However, several Spanish collaborators have recently used this method to survey a Catalonian breeding colony of the locally-protected black-headed gull (Chroicocephalus ridibundus). They had two goals: first and foremost, to acquire accurate data on how many birds were living and nesting at the recently-established Estany d'Ivars i Villa-Sana gull colony, and, second, to compare data collected using the unmanned aircraft systems (UAS) technique to those obtained via a standard human visit to the colony.
(Black-headed gull, Chroicocephalus ridibundus)
Although UAS sounds like something that involves military-level complexity and precision, the device deployed in the current study comprised only a few, commercially-available products: a radio-controlled model aircraft, fitted with a radio-controlled digital camera and a GPS unit. The plane was also equipped with a "first person view flight system" consisting of a small camera in the plane's cabin that allowed the human controller to visualize the aircraft's position in real time and to keep an eye on geographical coordinates, altitude, speed, and course. Data from the GPS unit were sent straight to a computer running Google Earth. This provided a sort of air traffic control, allowing the researchers to keep an eye on the UAS's progress remotely. All in all, the entire rig weighed about 2 kg and cost less than US $2,000.
The scientists selected flight parameters that enabled them to capture the entire colony in a single image; in this case, they flew the UAS at 30-40 km/h at a height of 30-40 m above the ground. They compared three different methods of data collection. First, they examined multiple images collected at each of two different flight passes on three different survey days. Second, they examined only the first image collected on the first day. Finally, they deployed a single surveyor on foot to census the birds using "standard" survey techniques.
(A Multiplex Twin Star II, the commercial radio-controlled airplane model used by the researchers to survey gull breeding colonies)
During the UAS flyovers, most gulls (98.8%) remained stationary, indicating that the birds were not overly disturbed by the aircraft's passing. Although there were some differences between the nest counts yielded by each of the three methods, the numbers were encouragingly similar: 244 nests counted during the sequential-image method, 227 definite nests and 54 probable nests counted during the single-image method, and 229 nests counted by the on-site surveyor. Agreement between the two UAS techniques was as high as 86%, and differences between the sequential-image and direct count methods were as low as 0.8%.
These results suggest that the UAS is a useful and accurate method for obtaining counts of birds/nests without disrupting the animals. Further, it allowed the researchers to easily census a breeding colony located on an island in the middle of a lagoon--not the most accessible of research locations. Because black-headed gull nests can have a range of sizes and linings, the researchers admit that they could have made some mistakes when evaluating the UAS images. However, errors are also possible during direct counts by surveyors; the advantage of the UAS technique is that multiple flyovers can be used to quantify those errors and adjust estimates accordingly.
(Black-headed gull breeding colony)
Although the sequential-imaging method provided more detail than the single-image method, the latter was still fairly accurate and could be useful in situations where focal animals are more easily disturbed, or where budget limitations restrict multiple flights. The researchers caution, however, that there are some potential drawbacks to the use of UAS. Both range and flight duration will be limited, requiring scientists to find a "home base" not too distant from the survey site. Another serious concern is situations that compromise image quality and resolution; wind and rain, in particular, can affect stability of the aircraft and obstruct the camera lens. As long as surveyors are willing and able to wait for clear weather, though, this should not be much of an issue. All in all, the authors of the study feel that their first attempts at "fine-scale bird monitoring" using UAS were a success, and they look forward to seeing how other scientists employ and refine the technique.
Sardà-Palomera, F., Bota, G., Viñolo, C., Pallaré s, O., Sazatornil, V., Brotons, L., Gomáriz, S., and Sardà, F. 2011. Fine-scale bird monitoring from light unmanned aircraft systems. Ibis 154:177-183.
Thanks to the following websites for providing the images used in this post: