Headed Woodpecker Picus Canus As Influenced by the Avail- Ability of Food
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Ornis Fennica 72:1-13. 1995 Seasonal patterns in home range and habitat use of the Grey- headed Woodpecker Picus canus as influenced by the avail- ability of food Jørund Rolstad & Erlend Rolstad Rolstad, J., Norwegian Forest Research Institute, HØyskoleveien 12, N-1430 Ås, Norway Rolstad, E., Norwegian Forest Research Institute, HØyskoleveien 12, N-1430 Ås, Norway Received 1 June 1994, accepted 7 February 1995 Seasonal patterns in diet, home range and habitat use of the Grey-headed Woodpecker Picus canus were recorded at the Varaldskogen study area, a managed boreal forest located on the Swedish-Norwegian border in southcentral Scandinavia. One success- fully breeding pair was radio-monitored throughout a year. Additional telemetry data were collected in summer from a male in a pair that failed breeding. In summer the birds almost exclusively preyed upon ant colonies in soil and rotten stumps located in young conifer plantations. In winter, when snow or frost prohibited ground feeding, birds were feeding on bark-dwelling insects in old pine and dead trees . The winter shift in diet and feeding behavior was accompanied by an approximately 100 times en- largement in home range size, increasing from 50-100 ha (n = 3) in summer to 4 500- 5 400 ha (n = 2) in winter. With reservations due to small sample size, our data suggests that availability of winter food is critical to the Grey-headed Woodpecker when snow or frozen ground prevents ground-feeding for soil-dwelling ants. 1 . Introduction reasons for its rarity, by and large, remain un- known. The Grey-headed Woodpecker Picus canus is As most of the species in the Picus genus, the widely distributed throughout temperate decidu- Grey-headed Woodpecker mainly relies on ants ous and conifer forests of the Palearctic faunal as staple food. However, it is considered less ant- region. Different subspecies also occur in the specialized than its European congeneric, the southeast Oriental region (Short 1982, Cramp Green Woodpecker P. viridis (Cramp 1985). 1985). Despite the wide distribution range, this Matsuoka and Kojima (1979) report seasonal diet medium-sized woodpecker consistently seems in the Grey-headed Woodpecker in Japan, where to occur at low densities (reviewed by Cramp the small black ant Lasius niger dominated the 1985). Although modern forestry practices have diet. Interestingly, when snow covered the been assumed to influence this species nega- ground, the proportion of L. niger in the diet de- tively (Ahlén & Andersson 1976), the ultimate creased to almost zero. Because probing and ORNIS FENNICA Vol. 72, 1995 gleaning are the main feeding techniques, snow layered stands up to 200 years old. The remnant presumably prevented the Grey-headed Wood- patches of old forest have been subject to selec- pecker from utilizing soil-dwelling ant colonies . tive cutting for fuel and timber, which leaves a Consequently, the diet shifted to other inverte- stand structure lacking big trees and snags. A brates, especially spiders, Clubonia spp., which more detailed description of the forest history were abundant under the bark of dead trees. and stand structure is reported by Rolstad & In Fennoscandia, the Grey-headed Wood- Wegge (1989a, 1989b). pecker is perhaps the least known woodpecker. Except for some notes about diet and nest-sites (Ehrenroth 1973, Wabakken 1973), information 3. Material and methods about seasonal habitat selection and home range is non-existent . Here we present radio-telemetry The material consists of two males and one fe- data from one pair of Grey-headed Woodpeck- male, belonging to two pairs spaced 7 km from ers, monitored throughout a year. Although our each other (Fig. 1). No other Grey-headed Wood- sample size is small, the results demonstrate dis- peckers were observed in the 15 000 ha study tinct seasonal shifts in diet, habitat use and home area in 1992. In spring 1993, two pairs were ranges, which indicates that the availability of located 12 and 18 km, respectively, to the south winter food is a critical factor limit that may of our 1992 southern nest. Their breeding per- woodpecker numbers. formance was not monitored. Hence, the breed- ing density of Grey-headed Woodpeckers in this tract was about 1-2 pairs per 10 000 ha. 2. Study area Birds were captured with a hoop net on a telescope pole at the roosting holes prior to in- The Varaldskogen study area is a 15 000 ha large cubation. They were fitted with a 7 gram (5-6% forest tract situated on both sides of the Norwe- of body mass, inclusive harness) SS-2 type, back- gian-Swedish border in the middle boreal zone pack transmitter (Biotrack, UK), attached with a (60°10'N, 12°30'E; Ahti et al . 1968). The topog- nylon harness enclosed in silicon rubber (Brander raphy is characterized by gentle hills and valleys 1968). Transmitting distance varied between 300 between 200 and 400 mabove sea level. Climate m and 2 km depending on the local topography. is moderately continental with mean tempera- Birds were reinstrumented at 3 month intervals tures of 16.2°C and -7.3°C in the warmest (July) according to the life length ofthe batteries . Daily and coldest (January) months . Yearly precipita- monitoring was conducted with a portable re- tion averages 613 mm. Normally, snow covers ceiver and a hand-held antenna. In winter, when the ground from mid-November to April-May . birds moved extensively around, a dense network The forest is dominated by two conifers, Scots of forest roads and aircraft surveys helped locali- pine Pinus sylvestris and Norway spruce Picea zation . abies, with scattered deciduous trees of birch At location A (Fig. 1) both sexes were cap- Betula spp., aspen Populus tremula, alder Alnus tured in March and April 1992. The pair suc- incana and rowan Sorbus aucuparia. Forested cessfully bred in a retained aspen on a large land covers 85% of the total area, with the rest clearcut . Eight eggs were laid and 7 young comprised of lakes, wetlands and a few scattered fledged, with 3 and 4 young following the male holdings and pastures . A detailed description of and female parent, respectively, for a 3 week vegetation types is given by Rolstad et al. (1988). period. One egg remained unhatched. This pair The forest has been intensively managed by was monitored continuously until April 1993 . means of stand replacement practices since the At that time we failed to reinstrument the female early 1950s. Clearcutting and replanting 5-50 ha so we lost radio-contact and did not observed units has created a landscape mosaic pattern of her again within the study area. In early May even-aged plantations less than 50 years old, 1993, the male was observed with an unmarked covering 75% of the forested area in 1993.The female at the 1992 nest site. However, shortly remaining 25% comprises uneven-aged, multi- after he disappeared out of the study area and we Rolstad & Rolstad: Home range and habitat use ofthe Grey-headed Woodpecker 3 failed to reinstrument the male, and although we intensively searched the location no birds were observed in the area in late summer, autumn and winter. In spring 1993 a single unmarked male displayed at the location, but no breeding attempt was recorded. Home range size, based on direct observations, or cross-bearings and triangulations closer than 100 m, was estimated by drawing a polygon among the successive outermost plots (Mohr 1965). Habitat selection was assessed by com- paring habitat use with availability . We assumed independency when successive daytime locations were temporally spaced at a minimum of 5 hours. However, less than 10% of the locations of a bird were recorded during the same day. Data on available habitats were extracted from detailed maps and computerized files from the forest owners . This information was carefully cross- checked in the field and from recent air photos to correct for recent forestry measures . We recorded forest stand age and dominating tree species in a 0.1 ha plot surrounding the direct observations and telemetry locations closer than 50 m. When data were pooled among birds, we weighed the available habitats with the number of locations obtained for each bird. Use of feeding substrate Fig. 1 . Seasonal home ranges of a pair (location A) was noted from the direct observations, and prey (location B) Grey-headed Woodpecker at and a male items taken were checked from the feeding site the Varaldskogen study area during 1992-93. Shaded areas indicate summer ranges and polygons indicate after the birds had left. winter ranges of the male (filled symbols) and female The diet was quantified by collecting fresh (open symbols) at location A. Stars denote nest sites, faecal droppings (n = 194) at the feeding sites or triangles daytime locations, circles roosting sites and roosting trees. Faecal droppings in roosting trees squares feeding tables. and nest remains were taken out with a portable vacuum cleaner.They were dried and food items were identified and counted using a binocular lost radio-contact. The unmarked female re- microscope . Most food items could be recog- mained displaying at the location, but also dis- nized from identifiable remains and they were appeared after a few days. categorized according to Appendix 1 . One group At location B (Fig. l) only the male was ra- of insects, Diptera, was impossible to count from dio-instrumented in early May 1992. This pair the droppings. Hence, they were only quantified made two unsuccessful breeding attemps in an as a proportion of biomass. Coleoptera never aspen on a new, large clearcut . The first attempt constituted more than 5% of the biomass intake resulted in the male threwing out two freshly laid in any season and it comprised a variety of dif- eggs.