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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