patterns. Predatory bar jack fish caused flight but not zebra displays, and squid usually paled and fled quickly (66%) from snapper. The speed of approach was the best predictor for flight and display responses to snapper, but for bar jack and parrotfish, the relative fish size and distance were the predictors for escalated responses. Paired dorsolateral mantle dots were produced when squid approached the sea bottom or hunted outside the group and in reaction to fish approaches; 56% of these were to the very common parrotfish. Reactive pairs of spots were selected from four possible mantle locations and they were significantly likely to be directional towards fish, presumably as startle/warning, but not directed towards conspecifics as indicators of predator presence. Thus the evasion techniques of the cephalopods and their ability to produce different display patterns on the skin show how an intelligent animal modifies an otherwise simple decision about which pairs of spots to select from four possible mantle locations and whether to flee from potentially dangerous animals.     

The ecology of fear and inverted biomass pyramids

In inverted biomass pyramids (IBPs) prey are outnumbered by their predators when measured by biomass. We investigate how prey should behave in the face of danger from higher predator biomass, and how anti-predator behavior (in the form of vigilance) can, in turn, affect the predator–prey system. In this study, we incorporate anti-predator behaviors into a Lotka–Volterra predator–prey model in the form of fixed and facultative vigilance. Facultative vigilance models behavior as a dynamic foraging game, allowing us to assess optimal behavioral responses in the context of IBPs using a dynamical fitness optimization approach. We model vigilance as a tradeoff between safety and either the prey's maximum growth rate or its carrying capacity. We assess the population dynamics of predators and prey with fear responses, and investigate the role fear plays on trophic structure. We found that the ecology of fear plays an important role in predator–prey systems, impacting trophic structure and the occurrence of IBPs. Fixed vigilance works against IBP structure by always reducing the predator–prey biomass ratio at equilibrium with increasing levels of vigilance. Facultative vigilance can actually promote IBPs, as prey can now adjust their vigilance levels to cope with increased predation and the costs associated with vigilance. This is especially true when the effectiveness of vigilance is low and predators are very lethal. In general, these trends are true whether the costs of vigilance are felt on the prey's maximum growth rate or its carrying capacity. Just as the ecology of fear, when first introduced, was used to explain why top carnivores are rare in terrestrial systems, it can also be used to understand how big fierce predators can be common in IBPs.