Erik Kleyheeg, Chris van Turnhout, Joost van Bruggen, Roland-Jan Buijs, Thijs Glastra & Pim Wolf (2025) Population trends and monitoring of rooftop-breeding Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus in the Netherlands. LIMOSA 98 (2): 80-91.

A growing proportion of Herring Gulls and Lesser Blackbacked Gulls in the Netherlands are breeding on rooftops, where many escape attention of regular bird monitoring schemes. Despite the relevance for policy and species management, the numbers and trends of rooftopbreeding gulls are largely unknown. In 2023, we aimed to improve our estimate of rooftop-breeding population sizes by collecting data of as many rooftop colonies as possible, using different types of volunteer counts, drone images and aerial photographs, and we evaluated which methods could improve the future monitoring of these populations. All methods combined, we counted 97 Herring Gull and 126 Lesser Black-backed Gull colonies on roofs (many were mixed), most of them in the coastal provinces, but also some more inland. Including estimates for known colonies that were not counted, we estimated a total of 3100-3300 Herring Gull breeding pairs and 6800-7800 Lesser Black-backed Gull breeding pairs on rooftops in the Netherlands. Historical counts combined with trend extrapolation yielded substantially fewer breeding pairs (1600-2300 Herring Gull and 3900-4400 Lesser Blackbacked Gull). Relative to the national population counts, this implies that 4-11% of Herring Gulls and 5-12% of Lesser Black-backed Gulls currently breed on rooftops. Of all monitoring methods evaluated, none clearly outperformed the others. Obtaining permission to enter rooftops is often difficult, obtaining proper views from elsewhere is often impossible, and the use of drones in urban areas is restricted by legislation. The availability and quality of commercial aerial photographs are variable and therefore unreliable, while obtaining aerial photographs for this specific purpose is expensive and analyses are time-consuming. We conclude that proper monitoring of rooftop-breeding gulls is challenging, especially in the light of the continuing range expansion. We propose a combination of methods: annual counts by volunteers to track the population trend and establishment of new colonies, combined with periodic collection of aerial photographs for validation, e.g. once every three to six years.

[pdf only for members] [dutch summary]