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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Breeding biology and population trend of Hawfinches Coccothraustes coccothraustes in Flevoland (1998)
Auteurs :	Rob G. Bijlsma, Auteur
Type de document :	Article
Dans :	Limosa (71(4) 1998)
Article en page(s) :	137-148
Résumé :	The breeding biology of Hawfinches in the recently reclaimed polders Noordoostpolder (woodland planted in 1944-55), Oostelijk Flevoland (planted in 1958-68) and Zuidelijk Flevoland (planted in 1987-96) was studied in 1989-97 (Tab. 1). Woodlands in Flevoland have mostly been planted on clayey and loamy soils, thus permitting the use of a large variety of deciduous trees (Tab. 1, 2). This contrasts with woodlands elsewhere in The Netherlands, which have been planted on poor sandy soils and mainly consist of coniferous trees (mostly Scots pine). During large-scale surveys in 1989-97, each area was covered with five complete visits between mid-March and late June/early July, on average spending 9.5 min/ha in territory mapping scarce and rare breeding birds. Special attention was paid to behaviour and nesting biology of Hawfinches. Densities thus obtained should be read as relative densities. Polder woodlands became occupied by Hawfinches between 8 and 18 years after planting (year of planting = 0). Initial settlement was slowest in the 1970s, when the Dutch Hawfinch population was not yet booming (started in mid-1980s), and fastest in the 1980s (countrywide increase). In the latter case, Hawfinches already reached relative densities of >1 pair/10 ha within a few years after settlement. The increase in density levelled off some 20 years after tree planting, at 2.0-2.6 pairs/10 ha in deciduous woodland (Fig. 1). The overall density in woodlands in Flevoland reached 1.5 territories/10 ha (Tab. 2), but locally much higher densities were found. For example, in 1990 26 nests were located on 26.8 ha of Carpinus betulus, Quercus robur, Fraxinus excelsior, Picea abies and P. sitchensis in Voorsterbos, i.e. 9.7 nests/10 ha. Flocks of up to 120 Hawfinches are a typical feature of woodlands in the Flevopolders in winter, mainly foraging on seeds of Carpinus betulus and Acer campestre. Flocks started to disperse from early March onwards, and settled in loose colonies in nearby suitable habitats. Hawfinches seemed to prefer Populus spec., Acer pseudoplatanus and mixed deciduous/coniferous stands, apparently avoiding coniferous stands (but not Larix spec.) and Salix spec. (Tab. 2). Nest site choice did not always reflect habitat choice, because Hawfinches preferentially nested at heights of 15 m. Low nests had better coverage, probably resulting in smaller predation rates. It is argued that the fast colonisation of, and high overall density in the Flevopolders have been caused by the planting of a large variety of seed and berry carrying trees and shrubs on fertile soils, resulting in excellent feeding and breeding conditions, high fecundity, good breeding success and probably a high survival rate in winter. Some of these conditions may also apply to other parts of The Netherlands, where woodland diversity has improved during the last few decades. Altogether, t he Dutch Hawfinch population has at least quadrupled since the mid-1970s, especially after 1985, and has reached a minimum of 15 000 pairs in the late 1990s.

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