Seasonal variation in home-range size, and habitat area requirement of the lesser spotted woodpecker (Dendrocopos minor) in southern Sweden
Introduction
This study concerns seasonal variation in home-range size and area requirement of foraging habitat of the lesser spotted woodpecker (Dendrocopos minor). A detailed knowledge about these factors, and of foraging preferences and habitat use, is essential for our ability to take appropriate measures for the conservation of threatened species. Like several woodpeckers it is affected by modern forestry and has declined in numbers in both Sweden (Nilsson et al., 1992) and Finland (Tiainen, 1985) at rates similar to that of the now endangered white-backed woodpecker (Dendrocopos leucotos). It is also scarce or decreasing in many other European countries (Mikusinski and Angelstam, 1993). In spite of a still wide distribution in the Palearctic region it has been surprisingly poorly known, which makes the study of habitat requirement urgent.
This work is part of a long-term study of the ecology of this smallest European woodpecker (body-mass 24 g) in a population in southern Sweden. It is highly dependent upon deciduous forests (Cramp, 1985, Olsson, 1998). From autumn to late spring it is a specialised forager, feeding almost exclusively on beetle larvae from dead branches in living, mainly deciduous, trees (Olsson, 1998). During spring in years when birch (Betula pendula, B. pubescens) and/or common alder (Alnus glutinosa) bloom, larvae of the moth Argyresthia goedarthella become an important food source (Olsson, 1998). After bud burst, its diet changes to include surface-living insects, which is also the main food for the nestlings (Török, 1990, Wiktander et al., 1994, Olsson, 1998). A new nest is excavated each year, mostly in the trunk of a dead deciduous tree, occasionally in the dead limb of a living tree (Cramp, 1985, Wiktander, 1998). Both parents participate in all stages of parental care. During our study, 16% of the breeding attempts involved a polyandrous female, which was simultaneously paired with two or three males in a season. In these cases, each male held a separate territory with a nest, the female laying a clutch in each male's nest in rapid succession and then dividing her parental care among all nests (Wiktander et al., 2000).
The prelaying season, i.e. from late March to egg-laying in mid-May, stands out as a critical period of the year for the lesser spotted woodpecker. Mortality rate during this period is significantly higher than in winter (Wiktander, 1998). Moreover, foraging and food availability in the prelaying season has a significant influence on the start of egg-laying and is also positively correlated with number of fledglings (Olsson et al., 1999). It therefore appears that the availability of prey in dead wood well in advance of breeding may be more decisive for overall reproductive success than the availability of surface-living prey during breeding. Because of these relationships we regard the prelaying season to be the relevant period to focus upon when estimating habitat area requirement.
Section snippets
Methods
Field work was conducted from March 1991 to June 1996 in a 125 km2 study area adjacent to lake Möckeln, situated in the boreo–nemoral region in southern Sweden (56°40′ N, 14°10′ E). About 40 km2 of the study area was covered by water. The land was mainly forested, but with smaller areas (about 10%) of agricultural land interspersed. Most of the forest was coniferous, mainly Norway spruce (Picea abies) but also some Scotch pine (Pinus silvestris), and subject to more or less intense commercial
Results
A three-way ANOVA showed that home-range size of the lesser spotted woodpeckers declined significantly from winter to nesting (F3,51=30.88, P<0.001; Table 1, Fig. 2), but did not vary between age-classes (F1,51=1.52, P=0.22) or sexes (F1,51=0.51, P=0.48). Moreover, in all but one of the individuals tracked over consecutive periods within a year, the home-range decreased from the earlier to the later period (Fig. 2).
Polyandrous females are not included in the above analysis. The home-range of
Home-range and territory size
Compared with other species inhabiting similar habitats, home-ranges of the lesser spotted woodpecker appear unusually large for a bird of this size. In a radio-tracking study of middle spotted woodpeckers (Dendrocopos medius; body-mass 58 g), Pasinelli (1999) found home-ranges in oak–hornbeam forest in Switzerland of 18.8 ha in winter, 11.6 ha in spring and 8.8 ha during nesting. In deciduous forest in our study area, the nuthatch (Sitta europea), similar in size to the lesser spotted
Acknowledgments
We wish express our gratitude to Anders Stagen, Krister Wahlström, Fredrik Östrand, Fredrik Haas, Leif Appelgren, Martin Stjernman and Ramón Martı́nez for many hours of help in the field, to Shahrzad Bakthiar for help with entering field data into the computer, and to all land-owners who let us work freely in their forests. Fredrik Haas also helped with preparing Fig. 1. The Regional Forestry Boards in Älmhult and Ljungby kindly let us use their forest data. The study was made possible
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