Investigating a link between bill morphology, foraging ecology and kleptoparasitic behaviour in the fork-tailed drongo

Despite the prevalence of vocal mimicry in animals, few functions for this behaviour have been shown. I propose a novel hypothesis that false mimicked alarm calls could be used deceptively to scare other species and steal their food. Studies have previously suggested that animals use their own species-specific alarm calls to steal food. However none have shown conclusively that these false alarms are deceptive, or that mimicked alarm calls are used in this manner. Here, I show that wild fork-tailed drongos (Dicrurus adsimilis) make both drongo-specific and mimicked false alarm calls when watching target species handling food, in response to which targets flee to cover abandoning their food. The drongo-specific and mimicked calls made in false alarms were structurally indistinguishable from calls made during true alarms at predators by drongos and other species. Furthermore, I demonstrate by playback experiments that two of these species, meerkats (Suricata suricatta) and pied babblers (Turdoides bicolor), are deceived by both drongo-specific and mimicked false alarm calls. These results provide the first conclusive evidence that false alarm calls are deceptive and demonstrate a novel function for vocal mimicry. This work also provides valuable insight into the benefits of deploying variable mimetic signals in deceptive communication.

Kleptoparasitism by attacks versus false alarm calls in fork-tailed drongos

Kleptoparasitism is a tactic used to acquire food opportunistically and has been shown to provide several benefits, including greater food intake rate and the acquisition of items not normally available during self-foraging. Host individuals may differ in their ability to defend themselves against kleptoparasitic attacks and therefore identifying those host individuals that are particularly vulnerable to attack could both provide energetic benefits and increase the efficiency of kleptoparasitism as a foraging strategy. Here, we show that the kleptoparasitic fork-tailed drongo (Dicrurus adsimilis) specifically targets juveniles when following groups of cooperatively breeding pied babblers (Turdoides bicolor). Drongos give alarm calls upon sighting a predator, thus providing extra predator vigilance to foraging pied babblers. However, drongos also use alarm calls to steal food items. During kleptoparasitic attacks, drongos give false alarm calls and then swoop down to steal food items dropped by alarmed babblers. Juvenile pied babblers are particularly vulnerable to attack because they (a) spend a longer period handling prey items prior to consumption and (b) respond to alarm calls primarily by immediately moving to cover, in contrast to adults who respond by looking up and visually scanning the surrounding area. Drongos attack juvenile babblers significantly more often than adults, with attacks on juveniles more likely to result in the successful procurement of a food item. This patterns of attack suggests that drongos are able to differentiate between individuals of different age when targeting pied babblers, thus increasing the efficiency of kleptoparasitism as a foraging strategy.

Great tits Parus major regularly gave alarm calls in winter without the presence of actual or potential predators. Such false alarm calls were given to deceive both conspecifics and heterospecifics. False calls were given when flock‐feeding sparrows Passer spp. monopolized a concentrated food source; if the food resource was dispersed false alarm calls were not used, independently of the presence of sparrows. Dominant great tits used alarm calls deceptively if a dominant conspecific was present on a concentrated food source but not if a subdominant individual was present; subdominant great tits were displaced by means of threat displays. Subdominant individuals gave false alarm calls both if dominant or subdominant conspecifics were present. False alarm calls were especially used when food was scarce or when great tits were feeding at a high rate, i.e. during snow storms and in the morning and the afternoon.

In many cases of interspecific kleptoparasitism, hosts develop defensive behaviors to minimize the impact of kleptoparasites. Because vigilance and defensive behaviors are often costly, selection should favor hosts that adjust the amount of investment needed to minimize losses to kleptoparasitism. However, examples of such facultative responses are rare. Here, we investigate the response of cooperatively breeding pied babblers (Turdoides bicolor) to the drongo (Dicrurus adsimilis), an avian kleptoparasite that regularly follows pied babbler groups, often giving alarm calls to alert the group to predators but also occasionally giving false alarm calls in order to steal food items. We show that pied babbler response to drongos varies markedly according to babbler group size. In small groups, where there are few individuals available to act as sentinels, babblers sentinel less when a drongo is present and respond strongly to drongo alarm calls. However, in large groups, where there are many individuals available to participate in predator vigilance, babblers sentinel more often when a drongo is present, rarely respond to drongo alarm calls, and aggressively displace drongos, with a consequent decline in the number of successful kleptoparasitism events. This behavior represent a facultative response to a kleptoparasite according to the costs versus benefits of tolerating their presence.

Audience effects are increasingly recognized as an important aspect of intraspecific communication. Yet despite the common occurrence of interspecific interactions and considerable evidence that individuals respond to the calls of heterospecifics, empirical evidence for interspecific audience effects on signalling behaviour is lacking. Here we present evidence of an interspecific audience effect on the alarm-calling behaviour of the kleptoparasitic fork-tailed drongo (Dicrurus adsimilis). When foraging solitarily, drongos regularly alarm at aerial predators, but rarely alarm at terrestrial predators. In contrast, when drongos are following terrestrially foraging pied babblers (Turdoides bicolor) for kleptoparasitic opportunities, they consistently give alarm calls to both aerial and terrestrial predators. This change occurs despite no difference in the amount of time that drongos spend foraging terrestrially. Babblers respond to drongo alarm calls by fleeing to cover, providing drongos with opportunities to steal babbler food items by occasionally giving false alarm calls. This provides an example of an interspecific audience effect on alarm-calling behaviour that may be explained by the benefits received from audience response.

False alarm calls are a substantial resource allocation problem for police administrators even though the issue tends not to be high profile. A panel design is utilized to describe how one municipal fine schedule affects the generation of false security alarm calls for police service by residential, commercial, and nonprofit property types. It was found that property type accounts for12% of the variation in annual false alarm generation and that, on average, false alarm production decreased substantially over time.

Feeling anxious? The mechanisms of vocal deception in tufted capuchin monkeys

Food theft by deceptive alarm calls in the fork-tailed drongo

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.

The suggestion that some members of mixed-species bird flocks use alarm calls when predators are not present in order to startle other species and thereby gain access to additional prey, first postulated by Munn (Munn CA. 1986. Birds that 'cry wolf'. Nature. 391:143--145.), has generated considerable interest due to its implication that the calling birds are intentionally deceiving listeners. Despite this interest, "false alarms" have been studied rarely and without detailed acoustical analysis. We explored whether false alarms are used by Greater Racket-tailed Drongos, which produce a distinctive set of notes when alarmed. We found that drongos did indeed make false alarms, defined as an alarm vocalization made either while the bird was on the perch or in the air, and followed within 10 s by a foraging attempt. However, the acoustic features of the call notes used, particularly of those calls made in the air, were more similar to aggressive calls, made when drongos chased each other, than to actual alarms produced when predators were present. Drongo foraging success was greater after false alarm calls than after silence or nonalarm vocalizations, and playback of false alarms in the air induced escape behavior in other species, though at a lower level than actual alarms. Thus, although drongos can use false alarms to startle other birds and gain foraging opportunities, such calls cannot be called "false" with certainty because they may also signal aggressive intent. Indeed, aggression and alarm may be intertwined in this family of birds as drongos actively mob or chase many predators, including bird-eating hawks. Copyright 2010, Oxford University Press.

Summary Most small birds inhabiting temperate latitudes in the Holarctic increase basal metabolic rate (BMR) in winter, a pattern thought to reflect the up‐regulation of metabolic machinery required for enhanced winter cold tolerance. In contrast, patterns of seasonal BMR variation in birds inhabiting subtropical latitudes are largely unknown. In this study, we investigate seasonal BMR changes in species from subtropical latitudes, and analyse global variation in the direction and magnitude of these responses. We estimated winter and summer BMR in five species resident in the Kalahari Desert, using flow‐through respirometry to measure O2 consumption and CO2 production in birds held overnight in a field laboratory. In all five species, mass‐specific BMR was significantly lower in winter than in summer, with mean reductions of 23% in African scops‐owls (Otus senegalensis), 30% in pearl‐spotted owlets (Glaucidium perlatum), 35% in fork‐tailed drongos (Dicrurus adsimilis), 29% in crimson‐breasted shrikes (Laniarius atrococcinneus), and 17% in white‐browed sparrow‐weavers (Plocepasser mahali). An analysis of global variation in seasonal BMR changes reveals that their magnitude and direction vary with latitude, ranging from pronounced winter increases at high latitudes where winters are extremely cold, to the opposite pattern in warmer, subtropical environments. Our empirical results for five species, taken together with the analysis of global variation, are consistent with the hypothesis that winter metabolism in subtropical environments is driven primarily by the need for energy and/or water conservation rather than cold tolerance.

ABSTRACT. Planktonic sarcodines, suspended in the water column, are conveniently grouped into three categories based on functional morphology: (1) gymnamoebae and their relatives, which lack major living or nonliving compartmentalizing barriers, (2) foraminifera, and testate amoebae enclosed by a test or shell with one or more major openings, but lacking extensive cytoplasmic compartmentalizing barriers, and (3) radiolaria, which exhibit distinct compartmentalization of the cytoplasm into functional zones. Differences in feeding strategies and trophic activity of members in the three groups reflect in part these differences in functional morphology. Members of all three groups form symbiotic associations with Monera and protists, including algae, thus partially offsetting interspecific trophic competition among species occupying the same water mass. Physiological and morphological adaptations supporting a symbiotic association are presented. C14‐labeling studies of endosymbiotic radiolarian species show a substantial contribution of carbon to the host. Rates of calcification (planktonic foraminfera) and silica deposition (radiolaria) are reported, based on morphometric analyses and isotopic labeling studies. Major distributional patterns in space and time for each of the three groups, and some ecological principles explaining these regularities, are presented as related to population growth dynamics, niche differentiation, water‐mass properties, and the role of symbionts in supporting highly diverse communities of species within the same locale in the water column.

We describe interspecific and intraspecific kleptoparasitic behaviour in Mallards Anas platyrhynchos, attempting to steal Zebra Mussels Dreissena polymorpha from other Mallards and Eurasian Coots Fulica atra. Both Coots and Mallards were most often attacked by Mallards perpetrating kleptoparasitic attacks when they handled large or intermediate-sized prey items. The probability of attack was nearly halved when the foraging birds had small versus large prey items in their bills. The overall probability of success of a kleptoparasitic attack was lowest when the attacked birds had small prey items, but higher if they had intermediate or large prey items. Kleptoparasitic attacks carried out on Coots were more often successful than those on Mallards. The results suggest that the optimal foraging theory, where animals tend to maximize net benefits, may be applicable to kleptoparasitic behaviour.

Birds evolved a diversity of head shapes, feeding behaviors and jaw muscles which drive the feeding apparatus. However, capturing the anatomical and functional diversity of these muscles remains challenging and we know little about how jaw muscles develop, function and evolve among lineages of the clade and this hampers our ability to understand skeletal tissue biology, joint function, and patterns of evolution in bird skulls. Here we use contrast imaging, 3D muscle fiber tracking and new data visualization methods to explore the morphology, architecture and functional significance of the jaw muscles of a sample of birds to highlight the potential of these approaches. First, we show differences in functional morphology of jaw muscles between a hatchling and adult Mallard duck (Anas platyrhynchos). Ducks show a marked change in adductor chamber shape during ontogeny including an elongated postorbital process, quadrate orbital process and retroarticular process along with a broad temporal fossa which likely track with changes in muscle function and cranial kinesis. Indeed, we found m. depressor mandibulae in baby and adult ducks shifts from a rostrocadual orientation to a more dorsoventral orientation that offers additional mechanical advantage to jaw opening and propalinal mandible movement as well as concomitant changes in the 3D resultant and pennation of other jaw muscle bellies. Second, we show differences between a long‐ and short‐faced, dabbling birds (mallard, green‐wing teal), long faced orthally‐biting bird (belted kingfisher) and a short‐faced, hard‐biting bird, the grey parrot (Psittacus erithacus). We expected position, architecture and biomechanics of homologous muscles to differ substantially between these avian species given their different behaviors and overall cranial morphology. Although some muscles showed marked differences in resultant and pennation, others, including protractor pterygoideus, the primary actuator of kinesis, were remarkably similar in morphology suggesting a mosaic of changes happening during avian evolution. Together, these new, 3D high fidelity anatomical data on jaw muscle functional morphology in birds will not only better illuminate ecomorphological evolution but also illustrate how other clades of birds and vertebrates adapt their muscle architecture to meet functional demands.