Perception and communication of predator threat in the black-capped chickadee (Poecile atricapillus)

The black-capped chickadee (Poecile atricapillus) is a type of North American songbird that produces multiple vocalizations, including their namesake chick-a-dee call. This call is used as a mobbing call to recruit and coordinate conspecifics in the presence of a predator. These parids perceive and respond to avian predators of varying size, as well as elicited conspecific mobbing calls. Specifically, smaller hawks and owls are of higher threat to chickadees as they easily maneuver through the trees, while larger hawks and owls cannot. The chick-a-dee call consists of four note types that can be repeated or omitted (A, B, C, and D), and chickadees produce more D notes per call in response to small, high-threat predators compared to large, low- threat predators. In Chapter 2, I extended work investigating the perception of arousal in vocalizations produced by three biological classes by testing whether two vocal learning species, humans and chickadees, can discriminate vocalizations of high- and low-arousal using operant discrimination go/no-go tasks. Chickadees (Experiment 1) and humans (Experiment 2) learned to discriminate between high- and low-arousal stimuli and significantly transferred the discrimination to additional giant panda, human, and black-capped chickadee vocalizations. I also conducted discriminant function analyses using four acoustic measures, and found that fundamental frequency likely played a role in responding during the task; however, these analyses also suggest roles for other acoustic factors as well as familiarity. Next, in Chapter 3, I conducted an operant go/no-go discrimination task to investigate the effect of signal degradation and anthropogenic noise on perceived threat. The results indicated that chickadees responded to natural low-threat owl stimuli more consistently across distance than high-threat owl stimuli, synthetic tones were responded to similarly compared to natural stimuli but at lower levels, and truck alarms were not responded to as though they were of high-threat suggesting that although acoustically-similar, chickadees can discriminate between high-threat owl calls and anthropogenic stimuli. In Chapter 4, I used operant conditioning techniques to investigate referential communication, which has been defined as the exchange of information giving reference to objects or events. Few studies have examined referential alarm calls in songbirds, yet we found that high- and low-threat mobbing calls were not treated similarly to the corresponding predator’s calls. Since owl stimuli are acoustically distinct in comparison to the conspecific stimuli, chickadees may have found predator stimuli easier to discriminate; this factor may have lead to the asymmetrical results. In Chapter 5, I conducted an operant go/no-go experiment investigating black-capped chickadees’ acoustic discrimination of predator threat. I confirmed that chickadees can discriminate between high- and low-threat predator calls, and further investigated how chickadees categorize mid-sized species’ calls via transfer of training according to perceived threat. Specifically, mid-sized broad-winged hawks were perceived to be of high threat whereas mid-sized short-eared owls were perceived to be of low threat. Mid-sized Cooper’s hawks and northern hawk owls, however, were not significantly differentially responded to, suggesting that they are of medium threat which supports the notion that perception of threat is along a continuum rather than distinct categories of high or low threat. Last, in Chapter 6, I exposed black-capped chickadees to stationary avian and mammalian mounts of various threat levels in the field. This study revealed that only chick-a-dee call production in response to avian mounts significantly differed, chickadees produced more chick- a-dee calls with 4-8 D notes in response to the high-threat avian mount compared to the high- threat mammalian mount, chickadees produced significantly more 4 D note calls in response to the small, sharp-shinned hawk in comparison to the large, red-tailed hawk, and that feeding was most reduced to high- and low-threat avian predator mounts compared to the foam control. Taken together, these studies reveal insights into chickadee perception and communication of threat. Specifically, these studies indicate that: chickadees are capable of discriminating between high and low arousal in vocalizations produced by multiple species (Chapter 2); signal degradation affects threat perception (Chapter 3); operant conditioning may not be an ideal paradigm for examining the referential elements of mobbing calls (Chapter 4); there is likely a continuum of threat according to predator size (Chapter 5); and antipredator responses of black- capped chickadees vary in response to avian and mammalian predators that pose a high, low, or no threat (Chapter 6).