Urban-dwelling birds face novel visual cues and soundscapes. To thrive in these challenging environments, individuals must correctly identify and calibrate threats posed by humans and their activities. We showed that Dark-Eyed Juncos (Junco hyemalis) residing in an urban habitat responded differently to the sounds that approaching people and objects make. A person approached juncos simultaneously playing the sounds of object types that normally move at different relative velocities: faster (bicycles), intermediate (skateboards and scooters), or slower (people walking). Juncos responded at significantly greater distances and moved further in relation to what sound cues would normally imply about the velocity of approach. Absolute stimulus volume was not a significant predictor of response across object type. The responses occurred without the presence of visual cues, suggesting that an auditory cue alone and without visual confirmation can produce an appropriate response. Overall, this shows that this population of urban juncos has the ability to respond appropriately to novel anthropogenic sound cues. The question remains as to how universal such abilities are across species, different urban situations, and in natural habitats.

The observer flushed birds on the ground that were foraging or otherwise at ease (e.g., not alarm calling or agitated near a nest), using standard protocols. The observer approached a focal individual in a straight line, dropping markers at locations where the approach began (the starting distance), where the observer was when the focal subject clearly moved away, and the distance to the spot from where that movement happened (the FID). Using the location from which birds flee avoids FID being confounded by incidental foraging movements. FIDs were measured based on converting steps between markers to meters (1 step = 0.85m, as measured during training before the experiment). To avoid eliciting further escape behavior, we visually estimated distance moved away in meters as the Euclidian distance determined by the horizontal and vertical distances the bird travelled. The immediate distance of targeted birds to cover (either bushes and trees, or structures such as walls and outdoor furniture) was recorded at the point in time when the approach began.

Each bird experienced 9 different sound stimuli. Each recording (10-15 seconds in length) was made using the Voice Memos app for smart phones as an MPEG-4. KL recorded the sounds while riding a bike, scooter, skateboard, or walking, in the presence of ambient background noise (i.e., without any extraneous sounds that might signal danger such as car horns, or singing or alarm-calling birds). Thus, the recordings themselves were of a constant activity and not of an object approaching or passing by. Hence, the sound of an ‘approaching’ stimulus was entirely created by the experimenter advancing towards the focal bird. Each stimulus type was replicated two to three times: in a quieter state when moving over smooth terrain such as a flat sidewalk, or a noisier state when moving over rough terrain such as a textured sidewalk or dry leaves. This captures a range of noise that such moving objects may often make in an urban setting. Playbacks were from the phone at same set volume level, held at same height, and perpendicular to the ground. Phones, rather than speakers or other large equipment, were used so that the advancing person would not look unusual. Because the played recordings would meld into the existing soundscape of the immediate area in UCLA where the approach occurred, the total auditory environment of each approach would not have been completely identical. Birds were not approached, however, in areas where construction or some other loud sound would have affected the ability to hear the recordings.

Decibel X, a noise meter application (https://skypaw.com/decibel10.html). 30-130dba; time weightings 200-500 milliseconds), was used to determine the mean loudness of each noise type across the duration of the recording when played. For each approach, the sound was turned on at the start of the approach and off when the bird moved away, which always occurred before the recordings ended. All birds were approached 5 consecutive times in a given day with the same sound playing (= 9 days of trials with 45 total approaches per bird).

The data are all in a single Excel file.