VAN DER KOOIJ H (2021) Counting nests of Purple Herons Ardea purpurea using drones. LIMOSA 94 (4): 146-154.

Purple Heron colonies have been surveyed for many decades in the Netherlands. Nests are situated in reed beds or bushes. Classically, colonies were surveyed from the ground, from the end of May to the beginning of June. Recently colonies have also been surveyed using a drone. In 2020, mainly due to Covid-19 issues, half of the Dutch breeding population was surveyed using drones. This article compares drone counts with the classical nest counts on the ground for three small (Genemuiden, IJsselstein, Giessendam) and three large (Nieuwkoop-Zuid, Kamerikse Nessen, Kinderdijk) Purple Heron colonies. Drones turn out to be suitable to survey bird colonies in reed beds and bushes. For small colonies that were counted using observations of commuting adults, drone counts provide higher numbers of nests. For larger colonies, drone counts slightly underestimate the number of nests.
The advantage using drones is the lack of disturbance in the colony during the survey. A disadvantage of using drones in large colonies is that, afterwards, the analyses of the pictures and recordings take relatively much time. Using drones in inaccessible colonies is indispensable (De Auken, Naardermeer, Kinderdijk), although pilots should have good knowledge of the approximate location of the nests and potential breeding habitat when flying the drone, otherwise part of the colony or single nests are missed out. We recommend to fly pre-programmed routes, to take pictures from right above the nests, and to fly at an altitude of about 15-20 m from the nest.

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