Ornis Fennica 77:65-70. 2000 Influence of large snow depths on Black Woodpecker Dryocopus martius foraging behavior Jørund Rolstad & Erlend Rolstad Rolstad, J., Norwegian Forest Research Institute, Høgskoleveien 12, N-1430 As, Norway; e-mail: [email protected] Rolstad, E., Norwegian Forest Research Institute, Høgskoleveien 12, N-1430 As, Norway Received 2 June 1999, accepted 14 October 1999 Large snow depths are assumed to constrain the Black Woodpecker's (Dryocopus martius) ability to feed on carpenter ants (Camponotus herculeanus) in stumps and logging debris following clearcutting. To document foraging behavior in a snow-rich area, we radio-marked three Black Woodpeckers in Nordmarka, southcentral Norway, and compared the results with data from a nearby snow-poor area at Varaldskogen (Rolstad et al. 1998). At snow depths below ca. 1 m, birds were feeding on carpenter ants in stumps and dead downed wood. At snow depths above 1 m birds increasingly fed on carpenter ants in the base of trunks of infested living trees, and on bark beetles (Ips typographus) and beetle larvae in dead standing trees. Home ranges at Nordmarka were markedly larger (mean = 449 ha, SD = 71, n = 3) than at Varaldskogen (mean = 226 ha, SD = 109, n = 23). One female at Nordmarka, killed by a pine marten (Martes martes), had lost 20% of body mass due to starvation . The results indicate that winter food limits Black Woodpecker populations in snow-rich managed forests, and we suggest that arboreal feeding on bark beetles renders the birds more vulnerable to goshawk (Accipiter gentilis) predation. 1. Introduction Tjernberg 1992). In northern regions carpenter ants (Camponotus spp.) constitute the staple win- The Black Woodpecker (Dryocopus martius) is terfood (Pynnönen 1939, 1943, Mikusinski 1997), the largest resident bird of Palearctic boreal for- which the birds excavate from colonies in dead ests, which is obligate insectivore during winter. wood and infested trees. In southcentral Scandi- During the last century it has expanded its range navia the clearcutting practice of modern forestry and increased in number in central and western seems to benefit the Black Woodpecker by creat- Europe, explained by an increase in the amount ing large amounts of stumps colonized by ants of coniferous forest (Mikusinski 1995). In con- (Rolstad et al. 1998). However, this applied to a trast, northeastern Europe, and northern Finland snow-poor area. Although the black woodpecker in particular, seem to have experienced a popula- is known to dig deep holes in the snow to access tion decline despite a slight increase in total for- subnival ant colonies (Pynnönen 1943), at some ested area and growing stock ofconifers (Demen- critical snow depth stumps and downed woods tiev et al. 1951, Järvinen et al. 1977, Ahlén & are expected to become inaccessible . In managed 66 ORNIS FENNICA Vol. 77, 2000 forests vertical structures like snags and rotten W w mm~_a) å 0 trees are being harvested in salvage silvicultural mwwc°m - operations. It can therefore be hypothesized that changes in the internal forest structure, combined N 1 c < M8 with severe snow conditions, constrain the winter ~° å in northern regions N N C- m diet of Black Woodpeckers 00 `7 CD w a (Mikusinski 1997, Rolstad et al. 1998). The hy- N _7 c a cT N w w (0 !R å Z w pothesis is supported in a Finnish birdcensus doc- w 7 O , f0 umenting an inverse relationship between snow " s depth in February and abundance ofBlack Wood- =o ~D' peckerthe following breeding season (Saari &Mi- co ~m ID. 0 w kusinski 1996). Likewise, a positive correlation OcnCCD -«D a between mean winter temperatures and a winter m index of Black Woodpecker numbers in a Swed- a ish bird census conforms to this explanation (Nils- CM OODD 0) :3 4, 0 (D <0 3 N son et al. 1992). 4 w 00 2: CD <D In this paper we present data on three radio- Q marked Black Woodpeckers from a snow-rich ö m area, Nordmarka in Norway, which supports the m c° ö cöi å ö m o hypothesis that this woodpecker's ability to find 0 ö food in managed forests is limited by large snow depths. Combined with previous data from the Va- cö =~ o m raldskogen study area, the findings point at a criti- v,cow <D ø) co d aw cal snow depth of ca. 1 m, above which ground ö feeding becomes prohibited. z 3 w o w åc 0 m co io a cQ c0D Q 2. Study areas, material and methods (n cn Q Three Black Woodpeckers, one male and two fe- 0 0 "' r y å males, were captured in roosting holes and radio- O O) N (D CD monitored during January-April of 1994 (Ta- w 0 ö m ble 1). All resided within a 5 000-ha forested tract, centrally located at 300-400 m a.s.l. within the z Normarka area, 25 km north of Oslo in south- 0 A A co c Ö. central Norway (60°06'N, 10°45'E, Fig. 1). Nord- w marka is in the middle boreal zone (Ahti et al. w 1968), and the forest is dominated by Norway o~å7 m:E w spruce (Picea abies), with Scots pines (Pinus syl- swå'' å scattered at bogs and on poor sites with w 0 vestris) å w. ~ _a~ exposed rocks. Birch (Betula pubescens), rowan N 0 «0 0?CD0° (Sorbus aucuparia), and alder (Alnus incana) oc- co 2 ~ ' N n 0 cur regularly along wetlands, rivers and lakes. w å .w0.. ~ (D 3 Aspen (Populus tremula) is rare, but occurs as m 3 :3 3 w single trees, or in small patches, presumably on w -o w ii N N N sites that previously have been burned or dis- N ö ö N 0 turbed. Snow covers the ground from mid-No- c vember to mid-May, with maximum snow depths 0 m0 of 1-2 m occurring in February-March . The cli- 5 m C- ö mate is humid with yearly precipitation averag- o. w Rolstad & Rolstad: Influence oflarge snow depths on Black Woodpecker 67 ing 1 200 mm. The Nordmarka data is compared with that of Varaldskogen (Rolstad et al. 1998), a 10 000-ha study area in the middle boreal zone (60°10'N, 12°30'E), 100 km east of Nordmarka (Fig. l). Va- raldskogen has a more continental climate, and snow depth rarely exceeds 1 m. Here we use data from 23 birds (15 males and 8 females) with >_ 20 radio-fixes during December-March 1990-1995 . A detailed description of the study area and dataset from Varaldskogen is reported in Rolstad and Rolstad (1995) and Rolstad et al. (1998). The woodpeckers were fitted with a 7-g (1 .8- 2.6% of body mass) radio transmitter (Biotrack, Wareham, UK) attached to the back with a nylon harness enclosed in silicon rubber. Expected bat- tery lifetime was 8 months, and average detecting distance of radio-signals was 5 km. Field moni- Fig. 1 . Map of home ranges and habitats of three toring and assessment of home ranges followed Black Woodpeckers monitored by radio-telemetry in the procedure of the Varaldskogen study (Rolstad Nordmarka, southcentral Scandinavia, January-April et al. 1998). The proportional use ofdifferent feed- 1994. Black areas show forest stands with trees killed ing substrates was obtained by approaching the by bark beetles. Grey shadings denote water-systems. birds at close distance . In most cases the birds Inset map shows the locations of the Nordmarka (N) and Varaldskogen (V) study areas. Confer Table 1 were not flushed, and the feeding sites were later for statistics . revisited to ascertain feeding substrate and source of prey items. Feeding sites were easily detected by fresh signs of wooden debris and prey remains previous spring and summer. Bark beetles oc- in the snow. Snow depth, averaged to the nearest curred in two life-history stages ; as immature bee- 10 cm, was measured at level ground at 5 sites in tles and as larvae that remained arboreal in win- open habitat, adjacent to the bird plots. The age ter. of forest stands was extracted from forest plan We recorded three types of feeding techniques documents of the forest owner, cross-checked in that were closely linked to type of prey and habi- the field by counting twig wreaths and annual rings tat (Fig. 2). First, up to snow depths of 120 cm, sampled with an increment borer. birds mainly excavated carpenter ants from colo- nies in stumps and downed woods, after remov- ing snow with lateral movements of their bill. 3. Results and discussion Birds often excavated stumps beneath branches of dense young spruce trees that were less cov- At Nordmarka snow depth increased from 100 cm ered with snow. These substrates were remains in mid-January to 150 cm in late-February and from loggings, and young spruce plantations (15- March. At Varaldskogen snow depth did not ex- 40 years) were the most frequented habitats. Sec- ceed 70 cm. We observed two main prey groups, ond, as snow accumulated, birds sought out large- carpenter ant (Camponotus herculeanus) and bark stemmed, living spruce trees where they excavated beetle (Ips thypographus). Otherprey groups were carpenter ants from colonies within the basis of occasionally recorded in negligible proportions the trunks. Birds often spent the whole day get- (e.g. Cerambycid larvae at Varaldskogen). Car- ting access tothese large colonies, and once opened, penter ants occurred in stumps, dead downed they revisited these sites at later events. Most of woods, and basal trunks of living spruce trees. these trees were in old forest stands .