BOELE A , DIJK AJ VAN, HUSTINGS F & ZOETEBIER D (2001) Regional differences in development of European Pied Flycatcher Ficedula hypoleuca numbers. LIMOSA 74 (3): 102-115.

Alter colonisation in the 20" century, European Pied Flycatcher has become a common breeding bird in the sandy eastern and southern parts of The Netherlands (Fig. 1,2). Highest densities are found in forests, urban areas and parks (Fig. 3) and it is assumed that more than 90% of the population breed in nestboxes. The breeding population in 1975-85 was estimated at 15000-25000 pairs. This paper focuses on regional trends in breeding numbers in the 1980s and 1990s. The results were derived from the long-term common bird census in The Netherlands ('Broedvogel Monitoring Project', BMP) The 336 plots from which data were available were assigned to region and habitat type (either deciduous or coniferous woodland). Trend analysis was carried out with the TRIM package (see e.g. van Strien & Pannekoek 1999).
     Trends in breeding populations varied according to region. Between 1984-99, numbers in the northern part of the country doubled, whereas in the central and southern regions, European Pied Flycatchers experienced a 50-70% decline. However, signs of a stabilisation (region Central) or recovery (region South) have become apparent more recently (Fig. 4) Differences in trends between deciduous and coniferous forests were more linked to region than to true differences in habitat (Fig 5, Tab. 1) This is surprising, as food availability is known to be generally better. In deciduous wood land and reproductive output (in other countries) is higher here compared to coniferous woodland.
     Striking similarities were found between trends in numbers of European Pied Flycatcher and Great Tit , which both compete for nextboxes (Tab. 2). The situation described here for The Netherlands is in line with findings in adjacent areas in Germany (Fig. 6)
     It is hypothesized that climatological factors might be an important underlying factor for the trends observed. As shown by Both & Visser (2001), spring temperatures in 1980-2000 have been subject to a steady increase. However, as European Pied Flycatchers return from their wintering areas at a more or less fixed date, the birds breeding in the central and southern part of the country might encounter more difficulties in raising their young, as the nestling period does not match peaks in insect abundance anymore. In the northern region, impact of this temperature change is probably smaller, allowing the birds an extra buffer. In this context, it is intriguing that years with low spring temperatures seem to coincide with a higher European Pied Flycatcher population index (Fig. 7) This might be linked to the later onset of breeding or a poorer body condition in Great Tits, the most important competitor for nestboxes. However, a direct relationship between trends in European Pied Flycatcher and Great Tit could not be proved at a national scale. Locally, such a link does exist (Fig. 8) Earlier assumptions of European Pied Flycatcher numbers being influenced by acid rain, do not seem to be met nowadays.European Pied Flycatcher Ficedula hypoleuca

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