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SPAANS B, LEOPOLD M & PLOMP M (2018) Using a drone to determine the number of breeding pairs and breeding success of Sandwich Terns Sterna sandvicensis. LIMOSA 91 (1): 30-37.

The number of breeding Sandwich Terns on the island of Texel increased during the past 10 years. With growing numbers, the dense colonies became more and more difficult to count from a distance. Visiting the colonies to count the nests would cause too much disturbance and was for that reason undesirable. Taking pictures from the air with a drone could be a method to count the breeding birds accurately. Therefore we tested the effect of a drone flying over the colony in 2013. The drone, flying at 15-20 meters height, appeared to cause hardly any visible disturbance to the birds. That is why we used a drone to make aerial photos of the colonies in 2014 and onwards. The number of breeding birds was counted afterwards on these photos on the computer. In this paper we describe our methods and the results of these counts. We flew with the drone in the early morning on calm days when almost all partners of the breeding individuals were out fishing. The advantage of this timing is that almost all birds present represent a nest. We corrected the final counts for the small fraction (3.2 – 7.8%) of non-breeders still present in the colony. The fraction of non-breeders was determined by counting the number of incubating and standing (non-breeding) birds from a distance using a telescope during the drone flight in a part of the colony that was well visible. Counts on photos made by a drone resulted in 1.4 to 3.5 times more nests compared to counts of the same area done with a telescope from a distance. The almost full-grown juveniles appeared to be well recognizable on the dronephotos. By counting these juveniles just before they fledged, we were able to determine breeding success, expressed as the average number of almost full-grown juveniles per pair. Because of its accuracy and the absence of disturbance, the use of a drone to count breeding pairs and juveniles in Sandwich Terns, and probably in many other species too, is by far the best option.

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limosa 91.1 2018
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