Experimental Design

Modelling visual perception in birds We used visual contrast models (details in Appendix S2) to estimate how Canada geese would perceive the stimuli (i.e. two aircraft) in relation to the visual background. Using this modelling approach, we tested a critical assumption of our behavioural experiments that Canada geese would perceive the aircraft with lights on as more contrasting than the aircraft with lights off. Using visual contrast models is important owing to the aforementioned differences between the avian and human visual systems. We calculated chromatic and achromatic contrasts (Endler 1990), which estimate the ability of the visual system to distinguish an object from the background using cues related to colour and brightness of visual stimuli, respectively (Vorobyev & Osorio 1998; Osorio, Miklósi & Gonda 1999). This approach requires information on (a) the sensitivity of the retina to different wavelengths, (b) the light reflectance patterns of the stimuli and the background environment, and (c) the spectral characteristics of the ambient light. Details on the parameterization and calculation of the visual contrast models are presented in Appendix S2.

Experimental site and equipment We conducted our experiment under semi‐natural conditions in a 9·3‐ha grass field in Erie County, OH, USA (41°22′N, 82°41′W) on 21 and 23 July 2009 between 0900 and 1715 hrs. (Appendix S1). We held each group of geese in a circular enclosure (∼229 m2) of 1·8‐m high synthetic, 5‐cm mesh fencing located in the centre of a 372‐m2 area of mixed grass (∼4 cm in height; Fig. 1). The enclosure was intended to mimic grasslands within airport property, and allowed the geese ample freedom for responding to aircraft approach. We used a standard fixed‐wing design, RC aircraft (Rascal 110; standard aircraft) and the Falco Robot GBRS©, designed to mimic a raptor (predator model), as our approach vehicles (Appendix S3). Engine noise was audible for each aircraft. However, we assumed that variations in wind conditions, noise from an on‐site power generator (used to power our server/video recorder system; Appendix S1), and the fact that aircraft approached the enclosure from an upwind direction reduced possible confounding effects of differential engine sounds. All approaches were video‐recorded (see Appendix S1). Figure 1 Open in figure viewer PowerPoint Diagram of the experimental site and approach scenario used for the exposure of captive groups of Canada geese to the approach of radio‐controlled (RC) aircraft. Camera positions are indicated by the numbers 1–6. The final leg of each standard‐aircraft approach began approximately 550 m from the enclosure, whereas the predator model approached from 420 m.

Experimental protocol We were limited to 58 birds and therefore used a repeated‐measures approach to the experiment. We exposed 14 groups of geese, four birds per group, to three treatments (standard aircraft with lights off, standard aircraft with lights on, and predator model; two birds were held as potential replacements). The first two treatments involved the standard aircraft either with lights off or on, with the order randomly determined. Standard‐aircraft approaches with lights on involved the alternating pulse (2 Hz) of two lights mounted on the landing gear (Appendix S3). A 2‐Hz pulse is considered safe for civil aviation pilots (Rash 2004). To assess response to the standard aircraft relative to antipredator behaviour, we exposed all groups to a third treatment consisting of the approach of the predator model. Tests prior to our experiment showed that the predator model generated antipredator behaviour in Canada geese (e.g. escape to water, aggregation of individuals; E. Fernández‐Juricic, unpublished data). Because our focus was the response of geese to the standard aircraft with lights on or off, we used the predator model consistently as our third and last treatment. Each goose group was allowed at least 15 min to acclimate to the enclosure. During acclimation the geese were, however, exposed to movement of the pilot and observer preparing aircraft for take‐off (approximately 60 m from the enclosure), as well as noise from the generator (Appendix S1). These geese were urban birds habituated to people, traffic, and associated noise. Accordingly, we observed no behaviour suggesting that the geese were overly disturbed, as each goose group quickly began exploring the enclosure and foraging. With the exception of take‐off and landing, the general flight scenario for each treatment was similar, entailing a downwind, base, and final flight legs (Fig. 1). Each goose group could hear and view aircraft departure. We launched the standard aircraft from a gravel road 60 m southeast of the enclosure and climbed it to altitude on the downwind leg, approximately 550 m west of the enclosure. While the aircraft was in flight, the pilot and observer were positioned behind a hide to the east of the enclosure (Fig. 1). After completing an approach, the standard aircraft was landed (on the same gravel road), retrieved by the pilot, then positioned for the second treatment, or removed and the predator model prepared for the third treatment. In contrast, we launched the predator model windward by hand from behind the hide (Fig. 1). Also, because of the smaller size and reduced visibility (from the pilot’s perspective), the pilot climbed the aircraft to altitude on the downwind leg approximately 420 m west of the enclosure to begin the final approach. The final leg for both aircraft was a fully powered and direct approach upwind, descending linearly from approximately 150–6 m and flaring upward upon reaching the western edge of the enclosure, then banking and climbing to position for the landing. The interval (mean ± SD) from take‐off until landing for treatments involving the standard aircraft (1·9 ± 0·5 min) exceeded that of the predator model (1·0 ± 0·2 min) because of the longer final flying leg of the former. Intervals (mean ± SD) between flights within group (i.e. across three treatments per group) were consistent (5·2 ± 0·5 min). However, owing to problems with our outside camera (no. 6, Fig. 1), we obtained ground speed estimates (see Appendix S1) for only 12 standard‐aircraft approaches with lights on, 11 standard‐aircraft approaches with lights off, and 11 predator model approaches. Aircraft approach speeds (mean ± SD) were similar (standard aircraft with lights on: 114·1 ± 13·2 km hr−1; lights off: 110·7 ± 8·0 km hr−1; predator model: 102·2 km ± 13·0 km hr−1).

Behavioural metrics We examined video footage of each group and measured behavioural responses relative to the time at which the aircraft was over the centre of the enclosure. Here, the same observer viewed video footage from each aircraft approach taken via camera five (Fig. 1) and measured the position of the aircraft relative to enclosure features. We recorded the time each individual within a group became alert and moved away (flight initiation) in response to aircraft approach (as per Blackwell et al. 2009a). We defined alert behaviour as the increase in vigilance‐related behaviours (e.g. rate and proportion of time head‐up scanning) in response to on‐coming aircraft. An alert response involved a transition in an individual’s behaviour from an undisturbed behaviour (e.g. pecking, preening, loafing, or general scanning) to a behaviour clearly directed towards the approaching aircraft; showing head up and neck extended, increased scanning, or crouching (Fernández‐Juricic, Jimenez & Lucas 2001; Blackwell et al. 2009a). Furthermore, alert behaviour in response to aircraft approach had to be maintained by an individual until (i) the aircraft was over the centre of the enclosure, or (ii) transition to flight behaviour. Those birds that showed an alert response to aircraft take‐off reverted to other behaviours before the aircraft began the final leg. We defined a flight response as a clearly differentiated transition in behaviour from, for example, loafing, pecking, foraging, or alert behaviour, to running, flight attempts, or sudden movement towards other members of the group in response to aircraft approach. For each individual bird within a group we recorded alert time as the time (seconds) required for the aircraft to reach the centre of the enclosure from the point at which an individual showed alert behaviour in response to aircraft approach (as defined previously). Similarly, flight‐initiation time was the time required for the aircraft to reach the centre of the enclosure from the point at which the individual initiated flight behaviour. Greater values of alert and flight‐initiation times indicate an earlier response to approaching aircraft. For individuals that maintained alert behaviour through the aircraft’s passage over the enclosure, without showing a flight response, we scored flight‐initiation time as zero. In instances where an individual showed no alert behaviour but initiated a flight response, we scored alert time as equivalent to flight‐initiation time. If a bird showed no alert or flight response, both time metrics were scored as zero. As an additional metric of antipredator behaviour in response to aircraft approach, we measured neighbour distances within each goose group at the point of aircraft take‐off and when the aircraft was over the centre of the enclosure. We used ImageJ (http://rsb.info.nih.gov/ij) to measure all pairwise neighbour distances from and to the centre of the body and distance between the head and the ground for each individual (individual height). We used pixels as our measurement unit and all distances were recorded from the same camera across trials. Because of distortion associated with distance of the individuals from the camera, we standardized pairwise distances as follows: distance between individual 1 and 2/[(individual 1 height + individual 2 height)/2]. We then used the standardized distances among all individuals to estimate an average neighbour distance per trial. We included ambient light intensity (μmol m−2 s−1), temperature, and wind as covariates in our models. We recorded ambient light intensity with a Li‐Cor (Lincoln, Nebraska, USA) LI‐250 Light Meter and LI‐190SA Quantum Sensor measured at completion of the first treatment per group. We also recorded temperature and wind speed using a WeatherHawk (Logan, UT, USA) 916 weather station.