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Snake phobia? Not for Fasciated Antshrikes!

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Fasciated Antshrikes regularly mob snakes and can escalate into deadly attacks

 
Ioana Chiver
Department of Integrative Biology and Physiology, University of California, USA
 
 
LINKED PAPER
Mobbing behavior and fatal attacks on snakes by Fasciated Antshrikes (Cymbilaimus lineatus). Chiver, I. Jaramillo, C.A. & Morton, E.S. 2017. Journal of Ornithology. DOI: 10.1007/s10336-017-1452-9. VIEW

It is always exciting to spot a flock of birds mobbing a potential predator. Owls or other raptors mostly skulk unnoticed except for the rare moments when they are discovered. The ensuing gathering of birds vocalizing loudly, approaching and retreating from the predator with conspicuous wing flicks and other postures, can be an incredible spectacle. Predator mobbing, although time consuming, serves an important function in deterring predators away from an area (‘move on’ hypothesis, Curio 1978). During the mobbing event, prey and predator assess each other (Morton 2017). Predators may vary in how quickly they abandon the area and for how long depending on the intensity of mobbing. Prey species may vary in mobbing intensity, sometimes escalating into attacks on the predator when these are judged less risky.

Most mobbing observations have focused on avian predators, however, for tropical birds, snakes are an important predator particularly on nests (Fig. 1). Little is known about birds mobbing of snakes and if there may be predator-prey communication similar to that occurring during mobbing of avian predators. The few observations to date indicate that birds do not pose a risk to snakes and mobbing mainly makes hunting unprofitable temporarily as birds learn of the snake’s location (Maklakov 2002, Mercado et al. 2002). Nevertheless, tropical birds have shared their habitat with a diverse group of snakes for a large slice of evolutionary history, so variety in interactions is to be expected.

chiver fig 1Figure 1 The “puff-bird” snake, Pseustes poecilonotus, is a common bird predator especially on nests of tropical forests species. It has several color morphs at the site, two of which are shown here © I. Chiver

We had set out to study the vocal behaviour of Fasciated Antshrikes, Cymbilaimus lineatus, a widespread Neotropical species of lowland rainforest in which, as in many species in this group of birds (family Thamnophilidae), females sing just as males do. We mapped territories by following singing adults, who forage mostly in the upper canopy and often in areas that contain large tangles of lianas. We monitored a total of 25 pairs near the town of Gamboa and the adjacent Soberania National Park (Pipeline Road), along the central part of the Panama canal. Strikingly, although Fasciated Antshrikes are relatively quiet and often hard to see, we noted five instances when pairs, along with other forest species, were loudly and conspicuously mobbing snakes. In one case, a large “puff-bird” snake, Pseustes poecilonotus, was found dead soon after a mobbing event near a nest we located! Gene, my co-author, had observed a pair kill a vine snake, Oxybelis sps., when he was doing field work in Panama in the ’60s and after conferring with other birdwatchers in the area, we found that at least three others had also observed Fasciated Antshrikes attack and kill snakes (Fig. 2).

chiver fig 2Figure 2 Female Fasciated Antshrike biting the head of a frog-eating snake, Leptodeira annulata
© M. Barria

How do they do it? And which other birds might also kill snakes? Previous reports pointed out that some relatively small birds, such as Puffbirds (Bucconidae), Motmots (Momotidae), Shrikes (Laniidae), and some Tyrant Flycatchers (Tyrannidae), which typically forage on large insects and small vertebrates may also occasionally hunt small snakes (Smith 1969, 1975, Frye and Gerhardt 2001). Hunting of relatively large insect prey is associated with morphological and cognitive adaptations to minimize the risk of injury during attack. Individuals in these species have strong beaks likely conferring a powerful bite and may also use a variety of cues, such as the location of the eyes, neck, and direction of movement, to grab prey below the neck and avoid being bitten (Smith 1973, 1976). Fasciated Antshrikes similarly have a large hooked beak and our observations further support that, when mobbing events escalate to attacks, individuals kill snakes by repeatedly biting them in the head and neck area. Further observations are needed to determine if other species with similar foraging habits may also attack and injure or kill predatory snakes. But we now know that snakes are likely under pressure to take note when forest passerines mob them, they can mean business!

Our first observation of mobbing occurred near a nest and prompted the question of whether Fasciated Antshrikes mob and attack snakes only when these pose an immediate threat to eggs or nestlings. We presented pairs with a plastic snake model that resembled P. poecilonotus in color and size to determine if they respond by mobbing both when nesting and when not nesting. We located 8 nests and we placed the snake 5-8 m from the nest. Along with the snake model, we played recordings of vocalizations that Fasciated Antshrike typically use during mobbing, which help the birds localize the snake. We similarly presented non-nesting pairs with the snake model and playback (set up 10 m from where the territorial pair was observed foraging). All pairs responded by mobbing the snake model with no difference between nesting and non-nesting pairs in how quickly or how closely they approached the model. This suggests that pairs benefit from mobbing snakes even when the current breeding attempt is not threatened, likely because they remain on the same territory long-term and the same predators remain a threat.

Interestingly, our observations also show that Fasciated Antshrikes attack snakes that are not known to present a threat to birds or their young. The principal nest predator, P. poecilonotus, has several color morphs and this may prevent learning to quickly discriminate between snakes that present a threat from ones that do not. Previous studies also suggested that snake mobbing could function to signal individual quality to conspecifics (Maklakov 2002). It could be that snake killing may also have signal value, indicating individual and/or territory quality, which could be important in maintaining the long-term pair bonds that characterize many tropical residents. More work is needed to untangle the different functions of snake mobbing, but our observations are a reminder that tropical environments set up the stage for novel interactions among species and these may be the key in the astonishing diversity of behaviour and morphology in organisms that live there.

References

Curio, E. 1978. The adaptive significance of avian mobbing. 1. Teleonomic hypotheses and predictions. Zeitschrift Tierpsychologie 48: 175–183.
 
Frye, G.G. & Gerhardt, R.P. 2001. Apparent cooperative hunting in Loggerhead Shrikes. Wilson Bulletin 113: 462–464. VIEW
 
Maklakov, A.A. 2002. Snake-directed mobbing in a cooperative breeder: anti-predator behaviour or self-advertisement for the formation of dispersal coalitions? Behavioural Ecology and Sociobiology 52: 372–378. VIEW
 
Mercado, J.E., Terranova, E. & Wunderle, J.M. 2002. Avian mobbing of the Puerto Rican Boa (Epicrates inornatus). Caribbean Journal of Science 38: 125–126. VIEW
 
Morton, E.S. 2017. Animal Vocal Communication: Assessment and Management Roles. 2nd ed. Cambridge University Press, Cambridge.
 
Smith, N.G. 1969. Avian predation of coral snakes. Copeia 1969: 402–404. VIEW
 
Smith, S.M. 1973. A study of prey-attack behaviour in young Loggerhead Shrikes, Lanius ludovicianus. Behaviour 44: 113–141. VIEW
 
Smith, S.M. 1975. Innate recognition of coral snake pattern by a possible avian predator. Science 187: 759–760. VIEW
 
Smith, S.M. 1976. Predatory behavior of young Turquoise-browed Motmots, Eumomota superciliosa. Behaviour 56: 309–320. VIEW

 
 
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About the author

Ioana Chiver is currently a post-doctoral student looking at the hormonal basis of the acrobatic courtship display of Golden-collared Manakins, Manacus vitellinus, a Panamanian forest species. She is especially interested in the mating behaviour of birds and for her doctoral work she studied the singing behavior and genetic mating system of Red-throated Ant-tanagers, Habia fuscicauda also a Neotropical forest bird.
 
View Ioana’s full profile
 

Image credit

Top right: Fasciated Antshrike, Cymbilaimus lineatus © I. Chiver
 

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