Rob Lensink, Chris van Turnhout, Nico Beemster, Arjan Boele, Albert de Jong, Henk Sierdsema & Sander Terlouw (2025) The increase and spread of the Cetti’s Warbler in the Netherlands; a first analysis since first settlement. LIMOSA 98 (1): 1-16.

After the first observation in 1968, Cetti's Warblers were found breeding in the Netherlands between 1969 and 1986, with a maximum of around 62 territories in 1978. After the severe winters of 1978/79 and 1984/85, 1985/86 and 1986/87 the species vanished from the Netherlands (and Belgium as well). In 2003 the species started to re-colonise the Netherlands. In the following years the number of territories increased and the distribution became wider. Until 2010 the population increase was 34% per year, thereafter at 8% per year. Nowadays the population is estimated at 4500-5500 (2022) and 5300- 6300 (2023) territories. The yearly change in breeding numbers in 2003-22 showed no relationship with the severity of the winters. This was unexpected because such negative correlation was found for Belgium and England. The climate in the Southwest-Netherlands is notably mild due to the influence of large water bodies, which presumably helped Cetti's Warblers to survive also moderately severe winters. Along with the increase in numbers, the number of occupied squares (5x5 km) increased as well. The relative yearly change in numbers was positively correlated with the relative yearly change in occupied squares, i.e. the spatial expansion was larger in years with a stronger increase in numbers. Based on annual distribution maps, the velocity of expansion was on average 4.9 km/year for 2003-22, with 3.1 km/year in 2010-16 and 16.4 km/year in 2016-22. The stronghold for Cetti's Warbler in the Netherlands is the Biesbosch and the adjacent Delta area. In this region the species is mainly breeding in dense vegetations with reed, thickets or shrub, and to a letter extent willow woods. Between 2010 and 2019 the densities of Cetti's Warblers increased in these habitats. Although willow woods remain the least preferred habitat, densities increased the most in this habitat. This habitat choice is representative for the rest of the Netherlands, with local differences due to the availability of habitats. Climate change, with increasing temperatures during winter, has strongly contributed to the northward expansion of the Cetti's Warbler in Northwestern Europe, through a better winter survival. This expansion has been notably fast, presumably because increased temperatures also allowed for a longer breeding season with multiple broods, and thus a high reproductive output. The northward expansion is believed to continue, and it is unknown what the future northern distribution range (as determined by winter temperatures) will be

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