1. Animal behaviors are often modified in urban settings due to changes in species assemblages and interactions. The ability of prey to respond to a predator is a critical behavior, but urban populations may experience altered predation pressure, food supplementation, and other human-mediated disturbances that modify their responsiveness to predation risk and promote habituation.

2. Citizen-science programs generally focus on the collection and analysis of observational data (e.g., bird checklists), but there has been increasing interest in the engagement of citizen scientists for ecological experimentation.

3. Our goal was to implement a behavioral experiment in which citizen scientists recorded antipredator behaviors in wild birds occupying urban areas. In North America, increasing populations of Accipiter hawks have colonized suburban and urban areas and regularly prey upon birds that frequent backyard bird feeders. This scenario, of an increasingly common avian predator hunting birds near human dwellings, offers a unique opportunity to characterize antipredator behaviors within urban passerines.

4. For two winters, we engaged citizen scientists in Chicago, IL, USA to deploy a playback experiment and record antipredator behaviors in backyard birds. If backyard birds maintained their antipredator behaviors, we hypothesized that birds would decrease foraging behaviors and increase vigilance in response to a predator cue (hawk playback) but that these responses would be mediated by flock size, presence of sentinel species, body size, tree cover, and amount of surrounding urban area.       

5. Using a randomized control–treatment design, citizen scientists at 15 sites recorded behaviors from 3,891 individual birds representing 22 species. Birds were more vigilant and foraged less during the playback of a hawk call, and these responses were strongest for individuals within larger flocks and weakest in larger-bodied birds. We did not find effects of sentinel species, tree cover, or urbanization.

6. By deploying a behavioral experiment, we found that backyard birds inhabiting urban landscapes largely maintained antipredator behaviors of increased vigilance and decreased foraging in response to predator cues. Experimentation in citizen science poses challenges (e.g., observation bias, sample size limitations, reduced complexity in protocol design), but unlike programs focused solely on observational data, experimentation allows researchers to disentangle the complex factors underlying animal behavior and species interactions.

The experiment consisted of two playback types (hawk and goldfinch) completed within a two-day period, approximately twice a month. On the first day, the participant would flip a coin to decide which playback type to conduct first. On the second day, the participants started with the opposite playback type.

During each playback, participants recorded two forms of data: flock sizes and focal individual behavioral observations (Fig. 2). Participants recorded flock size (the maximum number of each species seen at the feeder) before and after each period and focal behavioral data during each of the three periods (Fig. 2). Although many factors influence an individual’s vigilance in a flock (e.g., food quantity and quality, age and dominance, competition, and distance-to-neighbor; Beauchamp, 2008), flock size is thought to mediate individual responses to predation risk and is readily measured by citizen scientists. 

We employed a focal-switch observation approach (Losito et al., 1989). For focal behavioral data collection, participants watched an individual bird for approximately 30 s and recorded the bird’s activities into a handheld voice recorder. Throughout the 5-min observation period (15 minutes for the three observation periods), participants observed as many individuals as possible. If few birds were present, they repeated observations on the same individual. Participants recorded four focal behaviors: flying from feeder, freezing, head up, and pecking. Flight from the feeder was broken into three additional categories: flying within the feeder area, flying to cover within the feeder area, or flying away.

Participants submitted their voice-recorded behavioral observations and flock size datasheets on a regular basis. We processed the voice-recorded observations and transcribed—for each focal bird—the species, playback type (hawk or goldfinch), period it was observed (pre-playback, playback, and post-playback), number of each behavior, time of day to the nearest hour, and observation duration.

We were also interested in whether antipredator behaviors varied by 1) the presence of a sentinel species, 2) flock size, 3) body mass, 4) amount of protective cover near the feeder, and 5) level of urbanization surrounding the site. To evaluate the effect of sentinel species, we derived a binary variable indicating whether or not a black-capped chickadee was present or absent or all periods within each playback experiment. Chickadees function as sentinels by producing antipredator mobbing calls that elicit strong responses in other species (Hurd, 1996; Turcotte & Desrochers, 2002). In calculating the presence of a sentinel species and flock size, we used the flock counts from before each playback period to characterize conditions at the start of each period. We obtained species-specific body mass (grams) from The Sibley Guide to Birds (Sibley, 2000). Finally, we calculated percentages of canopy cover within 100 m of sites and impervious cover within 3 km of sites from The National Land Cover Database (Coulston et al., 2012; Homer et al., 2015; Jin et al., 2013; Song, 2005; Xian et al., 2011) as proxies for the amount of protective cover near feeders and the level of urbanization within the surrounding landscape, respectively.