BULT H (2018) Decline of the Black Woodpecker Dryocopus martius on the ‘Brabantse Wal’: reality or census effect?. LIMOSA 91 (4): 145-156.

In 1990-96 50-70 territories of Black Woodpecker were mapped on the Brabantse Wal (Fig. 1) using guidelines of the breeding bird monitoring program (BMP) in the Netherlands. Various factors may evoke overestimation of Black Woodpecker populations. The birds traverse vast, often overlapping home-ranges. Furthermore, both sexes use the same vocal and instrumental signals to demarcate territories, to defend roost-sites and to communicate with each other. The species was mapped again in 2017. To avoid overestimation of numbers, simultaneous displays, normally the key to territory mapping, were considered as intra-pair interactions when gender of the birds could not be determined, unless they were more than 1 km (the fusion distance) apart. The number of territories found in 2017 (23- 34) was much lower than in 1990-96.

To find out whether this decrease could be explained merely by overestimation of the population in the past, we analysed observation frequency (birds per hour of fieldwork) of Black Woodpeckers in our basic data for the national monitoring schemes of breeding (BMP) and wintering (PTT) birds. In breeding bird surveys the number of observations per hour correlated positively with the number of territories (Fig. 2) and declined gradually over time (Fig. 3). In seven sample plots for breeding birds it dropped from 0.26 (SD 0.07) birds per hour in 1990-92 to 0.13 (SD 0.10) in 2015-16 and this coincided with a 50% decline of the number of territories, strongly resembling the pattern for the whole study area (Fig. 4). During the winter counts in 1987-90 and 1991-2000 a mean of respectively 0.75 and 1.28 birds per hour was observed, dropping to 0.51 during 2001-10 and decreasing further to 0.24 birds per hour in 2011-16 (Fig. 6).

The combined data thus point to a substantial decline of the local Black Woodpecker population. Conversion of coniferous woodland into a more deciduous type by forestry management, decline of habitat by cutting pine stands to create open “natural” habitat, aerial deposition of nitrogen compounds, compromising the availability of ants (Fig. 5), their principal food, and predation by Goshawk and Pine Marten are discussed as putative explanations.

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