LENSINK R., J.W. DE JONG & J.C. KLEYHEEG-HARTMAN (2015) An aerial survey of roof-breeding gulls in the city of The Hague in 2010. LIMOSA 88 (3): 114-124.

This paper describes the use of aerial photographs as a novel technique for surveying gulls breeding and resting on roofs in a large city, The Hague in the west of the Netherlands. The survey, conducted in 2010, was commissioned by the city as a basis for (evaluation of) a policy to reduce nuisance perceived by due to increasing numbers of breeding gulls. The aerial photographs were taken with a laptop-operated camera mounted under a small airplane, that flew parallel transects over the city at an altitude of c. 2000 feet (Fig. 2).A first set of flights in late April yielded few breeding gulls but showed that a resolution of 3x3 cm on the ground was needed to distinguish gulls and gull nests. A second set was flown during the egg stage at the end of May, with a further flight in early June to fill in gaps between the transects of photos that arose through displacement and movements of the airplane due to wind and turbulence. Based on the coordinates of each picture, pictures were stitched together digitally to strips, which were scanned visually for breeding, resting and flying gulls. Nests were identified on the basis of behaviour of the gulls (pairs) and colour of the nest site. 422 pairs of breeding gulls were located; 30% Lesser Blackbacked Gull Larus fuscus and 70% Herring Gull Larus argentatus. More than 95% of all nests were found on flat roofs with gravel. Breeding was concentrated in some parts of the city. After correction for the 39% of the area not photographed, the estimate for the entire city is 603 pairs (Tab. 2), but up to 20% of pairs may have been missed on the photographs, due to structures obstructing the view, shading, or blurring of pictures. We thus estimate that 600-720 breeding pairs were present in the city, as well as 4000-5000 non-breeding individuals of both species combined. There was only a weak positive relationship between the estimated number of breeding pairs in different city districts and the number of gulls seen resting there (Tab. 3), whereas the latter was strongly correlated with the number of flying gulls (Tab. 4, Fig. 4). Between 2000 and 2010 the number of breeding gulls in The Hague had increased by >20% yearly. These figures contrast with the national trends, a decrease in Herring Gull since the 1980s and a stabilisation in Lesser Black-backed Gull. Roof breeding may increase due to high breeding success, caused by a scarcity of predators and a wide spacing of nests which probably limits intraspecific predation.

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