Alternative 1- Off-the-shelf drone (DJI Inspire 2)
Cost: The DJI Inspire 2 unit used in this study sells for about 3,000 €. If an iPad or smartphone is not already available to use as the interface for the user and the control software, then an additional investment of at least around 500 €. Aside from the costs associated with compensation of the researchers/conservationists who operate the drone (around 20 € per hour), the costs of the drone method are primarily capital costs. With travel, it must be expected that a researcher will need half a day (3.5 hours) to gather drone data. This brings the operation cost to 3.5 hours/day x 20 €/hour or 70 € /day.
Temporal frequency: The frequency at which measurements can be taken by a human-operated drone are ultimately tied to weather conditions and how often human operators can bring the drone out to the study site. On average, one can expect to obtain 15 measurements per month.
Disturbance: For the altitudes at which the Inspire 2 was flown in this study (10 to 50 m), disturbance was noted to be very high. Particularly, oystercatchers were very disturbed. The drone surveys coincided with the oystercatcher mating season, which left the oystercatchers on high alert. Once the drone was in the air, large numbers of oystercatchers (15 to 20) would swarm and attempt to intimidate the noisy drone. Fortunately, due to the Inspire 2’s obstacle- avoiding sensors, no oystercatchers were physically harmed (though, whether this stress negatively impacted their mating is not known). Although, the oystercatchers eventually became accustomed to the drone, the extent of the initial disturbance is too important to ignore. Other seabird species on the tidal flat (e.g. spoonbills, seagulls, sandpipers) were not as dramatically affected, but still moved away from the drone such that images containing live seabirds were rarely captured.
Deployability: As noted above, multi-rotor drones such as the Inspire 2 are only able to operate in certain weather conditions. Due to equipment sensitivity, the Inspire 2 cannot be operated in rain. Drones are also unable to be used in high winds. For the Inspire 2, the upper windspeed limit is 10 m/s (DJI, n.d.). In the author’s experience, three out of four days on average had sufficient conditions for drone use.
Spatial Coverage: The drone method offers the best spatial coverage of the three alternatives considered here. Drone images taken at even moderate altitudes can cover hundreds of metres of terrain. When paired with the mobility of the drone, the spatial extent of the images taken from moderate to high altitudes can obtain coverage on the scale of kilometres in a single trip (generally, 20 minutes long). If the specific case of flying at an altitude of 30 m is taken, the drone images have a footprint of about 35 m x 35 m when angled straight down. In a single flight, then, the drone can reasonably cover a ground track of 3000 m x 35 m. Given that multiple flights can be performed in a single outing, a conservative estimate of daily spatial coverage would be 9000 m x 35 m or 315,000 m2.
Resolution: In remote sensing there exists an inverse relationship between spatial coverage and resolution. Drones are no exception to this rule. However, if an altitude of 30 metres is chosen, the resolution of the images is high enough to identify to species of medium and large birds (as seen in this work).