Exposure affects the risk of an owl being mobbed – experimental evidence

Mobbing is a widespread anti‐predator strategy in birds, and predators are generally expected to avoid mobbing. For example, observational studies suggest that the cryptic roosting behaviour of nocturnal predators, such as many owls, may be a strategy to limit mobbing. In this paper, we present the results of the first experimental study investigating to what degree roost exposure influences the risk of being mobbed, and the intensity of a mobbing incidence once initiated. To determine these factors, we used an experimental setup with taxidermic mounts of tawny owls Strix aluco in Grib Skov forest, Denmark. The risk of an owl being mobbed during a 50 min morning survey period increased with the exposure of its roosting position, from 24% when hidden to 85% when openly exposed. The corresponding increase in the afternoon was from 6% to 36%. This suggests that an owl may minimize the mobbing rate by reducing the encounter rate with potential mobbers through its choice of roost. Once initiated, the duration of the mobbing (a proxy for the presumed cost of being mobbed) was independent of the roosting position of the mounted owl, but was positively correlated with the number of birds in the mob.     

The Adaptive Significance of Avian Mobbing V. An Experimental Test of the ‘Move On’ Hypothesis

This paper presents the first experimental study of the effects of mobbing on predators. Two captive little owls Athene noctua and a tawny owl Strix aluco, having stabilized activity patterns and roosting sites, were singly confronted with one or two mobbing blackbirds Turdus merula. The little owls showed marked stress (gular fluttering, panting and panic flights), abandoned chosen roosts and remained unsettled for several days. A permanent alternative roost was chosen after one mobbing encounter, other encounters resulted in several days of ‘floating’ before returning to the original sites. One little owl used ‘camouflage’ by ‘freezing’ during repeated harassments, the other tended to retaliate if mobbing was close and weak. The tawny owl seemed to ignore the rather weak blackbird mobbing, but abandoned its roost for up to 36 h when mobbed by larger numbers of woodland birds at close range. It is concluded that mobbing causes profound distress, especially in small, vulnerable predators like the little owl and forces it to change its daytime roost. If abandoning the area is not possible, time and energy are expended in increased vigilance as a protection against further mobbing attacks. Thus there is strong support for the ‘Move on’ hypothesis. Results also support the ‘Perception advertisement’ hypothesis, implying that the detected hunter deliberately forfeits the surprise element. Which effect prevails depends on predator evaluation of the signals involved.     

The biological function of snake mobbing by Siberian chipmunks: II. Functions beneficial for the mobbers themselves

The functions of snake mobbing by Siberian chipmunks were studied in the context of potencial benefit to the mobbers themselves. In experiment I, the threatening effect of the mobbing was examined, and following results were obtained. The snake retreated almost always into a shelter box when it was mobbed, and the time span from the beginning of the mobbing until the completion of the snake's retreat into the box was shorter when the snake was mobbed by 2 chipmunks than when mobbed by a single one. These results indicate that mobbing has a threatening effect against the snake that increases when more chipmunks mob. In experiment II, I examined whether chipmunks get information about the snake during the mobbing. The experiment showed that the chipmunks performed tail shakings after the mobbing more frequently at large snakes than at small ones, and also indicated that mobbing functions to obtain useful information about snakes. This study clarified that the snake mobbing by Siberian chipmunks provides benefits to the mobbers themselves.     

Effects of avian mobbing on roost use and diet of powerful owls,Ninox strenua

We observed the species and numbers of mobbing birds and their effects on a large, nocturnal, bird-eating predator, the powerful owl, together with the pattern of owl predation on mobbing and non-mobbing species. Owls were mobbed on 35 occasions by seven of 44 species of forest birds at a site composed of open forest (88% by area) and rainforest (12%). The majority of bouts involved individuals of a single species, although mixed groups were observed on nine occasions. Regular mobbers were between 4 and 26% of the owls’ body weight. Owls abandoned their daytime roosts during 20% of bouts and responded by calling or actively monitoring mobbers during 54% of bouts. Mobbing appeared to explain why owls roosted in rainforest significantly more often than expected by its availability, mobbing being significantly less frequent in rainforest than in open forest. Only one mobbing species regularly occupied rainforest and the canopy of roosts in rainforest was denser than that in open forest, thus reducing the chances of an owl being detected by potential mobbers. Twelve species of forest birds were within the range of prey size of the powerful owl (75–800 g): six were mobbers and six non-mobbers. The frequency of owl predation on non-mobbers was 8.75 times that on mobbers. The species in this study took a high risk by mobbing a very large predator, but benefited by greatly reducing their chances of predation.     

Seasonality of Mobbing Intensity in the Pinyon Jay

In the wild, pinyon jays (Gymnorhinus cyanocephalus) mob owls at all times of year, but our observations of wild birds suggested that the mobbing response was strongest during the breeding season. To test this, we placed four groups of five wild caught adult pinyon jays in environmental chambers during late autumn when their testes were regressed. These birds received photoperiods of 12, 12, 13 and 14 h for 30 days prior to being exposed singly to a live great horned owl (Bubo virginianus). Three groups of five birds were maintained under natural photoperiods in outdoor aviaries and were tested with the owl in Aug., Sep. and Nov. Birds on long artificial photoperiods mobbed more strongly than birds exposed to shorter photoperiods. The jays under natural photoperiods showed the opposite response: those tested in November mobbed significantly more strongly than birds tested in August. Jays with the weakest mobbing responses were molting, and those with the strongest responses seemed to be closest to reproductive readiness. There was no correlation between gonad size and mobbing intensity, but our results suggest a strong relationship between reproductive readiness and mobbing intensity in pinyon jays.     

Mutualism,reciprocity, or kin selection? Cooperative rescue of a conspecific from a boa in a nocturnal solitary forager the gray mouse lemur

Predator mobbing is a widespread phenomenon in many taxa but the evolution of cooperative mobbing as an adaptive behavior is still subject to debate. Here, we report evidence for cooperative predator defense in a nocturnal solitarily foraging primate, the gray mouse lemur (Microcebus murinus). Several mouse lemurs mobbed a snake that held a non-related male conspecific until he could escape. Evolutionary hypotheses to explain cooperative mobbing include (1) by-product mutualism, when individuals defend others in the process of defending themselves; (2) reciprocity, where animals achieve a higher fitness when helping each other than when they do not cooperate; and (3) kin selection where animals help each other only if they share genes by common descent. Owing to the solitary activity of this species, reciprocity seems to be least likely to explain our observations. By-product mutualism cannot be ruled out entirely but, if costs of snake mobbing are relatively low, the available detailed socio-genetic information indicates that kin selection, rather than any of the other proposed mechanisms, is the primary evolutionary force behind the observed cooperative rescue.     

Exposure affects the risk of an owl being mobbed – experimental evidence

Mobbing is a widespread anti-predator strategy in birds, and predators are generally expected to avoid mobbing. For example, observational studies suggest that the cryptic roosting behaviour of nocturnal predators, such as many owls, may be a strategy to limit mobbing. In this paper, we present the results of the first experimental study investigating to what degree roost exposure influences the risk of being mobbed, and the intensity of a mobbing incidence once initiated. To determine these factors, we used an experimental setup with taxidermic mounts of tawny owls Strix aluco in Grib Skov forest, Denmark. The risk of an owl being mobbed during a 50 min morning survey period increased with the exposure of its roosting position, from 24% when hidden to 85% when openly exposed. The corresponding increase in the afternoon was from 6% to 36%. This suggests that an owl may minimize the mobbing rate by reducing the encounter rate with potential mobbers through its choice of roost. Once initiated, the duration of the mobbing (a proxy for the presumed cost of being mobbed) was independent of the roosting position of the mounted owl, but was positively correlated with the number of birds in the mob.     

Behavioral responses to conspecific mobbing calls are predator‐specific in great tits (Parus major)

When facing a predator, animals need to perform an appropriate antipredator behavior such as escaping or mobbing to prevent predation. Many bird species exhibit distinct mobbing behaviors and vocalizations once a predator has been detected. In some species, mobbing calls transmit information about predator type, size, and threat, which can be assessed by conspecifics. We recently found that great tits (Parus major) produce longer D calls with more elements and longer intervals between elements when confronted with a sparrowhawk, a high‐threat predator, in comparison to calls produced in front of a less‐threatening tawny owl. In the present study, we conducted a playback experiment to investigate if these differences in mobbing calls elicit different behavioral responses in adult great tits. We found tits to have a longer latency time and to keep a greater distance to the speaker when sparrowhawk mobbing calls were broadcast. This suggests that tits are capable of decoding information about predator threat in conspecific mobbing calls. We further found a tendency for males to approach faster and closer than females, which indicates that males are willing to take higher risks in a mobbing context than females.     

Syntax manipulation changes perception of mobbing call sequences across passerine species

Many species approach predators to harass them and drive them away. Both the intensity of this antipredator strategy and its success are positively related to the size of the group that carries out this mobbing. To recruit individuals to the mob, members of prey species produce mobbing calls. In some songbirds—the Japanese tit, Parus minor, and the southern pied babbler, Turdoides bicolor—mobbing calls are structurally complex and it has been suggested that they convey information by means of compositional syntax, when meaningful items are combined into larger units. These two species combine alert and recruitment calls into an alert and recruitment sequence when attracting conspecifics to cooperate in mobbing a predator. Whether this rudimentary, two‐call, compositional structure is used by other bird species in mobbing calls and how it can alter the ability of heterospecifics to adequately recognize mobbing calls is not well understood. Heterospecifics’ responses to mobs are critical to the success of the mobbing strategy, so it is of great importance to understand whether and how syntax influences these responses. To address these questions, we conducted two playback experiments. Firstly, we investigated whether the great tit, Parus major, extracts different meanings from different individual motifs (i.e., component calls), and from combined motifs in both natural and artificially reversed order. We found that great tits extract different meanings from the two motifs involved in mobbing calls and that they also discriminate for motif order reversal in the mobbing call sequence. Secondly, we investigated whether heterospecifics (the coal tit, Periparus ater, and the common chaffinch, Fringilla coelebs) are sensitive to syntax alteration of great tit mobbing calls. While chaffinches did not respond to great tit mobbing calls, coal tits were sensitive to mobbing call sequence reversal although they did not react in the same way as conspecific subjects. Overall, whereas our results indicate that tits are sensitive to call reversal, this is not to say that tits actually use compositional syntax to increase the information content.     

Are Blackcaps (Sylvia atricapilla) defending their nests also calling for help from their neighbours?

One hypothesized function of conspicuous mobbing of intruders by bird nest owners is to attract neighbouring birds (“calling for help” hypothesis) or third-party predators (“attract the mightier” hypothesis). These may help the nest owners by distracting and/or attacking the mobbed intruder. To date, these hypotheses have been studied solely during the mobbing of predators. Here, for the first time, I have studied mobbing attraction in the context of brood parasitism. I experimentally tested the Blackcap (Sylvia atricapilla), a small passerine with a highly aggressive and conspicuous nest defence behaviour. I elicited the aggressive responses of Blackcaps by presenting stuffed dummies of the brood parasitic Common Cuckoo (Cuculus canorus) and controls near their nests. At 32% of the nests (n = 75), the responses of the Blackcaps to dummies attracted up to 15 birds per trial from 21 passerine species. Most of the attracted birds were heterospecifics and rarely participated in mobbing; thus the “calling for help” hypothesis was not supported. No potential predators of the Cuckoo were attracted despite them living in the study area and despite prolonged mobbing by Blackcaps; thus rejecting the “attract the mightier” hypothesis. I argue that this hypothesis is unlikely to apply to typical avian predators during nest predation acts because these only last for several seconds. The number of attracted birds was a positive function of the owner’s intensity of nest defence as measured by the rates of alarm calling but not visual cues (rates of attacks). Suitable and unsuitable Cuckoo hosts did not differ in their behaviour in the vicinity of defended nests. The observed pattern of the positive correlation between the intensity of nest defence and the number of attracted birds is most likely a proximate by-product of the conspicuous nest defence by Blackcaps (but may well be adaptive for recruiting neighbours themselves). Thus, the mobbing behaviour of the Blackcap is directed towards the brood parasite and not towards other audience predators or potential recruits to the mob.     

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.     

Group Mobbing Behaviour and Nest Defence in a Cooperatively Breeding Australian Bird

Cooperatively breeding noisy miners (Manorina melanocephala) are well known in Australia for their persistent and very vocal group mobbing of heterospecifics. Here I investigated the nature of this extraordinary behaviour, in particular its role in nest defence, in a colour banded population of noisy miners in south‐east Queensland, Australia. I focused on two questions. First, did the intensity of mobbing vary according to factors such as the threat to the nest, or the ‘value’ of a clutch? Secondly, what role did group mobbing play in the success of a nest? To answer these questions, I experimentally manipulated the nest defence behaviour by placing one of three stuffed models near active noisy miner nests. The response of noisy miners to intruders was not indiscriminate. However, I found that the number of birds that mobbed a model did not simply reflect the potential threat posed. The response of noisy miners to raptors and other potential nest predators may have reflected their rarity as well as the threat posed. The number of mobbers did not vary with the age or size of a brood. In this study, the fate of nests was independent of the number of mobbers or visitors at nests. Finally, up to 80% of mobbers were never seen to make any other type of contribution to a nest, and many could not be related to the brood that they were ‘defending’. Hence, for some noisy miner ‘helpers’ the benefits that they accrued were probably not wholly dependent on the survival of the broods. I suggest that, in this gregarious species, mobbing behaviour at the nest may be a display of social status or individual quality. This hypothesis warrants further investigation.     

Wild fledgling tits do not mob in response to conspecific or heterospecific mobbing calls

Mobbing, where birds harass a predator through a combination of vocalizations and stereotyped behaviours, is an effective anti-predator behaviour for many species. Mobbing may be particularly important for juveniles, as these individuals are often more vulnerable than adults. Although the component behaviours of mobbing are often considered to be un-learned, there are few confirmatory data, and the developmental trajectory of mobbing is unknown. In this study, we tested whether conspecific or heterospecific mobbing calls initiated mobbing behaviour in juvenile Blue Tits Cyanistes caeruleus. We located wild adult and recently fledged juvenile Blue Tits and presented them with playback recordings of adult conspecific (Blue Tit) and heterospecific (Great Tit Parus major) mobbing alarm calls. Although adult birds readily mob in response to these types of playbacks, juveniles did not exhibit characteristic mobbing behaviour. Some juveniles did, however, exhibit individual components of mobbing behaviour found in mobbing, despite not producing adult-like mobbing behaviour in response to either conspecific or heterospecific playback. These results suggest that, although birds might be capable of mobbing as juveniles, the associations between the non-vocal stereotyped mobbing behaviours and mobbing calls may be learned.     

Mobbing calls signal predator category in a kin group-living bird species

Many prey species gather together to approach and harass their predators despite the associated risks. While mobbing, prey usually utter calls and previous experiments have demonstrated that mobbing calls can convey information about risk to conspecifics. However, the risk posed by predators also differs between predator categories. The ability to communicate predator category would be adaptive because it would allow other mobbers to adjust their risk taking. I tested this idea in Siberian jays Perisoreus infaustus, a group-living bird species, by exposing jay groups to mounts of three hawk and three owl species of varying risks. Groups immediately approached to mob the mount and uttered up to 14 different call types. Jays gave more calls when mobbing a more dangerous predator and when in the presence of kin. Five call types were predator-category-specific and jays uttered two hawk-specific and three owl-specific call types. Thus, this is one of the first studies to demonstrate that mobbing calls can simultaneously encode information about both predator category and the risk posed by a predator. Since antipredator calls of Siberian jays are known to specifically aim at reducing the risk to relatives, kin-based sociality could be an important factor in facilitating the evolution of predator-category-specific mobbing calls.     

Avian Mobbing Response is Restricted by Territory Boundaries: Experimental Evidence from Two Species of Forest Warblers

Predator mobbing has been viewed as an adaptation to reduce the risk of predation, however, factors influencing mobbing behaviour are still debated. We report on the results of an experiment with Dendroica caerulescens and Dendroica virens designed to determine (1) whether mobbing response by forest songbirds during the breeding season is restricted by territory boundaries, (2) the distance songbirds will move in response to anti‐predator mobbing calls, and (3) whether reproductive status, age, and time of the breeding season determine the distance moved to mob. We did not detect an effect of reproductive status, age, or time of breeding season on the distance moved by birds to mob. All birds responded to the mobbing playback within their territory (defined by territorial defence in relation to specific song playbacks). The maximum distance moved within a territory to engage in mobbing ranged from 25 to 175 m ( = 72 ± 6 m). Three of 37 birds responded to playbacks outside their territory boundaries. In all three cases, maximum movement distances outside territories were short (25 m). Thus, for two species of warblers, mobbing is highly constrained by territory boundaries during the breeding season. This finding is congruent with arguments that mobbing is primarily a selfish behaviour, at least with respect to conspecifics. Our results also provide support for the ‘move‐on’ hypothesis.     

Risky Business? Lethal Attack by a Jaguar Sheds Light on the Costs of Predator Mobbing for Capuchins (Cebus capucinus)

While some primates attempt to avoid predation by fleeing or hiding, others actually approach, harass, and sometimes attack potential predators, a behavior known as mobbing. Mobbing seems to invite obvious risks, but predators have rarely been observed to injure or kill animals that harass them. The true costs of predator mobbing and the functional significance of this behavior remain poorly understood. Here, we report a fatal attack by a jaguar (Panthera onca) on a mobbing capuchin (Cebus capucinus) that we observed on Barro Colorado Island, Panama. This rare observation illustrates that mobbing predators such as large felids can, indeed, be costly. However, to understand fully the costs of mobbing, the energetic and opportunity costs that primates incur when they harass predators must also be considered.     

Localization of Passerine Seeet and Mobbing Calls by Goshawks and Pygmy Owls

Goshawks and pygmy owls responded to recordings of passerine alarm calls by correctly orienting to their source. The seeet, or “aerial predator” alarm call which is generally assumed to be “non-localizable”, while it elicited fewer responses than did mobbing calls, was nevertheless accurately localized by all birds that did respond. The evolution of alarm calls is discussed in terms of efficient prey communication, following Darwin's “antithesis principle”, rather than predator selection for non-localizability.     

The biological function of snake mobbing by Siberian chipmunks: I. Does it function as a signal to other conspecifics?

The signal effect of mobbing of snakes (a signal effect alerts conspecifics of the presence of a predator) by Siberian chipmunkEutamias sibiricus asiaticus was examined. A pair of chipmunks, familiar with each other, were moved from their rearing pen into a test box, where they were kept undisturbed for acclimatization. Then the anterior portion (or head) of an anesthetized Japanese rat snake was exposed in a corner of the box. Several minutes later, one of the chipmunks discovered the snake and began mobbing it. This was followed by the discovery of the snake by the other chipmunk. Although the time span from the introduction of the snake to its discovery by the first chipmunk was quite different between trials, the time elasped from the snake discovery by the first chipmunk to that by the other chipmunk was relatively constant and significantly shorter than the former time span in all trials. These results strongly support the premise that the mobbing has a signal effect.     

Birds respond similarly to taxidermic models and live cuckoos <Emphasis Type="Italic">Cuculus canorus</Emphasis>

Stuffed birds are widely used in research for identifying effects of predators and nest parasites on bird behaviour, studying levels of aggression and the size of territories. However, the fact that these models do not move or vocalize may question the results of such studies and open them to criticism. One solution would be to determine how the results of research using stuffed dummies correlate with the response of wild animals to enemies under the same environmental conditions. In a first attempt, we examined the correlation between the intensity of mobbing of a dummy cuckoo Cuculus canorus and interactions with live cuckoos in the field during the breeding season in western Poland. A total of 39 bird species mobbed cuckoo dummies; all 39 were found to attack live cuckoos, while 24 species (61.5%) did so during experiments using a dummy. The number of individual birds involved in mobbing a dummy was positively correlated with the number of individuals attacking real cuckoos in the same areas, even when the most commonly mobbing species, the barn swallow Hirundo rustica, was excluded from the analyses. However, we did not find significant differences in frequency of mobbing behaviour depending on cuckoo behaviour described as flight or sitting, or calling rather than remaining quiet. Therefore, we conclude that the use of a dummy for studying mobbing of hosts and non-hosts of the cuckoo provide results that are similar to those made in response to the behaviour of live hosts.     

Threat-sensitive anti-predator defence in precocial wader,the northern lapwing <Emphasis Type="Italic">Vanellus vanellus</Emphasis>

Birds exhibit various forms of anti-predator behaviours to avoid reproductive failure, with mobbing—observation, approach and usually harassment of a predator—being one of the most commonly observed. Here, we investigate patterns of temporal variation in the mobbing response exhibited by a precocial species, the northern lapwing (Vanellus vanellus). We test whether brood age and self-reliance, or the perceived risk posed by various predators, affect mobbing response of lapwings. We quantified aggressive interactions between lapwings and their natural avian predators and used generalized additive models to test how timing and predator species identity are related to the mobbing response of lapwings. Lapwings diversified mobbing response within the breeding season and depending on predator species. Raven Corvus corax, hooded crow Corvus cornix and harriers evoked the strongest response, while common buzzard Buteo buteo, white stork Ciconia ciconia, black-headed gull Chroicocephalus ridibundus and rook Corvus frugilegus were less frequently attacked. Lapwings increased their mobbing response against raven, common buzzard, white stork and rook throughout the breeding season, while defence against hooded crow, harriers and black-headed gull did not exhibit clear temporal patterns. Mobbing behaviour of lapwings apparently constitutes a flexible anti-predator strategy. The anti-predator response depends on predator species, which may suggest that lapwings distinguish between predator types and match mobbing response to the perceived hazard at different stages of the breeding cycle. We conclude that a single species may exhibit various patterns of temporal variation in anti-predator defence, which may correspond with various hypotheses derived from parental investment theory.