Functionally Referential Alarm Calls in Tamarins (Saguinus fuscicollis and Saguinus mystax) – Evidence from Playback Experiments

Studies on primate vocalisation have revealed different types of alarm call systems ranging from graded signals based on response urgency to functionally referential alarm calls that elicit predator‐specific reactions. In addition, alarm call systems that include both highly specific and other more unspecific calls have been reported. There has been consistent discussion on the possible factors leading to the evolution of different alarm call systems, among which is the need of qualitatively different escape strategies. We studied the alarm calls of free‐ranging saddleback and moustached tamarins (Saguinus fuscicollis and Saguinus mystax) in northeast Peru. Both species have predator‐specific alarm calls and show specific non‐vocal reactions. In response to aerial predators, they look upwards and quickly move downwards, while in response to terrestrial predators, they look downwards and sometimes approach the predator. We conducted playback experiments to test if the predator‐specific reactions could be elicited in the absence of the predator by the tamarins’ alarm calls alone. We found that in response to aerial alarm call playbacks the subjects looked significantly longer upwards, and in response to terrestrial alarm call playbacks they looked significantly longer downwards. Thus, the tamarins reacted as if external referents, i.e. information about the predator type or the appropriate reaction, were encoded in the acoustic features of the calls. In addition, we found no differences in the responses of S. fuscicollis and S. mystax whether the alarm call stimulus was produced by a conspecific or a heterospecific caller. Furthermore, it seems that S. fuscicollis terrestrial alarm calls were less specific than either S. mystax terrestrial predator alarms or either species’ aerial predator alarms, but because of the small sample size it is difficult to draw a final conclusion.     

Effects of stoat's presence and auditory cues indicating its presence on tree seedling predation by meadow voles

Predators may control the impact of herbivores on their plant resources by 1) decreasing herbivore numbers, 2) imposing predation risk affecting foraging behavior. The goal of the present study was to examine the effects of a predator and auditory cues indicating its presence on the rate of tree seedling (Acer rubrum, Betula lenta) consumption by meadow voles (Microtus pennsylvanicus). The first of our experiments involved introduction of a stoat (Mustela erminea) into an enclosed vole population and the second a playback of recordings of vole distress calls, movements of a stoat and its vocalizations. In both experiments we manipulated vegetation cover and the availability of food next to the experimental seedlings to assess the effects of microhabitat under the different risk situations. The results of the first unreplicated experiment suggested an increased rate of seedling predation in the presence of the stoat. Consistent with these results, the playback of predator sounds in the second replicated experiment caused an increased rate of seedling predation compared to control plots with no recordings. A mowed circle around a seedling station, representing increased risk of predation on the voles, decreased seedling consumption. This effect was modest in the playback treatment. We suggest the results to be due to displacement behavior by the voles exposed to prolonged risk and conflicting demands of foraging and avoiding predators. Alternatively, as suggested by the model of Lima and Bednekoff, prolonged risk of predation forced the voles to decrease their levels of vigilance during low‐risk playback breaks. The modest inhibitory effect of cover removal on seedling predation in the playback treatment is consistent with this interpretation. The results confirm recent evidence for trophic cascades mediated by behavioral interactions between predator and prey. They are novel in suggesting that the presence of predation risk can increase the inhibitory effects of consumers on their resources.     

Model vs. playback experiments: The impact of sensory mode on predator-specific escape responses in saki monkeys

Although experimentally simulating predator presence helps improve sample sizes in studies of free-ranging animals, few studies have examined whether auditory playbacks and visual models produce similar results. Additionally, it is unclear if anti-predator strategies are specific to predator hunting styles in understudied Neotropical pitheciid primates, limiting what we can generalize about this phenomenon across this taxonomic order. We conducted predator simulation experiments to assess whether wild Rylands' bald-faced saki monkeys (Pithecia rylandsi) recognize predators based solely on acoustic cues, exhibit predator-specific responses to different predator types, and vary responses to presentations in different sensory modes. In our playback experiments, sakis had weak responses to non-predator control vocalizations compared to jaguar growls and harpy eagle shrieks. In most predator playbacks, subjects' first glance corresponded to the direction from which simulated predators would typically attack (above vs. below). However, although sakis exhibited appropriate movement responses to harpy playbacks (i.e., descending canopy), they exhibited no clear movement patterns when presented with jaguar playbacks. In contrast, jaguar model experiments consistently elicited fast approaches, mobbing-style responses, and long alarm calling bouts. Thus, if we had relied on playbacks alone, we might have concluded that sakis have only generalized responses to terrestrial ambush predators. In fact, in all variables measured (e.g., latency, number of calls, and response duration), models of both predator species elicited stronger reactions than playbacks. Results indicate that bald-faced sakis can identify predators based solely on vocalizations, but do not exhibit predator-specific escape responses to terrestrial predators based on acoustic cues alone. The differential response to playbacks and models calls into question the reliability of using acoustic-only stimuli to assess the specificity of anti-predator behavior to predator hunting styles in some primate species.     

Non‐Selective and Time‐Dependent Behavioural Responses of Common Toads (Bufo bufo) to Predator Acoustic Cues

Acoustic predator recognition has rarely been studied in anurans, in spite of the fact that hearing is widespread in these animals and that it has been demonstrated to play an important role in both arthropods and other vertebrates. Using field playback experiments, we tested the hypothesis that adult common toads (Bufo bufo) are capable of recognizing natural vocalizations of a common predator, the Eurasian otter (Lutra lutra), and show antipredator responses. We found that toads exposed to both natural (two types of otter sounds) and synthetic stimuli [white noise (WN) and otter sound‐based amplitude modulated WN] increased time allocated to locomotion and escape behaviour. These responses were correlated with time elapsed from sunset to the onset of testing and were independent from the type of acoustic signal, toad features and other environmental factors monitored. We conclude that B. bufo has not developed a selective recognition of predator vocalizations, suggesting that low‐frequency or seismic sounds associated with predator movements may provide anurans with better cues about an approaching risk. We propose that the time‐dependent response to acoustic stimuli of common toads represents a case of threat‐sensitivity and demonstrates that it can occur even when the response to the threat is not predator specific.     

Anthropogenic noise compromises antipredator behaviour in European eels

Increases in noise‐generating human activities since the Industrial Revolution have changed the acoustic landscape of many terrestrial and aquatic ecosystems. Anthropogenic noise is now recognized as a major pollutant of international concern, and recent studies have demonstrated impacts on, for instance, hearing thresholds, communication, movement and foraging in a range of species. However, consequences for survival and reproductive success are difficult to ascertain. Using a series of laboratory‐based experiments and an open‐water test with the same methodology, we show that acoustic disturbance can compromise antipredator behaviour – which directly affects survival likelihood – and explore potential underlying mechanisms. Juvenile European eels (Anguilla anguilla) exposed to additional noise (playback of recordings of ships passing through harbours), rather than control conditions (playback of recordings from the same harbours without ships), performed less well in two simulated predation paradigms. Eels were 50% less likely and 25% slower to startle to an ‘ambush predator’ and were caught more than twice as quickly by a ‘pursuit predator’. Furthermore, eels experiencing additional noise had diminished spatial performance and elevated ventilation and metabolic rates (indicators of stress) compared with control individuals. Our results suggest that acoustic disturbance could have important physiological and behavioural impacts on animals, compromising life‐or‐death responses.     

Simulated Birdwatchers’ Playback Affects the Behavior of Two Tropical Birds

Although recreational birdwatchers may benefit conservation by generating interest in birds, they may also have negative effects. One such potentially negative impact is the widespread use of recorded vocalizations, or “playback,” to attract birds of interest, including range-restricted and threatened species. Although playback has been widely used to test hypotheses about the evolution of behavior, no peer-reviewed study has examined the impacts of playback in a birdwatching context on avian behavior. We studied the effects of simulated birdwatchers’ playback on the vocal behavior of Plain-tailed Wrens Thryothorus euophrys and Rufous Antpittas Grallaria rufula in Ecuador. Study species’ vocal behavior was monitored for an hour after playing either a single bout of five minutes of song or a control treatment of background noise. We also studied the effects of daily five minute playback on five groups of wrens over 20 days. In single bout experiments, antpittas made more vocalizations of all types, except for trills, after playback compared to controls. Wrens sang more duets after playback, but did not produce more contact calls. In repeated playback experiments, wren responses were strong at first, but hardly detectable by day 12. During the study, one study group built a nest, apparently unperturbed, near a playback site. The playback-induced habituation and changes in vocal behavior we observed suggest that scientists should consider birdwatching activity when selecting research sites so that results are not biased by birdwatchers’ playback. Increased vocalizations after playback could be interpreted as a negative effect of playback if birds expend energy, become stressed, or divert time from other activities. In contrast, the habituation we documented suggests that frequent, regular birdwatchers’ playback may have minor effects on wren behavior.     

Avian biodiversity in the coastal wetlands of the Okahukura Peninsula

The Tapora Landcare Group, operating on the Okahukura Peninsula, has the long-term goal of making this region predator fenced. The aim of this study was to obtain information on the current status of avian biodiversity and the bird community across the band of coastal wetlands on the Okahukura Peninsula. Bird counts were conducted and playback lures used to detect three cryptic wetland species: fernbirds (Bowdleria punctata); spotless crakes (Porzana tabuensis); and banded rails (Gallirallus philippensis). Fernbirds and banded rails were detected at seven of the eight wetland sites sampled whereas spotless crakes were detected at two sites. The native species with the highest relative abundance across the eight sites were silvereyes (Zosterops lateralis) and South Island pied oystercatchers (Haematopus finschi). Changes in avian biodiversity over time in the region can now be monitored, and comprehensive long-term data on the status of avian biodiversity over time obtained.     

Mobbing behaviour in a passerine community increases with prevalence in predator diet

Mobbing behaviour against predators is well documented but less is known about the factors influencing variation in behavioural response between prey species. We conducted a series of playback experiments to examine how the mobbing responses of prey species differed according to their relative risk of predation by the Eurasian Pygmy Owl Glaucidium passerinum, a predator of passerines. We found that mobbing among 22 passerine prey species was positively correlated with their prevalence in the Pygmy Owl diet. To compare mobbing behaviour between two seasons, we conducted playback experiments during spring (breeding season) and autumn (non‐breeding season). Contrary to previous studies, we found that mobbing intensity was greater during autumn than in spring. Our study shows a differential mobbing response of 22 species to the calls from one predator species and underscores the importance of considering seasonal variation in mobbing behaviour. Mobbing response differences observed among bird species strongly suggest different cooperation behaviour at the community level.     

Prey Responses to Predator's Sounds: A Review and Empirical Study

Many animals assess their risk of predation by listening to and evaluating predators' vocalizations. We reviewed the literature to draw generalizations about predator discrimination abilities, the retention of these abilities over evolutionary time, and the potential underlying proximate mechanisms responsible for discrimination. Broadly, we found that some prey possess an ability to respond to a predator after having been evolutionarily isolated from a specific predator (i.e., predators are allopatric) and that some prey are predisposed to respond to certain types of predators that they coevolved with but without having ecological experience. However, these types of studies are lacking, and relatively, few studies have examined predator discrimination abilities in ungulates. To begin addressing these knowledge gaps, we performed field experiments on Mule deer (Odocoileus hemionus) in which we investigated the ability of deer to discriminate among familiar predators [coyotes (Canis latrans) and mountain lions (Puma concolor)] and an evolutionary relevant predator with which deer have had no recent exposure [locally extinct wolves (Canis lupus)]. We found that Mule deer respond to and discriminate among predators based on predator vocalizations and have retained an ability to respond to wolves that have been extinct from the study area since the early 20th century. Previous playback studies have shown that responses vary among human‐habituated and non‐habituated populations and differ according to human proximity. Deer greater than 0.5 km from human residences allocated more time to heightened responses both before and after stimulus playback. Our findings may help predict how prey–predator interactions may change as a result of the recovering wolf population with a basis in ecological and evolutionary experience in predator discrimination and desensitization.     

Eavesdropping of an African ground squirrel on the heterospecific alarm calls of a noisy ground-nesting bird

Animals gather information about their environment from a variety of sources to enable adaptive decision-making behaviour. Eavesdropping on heterospecific alarm calls enhances predator avoidance, reduces time spent vigilant and allows for more time on daily activities such as foraging. If the information is relevant and reliable, individuals that respond to heterospecific signals may benefit from a wider range of information at a low marginal cost. The Cape ground squirrel (Xerus inauris) and crowned lapwing (Vanellus chilensis) are ground-dwelling species that are taxonomically distant but share similar predators, habitat and anti-predatory behaviours. We used playback experiments of the alarm calls produced by conspecifics and lapwings to investigate the vigilance responses of adult female Cape ground squirrels. Squirrels responded with greater vigilance to both squirrel and lapwing alarm calls, and no changes of vigilance levels were observed in response to a control sound. However, contrary to our predictions, changes in vigilance and time to relax did not differ between conspecific versus heterospecific playbacks. The results from our study suggest that squirrels perceive lapwing alarm calls as relevant and reliable information and that responding to it could increase their survival.     

Singing in the Face of Danger: the Anomalous Type II Vocalization of the Splendid Fairy-Wren

Males of certain species of fairy-wrens (Aves: Maluridae) emit a unique vocalization, the Type II vocalization, in response to the calls of potential predators. We conducted field observations and playback experiments to identify the contexts in which the Type II vocalization is emitted by splendid fairy-wren ( Malurus splendens ) males, and to examine social and genetic factors that influence its occurrence. In field observations and controlled playback experiments, Type II vocalizations were elicited most consistently by calls of the predatory gray butcherbird ( Cracticus torquatus ). Some vocalizations from other avian species also elicited Type II vocalizations, and the majority of these were vocalizations from avian predators. Splendid fairy-wrens are cooperative breeders, and males that responded with Type II vocalizations to playbacks of butcherbird calls tended to be primary rather than secondary males, had larger cloacal protuberances, and were older than those that did not respond. In addition, secondary males that were sons of resident females were more likely than non-sons to respond with a Type II vocalization. In another playback experiment, females responded similarly to the Type I song and Type II vocalizations of their mates. Although the Type II vocalization is emitted primarily in response to predator calls, it is inconsistent with an alarm call explanation. Patterns of reproductive success among Type II calling males suggest that it does not function as an honest signal of male quality. At present, the function of the vocalization remains anomalous, but indirect fitness benefits may play a role in its explanation.     

Experimental evidence for edge-related predation in a fragmented agricultural landscape

Abstract This study tested the hypothesis that increased predation of experimental nests occurs close to a forest edge in a fragmented agricultural landscape. Artificial nests and eggs of willie wagtails Rhipidura leucophrys and superb fairy-wrens Malurus cyaneus were used in experiments to assess the extent and nature of predation occurring throughout the known breeding seasons of these species. Predators were identified by the imprints they left in plasticine eggs, and by remote photography. Surveys of avian predators were undertaken to investigate the relationship between predation intensity and predator distribution and abundance. Avian predators accounted for almost all predation for which a predator could be identified (96%). Five of seven predator species photographed attacking wagtail nests were corvids or artamids. Fairy-wren nests suffered relatively low rates of predation (29%) compared to wagtail nests (87%). Increased predation at the habitat edge was recorded for wagtail nests only; predation was correlated with the distribution and abundance of predatory avian species. The different extent and pattern of predation on fairy-wren nests could be explained by problems in detecting predation by mammals, and by possible failure of avian predators to locate the cryptic nests.     

Corn bunting (<Emphasis Type="Italic">Miliaria calandra</Emphasis>) males respond differently to alternating and overlapping playback of song

Interactive playback experiments were used to study the signal value to the corn bunting, Miliaria calandra, of alternating and overlapping singing. We subjected 15 males to two stimuli that differed in the temporal pattern of song playback (alternating or overlapping). We measured eight characteristics of the males’ response in two categories—song output and movements. Overlapping and alternating playback elicited a similar song response, characteristic of highly aroused males. Song response correlated positively with males’ singing activity before playback, irrespective of stimulus. There were significant differences between latency of approach to the loudspeaker and number of flights. Birds approached the loudspeaker more quickly and spent more time close to it when playback alternated with their songs. The results suggest overlapping song could be interpreted as a stronger threat but elicits a more cautious, rather than stronger, response than the alternating pattern. Males were found to shorten songs during the playback compared with songs sung before and after stimulation. The only predictor of degree of song shortening was song activity before the playback began. It should, therefore, be regarded as a signal which is related to escalated, close-distance counter-singing.     

Predation as a cost of sexual communication in nocturnal seabirds: an experimental approach using acoustic signals

Conspicuous behaviour, such as sexual advertisement, exposes animals to predation; mate attraction thus often conflicts with antipredator behaviour. We investigated whether an avian predator, the brown skua, Catharacta antarctica l?nnbergi, uses the mate attraction calls of colonial seabirds, the petrels. The majority of petrels attract mates at night and vocalizations are their main way of communicating. At our study sites, skua predation on nocturnal petrels was heavy, and concentrated particularly on a single species, the blue petrel, Halobaena caerulea. Using playback experiments, we showed that skuas can use male petrel calls as a cue for prey location and selection. This listening behaviour of skuas probably imposes a major constraint on advertising petrels, and especially on single males which face a trade-off between attracting females (which respond by calling in flight) and avoiding predation. We also investigated the consequences of this predation risk on the behaviour of petrels: a second set of playback experiments showed that the most heavily preyed on petrel species could use skua territorial calls to infer predation risk and stop calling thereafter, which may reduce conspicuousness and predation risk. Copyright 2000 The Association for the Study of Animal Behaviour.     

A Test of the Multi-Predator Hypothesis: Rapid Loss of Antipredator Behavior after 130 years of Isolation

Many species find themselves isolated from the predators with which they evolved. Isolation often leads to the loss of costly antipredator behavior, which may have adverse consequences if the population should later come into contact with predators. An understanding of both the mechanism (i.e. the degree to which antipredator behavior depends on experience), and of the time course of loss is important to be able to predict how a population will respond to future contact. We studied ‘group‐size effects’– the way in which animals change the time they allocate to antipredator vigilance as a function of group size – and visual and acoustic predator recognition in a population of tammar wallabies (Macropus eugenii), a cat‐sized (6–10 kg) macropodid marsupial. To study group size effects we observed wallabies foraging in four populations – three with some sort of predator and a New Zealand population that was isolated from all predators for about 130 yr. To study predator recognition, we observed the response of New Zealand wallabies to the presentation of a model or taxidermic mount of mammalian predators, and to the broadcast sounds of mammalian and avian predators. We compare these predator recognition experiments with results from a previous study of Kangaroo Island (South Australia) tammars. Complete isolation from all predators for as few as 130 yr led to the loss of group size effects and a rapid breakdown in visual predator recognition abilities. Our results are consistent with a key prediction of the multi‐predator hypothesis – namely, that the isolation from all predators may lead to a rapid loss of antipredator behavior.     

Observational and Experimental Evidence for the Function of Tail Flicking in Eurasian Moorhen Gallinula chloropus

Tail movements such as wagging, flicking or pumping are reported from many bird species but their adaptive functions remain poorly understood. To investigate whether tail flicking functions as an alarm signal, either to predators or neighbouring birds, or as a signal of submission to conspecifics, I observed this behaviour in moorhen in a natural context, and conducted playback experiments using vocalizations of predators, conspecifics and heterospecifics. I found positive relationships between flicking and vigilance and nearest neighbour distance, and negative relationships between flicking and moorhen flock size and total flock size. Moorhen at the edge of a flock flicked at a higher rate. Single moorhen flicked more often compared with individuals in groups, both in single‐ and mixed‐species flocks, and there was a tendency that single moorhen flicked more often than single moorhen within a mixed‐species flock. Moorhen responded differently to conspecific and predator calls. While in both cases vigilance increased, tail flick rate was higher during predator playbacks and lower during conspecific playbacks. Furthermore, moorhen remained rather motionless when conspecific calls were played back, but not during predator calls, and, moorhen resumed to a baseline level of tail flicking more quickly after the playback of conspecific calls. Taken together, the results suggest that flicking may be considered as a honest signal of vigilance directed towards ambushing predators.     

Analysis of a Blainville's beaked whale's movement response to playback of killer whale vocalizations

Increasing evidence links exposure to Navy sonar with certain mass stranding events of deep diving beaked whales. Although the cause of these strandings is unknown, one theory suggests that the animals confuse the sonar signals with vocalizations of killer whales, a known predator. Here we analyze the movement patterns of a tagged female Blainville's beaked whale in reaction to playback of killer whale predation calls. During a deep foraging dive, the whale was exposed to a playback of killer whale vocalizations with the source level slowly increased until the whale prematurely ceased foraging. The heading data from the tag were analyzed using a rotation test with a likelihood ratio calculated for a nonparametric kernel density estimate. We found a significant difference (P < 0.005) in the distribution of Δheading (the change in heading averaged over 200 s) after the cessation of the killer whale playback. A test of the angular standard deviation (SD) of the Δheading showed that after the playback, the SD was significantly reduced (P = 0.0064), which indicates that the animal maintained a straighter than normal course for an extended period of time. The prolonged directed avoidance response observed here suggests a behavioral reaction that could pose a risk factor for stranding.     

Anti‐Predator Behaviour in a Nocturnal Primate,the Grey Mouse Lemur (Microcebus murinus)

Although one‐third of all primates are nocturnal, their anti‐predator behaviour has rarely been studied. Because of their small body size, in combination with their solitary and nocturnal life style, it has been suggested that they mainly rely on crypsis to evade predators. However, recent studies revealed that nocturnal primates are not generally cryptic and that they exhibit predator‐specific escape strategies as well as alarm calls. In order to add to this new body of research, we studied anti‐predator strategies of nocturnal grey mouse lemurs experimentally. In order to elicit anti‐predator behaviour and alarm calls, we conducted experiments with a carnivore‐, snake‐ and raptor model. We also conducted playback experiments with mouse lemur alarm calls to characterize their function. In response to predator models, they exhibited a combination of anti‐predator strategies: in response to carnivore and snake models, mouse lemurs monitored the predator, probably to assess the potential risk that emanates from the predator. In response to raptor models they behaved cryptically and exhibited freezing behaviour. All mouse lemurs, except one individual, did not alarm call in response to predator models. In addition, during playback experiments with alarm calls, recorded during real predator encounters, mouse lemurs did not emit alarm calls nor did they show any escape behaviour. Thus, as in other nocturnal primates/mammals, mouse lemurs do not seem to rely on routinely warning of conspecifics against nearby predators.     

Short-Term Behavioural Responses of Impalas in Simulated Antipredator and Social Contexts

Prey animals often have to trade off foraging against vigilance. However, vigilance is costly and individuals are expected to adjust their vigilance and its cost in relation to social cues and their predation risk. To test this, we conducted playback experiments in the field to study how lions’ (Panthera leo) roars and male impalas’ (Aepyceros melampus) territorial vocalizations affected the vigilance and foraging behaviours as well as movements of female impalas. Our results show that impalas adjusted their activities in different ways depending on the vocalizations broadcast. After lions’ roars were played, female impalas increased their vigilance activity (in particular increasing their high-cost vigilance – vigilance without chewing), decreased their bite rates and increased their movements, whereas male impalas’ vocalizations caused females to decrease their vigilance (decreasing their low-cost vigilance – vigilance while chewing) and increase their movements without affecting their bite rates. Therefore, it appears that predators’ vocalizations stimulate anti-predator behaviours such as vigilance and movement at the expense of foraging, whereas males’ vocalizations increase individuals’ displacements at the expense of vigilance. Overall, this study shows that both predator and social cues have direct effects on the behaviour of gregarious prey and need to be considered in future studies.     

Parent birds assess nest predation risk: influence of cavity condition and avian nest predator activity

Skutch hypothesized that nest predators visually assess parental activities to locate a prey nest, whereas parents modify fitness‐related traits to reduce the probability of nest predation. We examined how cavity condition and parental activity interact with avian nest predators to shape the nest success of two coexisting parid species, marsh tits Poecile palustris and oriental tits Parus minor, breeding in nest‐boxes during the incubation period. Nest‐boxes were manipulated to create a prolonged risk of nest predation (entrance diameter 2.6 cm control vs 5.5 cm treatment) soon after clutch completion. To measure changes in parental behavior, we also simultaneously simulated a pulsed risk of nest predation, using sound playbacks of a coexisting control bird and an avian nest predator. We found that the parent tits merely responded the pulsed risk, presumably due to an environment with high avian nest predator encounters, compared to the prolonged risk. Instead, both species spent more time on vigilance at the nest, only under prolonged risk conditions. The activity of corvids near the nest‐box was higher in the marsh tit than that in oriental tits. This activity was also higher in the treatment nest box than that in the control nest‐box. Nest predation during the incubation period was higher in marsh tits than in oriental tits, presumably due to higher and more plastic vigilance in oriental tits, compared to marsh tits. Our results highlight that the differences in cavity condition and parental activities at the nests of two coexisting non‐excavators may contribute to differential nest predation by attracting avian nest predators.