The Condor 102:267-274 0 The Cooper Omithologuztl Society Zoo0

INFLUENCE OF TREEFALL GAPS ON DISTRIBUTIONS OF BREEDING WITHIN INTERIOR OLD-GROWTH STANDS IN BIAtOWIEiA FOREST, POLAND ’

ROBERT J. FULLER British Trust for Ornithology, The Nunnery, Theford, Norjolk IP24 2PU, UK, e-mail: [email protected]

Abstract. Breeding birds were counted using point counts at 50 treefall gaps and 50 closed-canopy sites within one of the largest tracts of old-growth forest in Europe. Numbers of species and individuals were slightly, but significantly, higher at gaps. Overall species composition differed substantially at gaps and non-gaps. Dunnock (Prunella modularis), Blackcap (Sylvia atricupilla), and Chiffchaff (Phylloscopus collybita) were significantly more abundant at gaps. Wood Warbler (Phylloscopus sibilatriw) and Red-breasted Flycatcher (Ficedulu parva) were significantly more abundant at non-gaps. Warblers (Sylviidae), ground insectivores, ground nesters, and short-distance migrants were significantly more abundant at gaps, but no species groups were more abundant at non-gaps. Eight species breeding in forest edges and young plantations in eastern Poland were not recorded in natural treefall gaps. The proportion of tropical migrant passerines was considerably higher in closed-can- opy stands (0.47) than at gaps (0.29), which contrasts with the situation in most managed European forests where the highest proportions of tropical migrants typically occur in young- growth forests. Key words: Bialowieia Forest, community structure, forest dynamics, old-growth, tree- fall &ps.

INTRODUCTION tional Park remain a singular exception (Falinski Resources used by some forest birds, including 1986, Tomialojc and Wesolowski 1990, Peterken food and preferred habitat structures,can be lo- 1996). These stands have never been systemat- calized within treefall gaps (Blake and Hoppes ically cut but they do have a history of human 1986, Martin and Karr 1986, Levey 1988). I ex- activity, including sporadicfelling and overgraz- amine how treefall gaps in Bialowieia National ing by artificially high numbers of deer and cat- Park, Poland, affected spatial variation in the tle in the early twentieth century. Natural pro- composition of bird assemblages within old- cesseshave been the only source of disturbance growth stands.This was done by comparing bird within the National Park since 1921. The mosaic numbers and habitat attributes at 50 treefall gaps of deciduous, coniferous, and mixed standscon- and 50 closed-canopy locations in 1988. I also tains gaps of varying sizes caused by storms. examine whether the bird speciesthat use edges METHODS or the young-growth stages of managed Euro- pean forests also use natural gaps. STUDY AREA This appears to be the first study of its kind Bialowieia National Park covers an area of 47 for a European temperate forest. Many previous km2 positioned at the core of a much larger area studies have considered responses of birds to of lowland managed forest (> 1,000 km2) ex- young-growth in managed forests, most com- tending over a plain straddling the Polish-Bela- monly the early stagesof tree growth after fell- rus border. Approximately 20% of the boundary ing (Glowacinski 1975, Ferry and Frochot 1990, of the National Park is formed by an agricultural Fuller and Henderson 1992), but not to natural clearing; the remaining boundary lies adjacent treefalls. Historical destruction of forests and in- to managed forest or swampy river valleys. The tensive management of the surviving fragments climate is subcontinental with average tempera- have eliminated opportunities for research on tures of -4.3”C in January and 17.6”C in July natural forest dynamics throughout most of Eu- with up to 92 days snow cover (Wesolowski and rope. The old-growth stands of Bialowieia Na- Tomialojc 1997). Tree communities include de- ciduous and coniferous stands (see Falinski I Received1 December1998. Accepted7 January 1986, Peterken 1996 for details). On the better 2000. drained soils, deciduous standsare dominated by

W71 268 ROBERT J. FULLER small-leaved lime ( cordatu) and hornbeam proximity (200-400 m) and be of similar stand (Curpinus betulus), with variable amounts of type with respect to soil type and tree species Norway maple (Acer plutunoides), pedunculate composition. At each location a point was cho- oak (Quercus robur), and Norway spruce (Piceu sen at which birds were counted (see below); in ubies). Deciduous standsin wet areas are mostly the case of gaps, this point was as close to the of alder (Alms glutinosu) and ash (Fraxinus ex- center of the gap as possible. For 9 of the 50 celsior). Coniferous stands tend to occur in the gaps, it proved impossible to match them with more acid and peaty areas, with spruce more nearby closed-canopy locations and in these cas- abundant on damper, organic soils, and pine (Pi- es an equivalent number of closed-canopy lo- nus sylvestris) on dry sand. The period of heavy cations in similar stand types were selectedsome grazing in the late 1800s and early 1900s ap- distance away (0.6-3.0 km). All closed-canopy peared to favor spruce regeneration, and the for- locations were a minimum of 80 m from the est now seems to be in a recovery phase where- edge of any treefall gap, excluding small canopy by spruce is being replaced by lime, hornbeam, gaps created by the collapse of individual trees. and Norway maple (Peterken 1996). Numbers of All 100 points (i.e., 50 gaps and 50 closed-can- red deer (Centus eluphus) increased during the opy) were at least 400 m inside the National 1980s but this does not appear to have greatly Park so they were not influenced by edge ef- inhibited regeneration (L. Tomialojc, pers. fects. A minimum spacing of 200 m between comm.). Canopy gaps occur in many sizes, from points was achieved. single fallen trees to blowdowns covering sev- eral hectares. Many of the present treefalls in COUNTING BIRDS Bialowieia National Park occur where there has Five-minute point counts were undertaken on been considerable blowdown of mature spruce. three occasions at each of the 100 locations. Each point received one morning count in each GAP DEFINITIONS AND SELECTION OF STUDY of the three periods: 11-17 May, 20-25 May, 3- LOCATIONS 7 June 1988. Counts commenced at dawn and All treefall gaps examined were created by were completed before 11: 15. The sequence blowdown. Gaps contained at least three fallen changed between the three counts so that all trees and were at least 40 m acrosstheir longest points received at least one count in the very dimension (gaps were frequently asymmetrical). early morning. Paired points were always count- Hence, small gaps created by isolated fallen ed within 30 min of one another. Birds were re- trees and gaps containing only standing dead corded in two distance bands: O-30 m (“near”) trees and no fallen trees were excluded. Edges and 30-50 m (“far”). The data were treated as of the gaps were usually clearly defined. Gaps fixed distance counts with radii of 50 m (Hutto typically contained substantial numbers of 1986). This helped ensure that gap counts did standing as well as fallen trees. Exact ages of not include large numbers of records from be- the gaps were unknown but they were all yond the edge of the gap. Detectability of ter- thought to be 2-10 years old, with the exception ritorial singing birds of most forest species is of four casesknown to be less than 2 years old. good at distances up to 50 m (Schieck 1997). Tree regeneration was highly variable, and typ- It was important to be sure that comparisons ically of lime and hornbeam, but was generally of bird communities between gaps and non-gaps no taller than 2 m. Even where tree regeneration were not confounded by differences in detect- was sparse, there was often a dense field layer ability which could be associatedwith the large of raspberry (Rubus idueus). All the gaps stud- differences in vegetation structure. G-tests (Zar ied were in dry areas, the majority (n = 44) 1996) were used to assesswhether the relative being in lime-hornbeam standswhere broadleav- numbers of near to far records differed between es formed at least 50% of the trees. Even in the the gaps and non-gaps. lime-hornbeam, the majority of fallen trees with- The main seasonal song periods of most spe- in the gaps were spruces. cies in Bialowieia Forest were covered by the A total of 50 treefall gaps were located and, count period. The main exceptions were Hazel as far as possible, these were matched with Grouse (Bonasa bonusiu), most woodpeckers, closed-canopy locations. Criteria for matching and possibly Marsh Tit (Purus pulustris). These were that pairs of locations should be in close species are most vocal in April and early May, POLISH TREEFALL GAPS 269 so they are probably under-represented in the tained by each location on axis 1, the main gra- counts. dient, were treated as indices of bird species composition. Forward stepwise multiple regres- VEGETATION SAMPLING sion was used to relate the number of bird spe- Vegetation was measured at all locations, cen- cies, the number of individuals, and scores on tered on the point. Canopy cover was estimated DCA axis 1 to vegetation variables. The vege- to the nearest 5% through a 28-mm camera lens. tation variables were those listed in Table 1 with Canopy height was estimated as the mean of the two exceptions. First, the number of root plates height to the nearest 2.5 m, of the two tallest was excluded because of a large number of ze- trees within 30 m using a clinometer. The spe- ros. Second, in the case of gaps, two additional cies of the nearest 20 trees that were >20 cm variables were used: the minimum and maxi- diameter at breast height (dbh) were recorded mum gap dimensions. Canopy cover and % and the proportion of broadleaved trees calcu- broadleaved trees were arcsine transformed and lated. Any other species of trees or woody tree height, understory density, snags, and fallen shrubswithin 10 m were also listed and together trees were log transformed. Results from step- with the data from the 20 large trees used to wise regression were compared with those from calculate tree and shrub richness. The density of all possible combinations of variables (PROC the understory (shrub layer foliage plus fallen RSQUARE; SAS Institute 1996). This con- trees) was estimated using a checkered sighting firmed that in all cases the stepwise regression board (Fuller and Henderson 1992). The dis- had selected the optimum combination of vari- tance at which the board was obscured by veg- ables in terms of maximizing R2. etation at eye height was estimated to the nearest Analyses were conducted separately for the 10 m on NE, SE, SW, NW bearings. An index 41 pairs of matched points, but in all cases the of shrub density was then derived from the re- patterns and conclusions drawn were the same ciprocal of the mean X 1,000. All trees of dbh as for the full data set. Therefore, results are > 20 cm were counted that were lying on or only presented for analyses of all 100 points. close to the ground and for which any part lay within 10 m of the point. Toppled, but support- RESULTS ed, trees were counted as fallen. All snagswithin VEGETATION 10 m were counted. A snag was delined as a Gaps and non-gaps differed greatly in structural standing dead tree or stump at least 2 m tall and aspectsof habitat but did not differ in the broad 220 cm dbh. Vertical root plates >2 m tall and composition of tree communities (Table 1). Can- within 10 m of the point were counted. Mini- opy openness, understory density, and numbers mum and maximum dimensions of the gaps of fallen trees, snags, and root plates were far were estimated to the nearest 10 m. higher at gaps than non-gaps. Tree height was slightly lower and tree and shrub richness slight- DATA ANALYSIS ly higher at gaps than non-gaps. There was no For each point, a single abundance value was difference in the ratio of standing broadleaved derived for each species by taking the highest trees between gaps and non-gaps. count recorded within 50 m of the point from any of the three visits. The resulting species DETECTABILITY AT GAPS AND NON-GAPS abundances were compared between gaps and For all species combined, the total number of non-gaps using chi-square goodness-of-fit tests near and far individuals recorded at gaps were with the Yates’ correction with the expectation 307 and 606 (ratio 0.51) and at closed-canopy that individuals should be equally abundant in points 262 and 546 (ratio 0.48) respectively (G, gaps and non-gaps. Detrended correspondence = 0.26, P > 0.50). This calculation also was analysis (DCA) was used, without downweight- made for all families of birds with at least three ing for rare species (Jongman et al. 1995) to species recorded: Picidae, Turdidae, Sylviidae, determine whether overall bird species compo- Muscicapidae, Paridae, Fringillidae (all G, 5 sition at gaps differed from non-gaps. This or- 1.8, all P > 0.10). Samples sizes were adequate dination approach arranges samples in multidi- in both gaps and at closed-canopy locations to mensional space defined by axes that represent test for differences in relative numbers of near gradients of bird speciescomposition. Scores at- to far counts in 16 species.Only for Wood War- 270 ROBERT J. FULLER

TABLE 1. Vegetation at gaps and non-gaps. Median values, with 25% and 75% quartiles in parentheses,are shown. Differences between gaps and non-gaps were tested by Mann-Whitney U-tests.

Gap Non-gap Variable (n = 50) (n = 50) 2 P Canopy cover (%) 15 (10-35) 90 (85-95) 8.54

bler (Phylloscopus sibilutrix) was there a signif- stronger for the number of species than for in- icant result (G, = 11.8, P < 0.001). At gaps, the dividuals suggests that the higher species rich- number of near and far individual Wood War- ness at gaps was not simply a consequence of a blers was 13 and 57 (ratio 0.23), respectively, larger number of individuals being recorded but at non-gaps the respective numbers were 57 there. and 84 (ratio = 0.68). The relatively low number Gaps and non-gaps differed substantially in of near records from gaps was almost certainly overall bird species composition, showing a consequence of the fact that Wood Warblers marked separation on DCA axis 1. The axis had avoid areas of dense understory. In conclusion, an eigenvalue of 0.244. Scores of gaps and non- there was no evidence that detectability was a gaps were significantly different on axis 1 serious confounding factor in this study. (Mann-Whitney U-test, z = 6.26, P < O.OOl), but were not significantly different on axis 2. COMPARISONS OF BIRD ASSEMBLAGES AT Hence, axis 1 represented a clear gradient in GAPS AND NON-GAPS species composition from gaps to non-gaps. More species and more individuals were record- Across all 100 points, the number of species ed at gaps than non-gaps. Median numbers of was positively related to understory density and species (? SD) were 10.6 ? 2.3 at gaps and 8.7 tree species richness, whereas number of indi- -C 2.7 at non-gaps (Mann-Whitney U-test, z = viduals increased with tree richness and the pro- 3.71, P < 0.001) and of individuals 12.5 5 2.6 portion of broadleaved trees (Table 2). The re- and 10.7 -C 3.1, respectively (z = 3.03, P < gression model explaining overall bird species 0.01). The fact that differences were slightly composition (as indexed by DCA axis-l scores)

TABLE 2. Regression models accounting for variation in number of species, number of individuals, and bird species composition, the latter as shown by sample scores on axis 1 of detrended correspondence analyses. Variables used were those listed in Table 1 together with minimum and maximum gap dimension for the analyses of gaps. Variables are listed in the order they were entered in the models with cumulative R2 and the sign indicating direction of the relationship in parentheses. All variables listed are significant at least at P < 0.05.

Number of species All sites” understory density (0.15+), number of tree species (0.20+) Gaps no variables significant Non-gaps number of tree species (0.21+) Number of individuals All sites number of tree species (0.09+), broadleaved trees (0.15+) Gaps no variables significant Non-gaps number of tree species (0.17+), broadleaved trees (0.24+) Bird species composition All sites canopy cover (0.36-), broadleaved trees (0.40+), number of fallen trees (0.45+) Gaps minimum gap dimension (0.3 1 -) Non-gaps Broadleaved trees (0.27-), understory density (0.39-)

d All sites;n = 100, Gaps: n = 50, Non-gaps: n = 50 POLISH TREEFALL GAPS 271

TABLE 3. Abundance of birds within selected groups defined by , feeding habits, nest site, and migratory status.Chi-square goodness-of-fit tests with the Yates’ correction were used to assessprobability of no difference between gaps and closed-canopylocations with an expectation of equal numbers in both. Groups are defined following TomiaIojC and Wesolowski (1990).

Gaps Non-gaps (n = 50) (n = 50) X2 P Families Picidae 30 19 2.0 ns Turdidae 81 81 0 Sylviidae 159 111 8.2 <0:;1 Muscicapidae 62 79 1.8 ns Paridae 70 54 1.8 ns Fringillidae 112 108 0.1 ns Feeding groups Ground insectivores 205 146 9.6

accounted for more than twice as much variation Spotted Woodpecker (Dendrocopos minor), as those for numbers of species and individuals Three-toed Woodpecker (Picoides tridactylus), (Table 2). The main gradient in bird species Tree Pipit (Anthus trivialis), and Bullfinch (Pyr- composition was explained in terms of canopy rhula pyrrhula). Two species were more abun- cover, proportion of broadleaves, and number of dant at closed-canopy locations: Wood Warbler fallen trees. Canopy cover and fallen trees both (x2] = 8.8, P < 0.01) and Red-breasted Fly- differed strongly between gaps and non-gaps, catcher (Ficedula parva) (x2, = 10.3, P < but the proportion of broadleaved trees did not 0.001). (Table 1). This suggests that much of the main Broader patterns in bird community compo- variation in bird species composition was ac- sition of gaps and non-gaps were examined with counted for by whether points were gaps or non- respect to groups of species defined by taxono- gaps, but that an additional element of variation my (families), feeding habits, nest site, and mi- was accounted for by tree species composition. gratory status (Table 3). To ensure that species Despite the general difference in bird species were appropriately classified and weighted by composition at gaps and non-gaps, few species groups, I followed the information in Table 1 of differed significantly in frequency or abundance TomialojC and Wesolowski (1990). Warblers between gaps and non-gaps. For 25 species it (Sylviidae), ground insectivores, ground nesters, was possible to test the null hypothesis that and short-distance migrants were more abundant numbers were equal at gaps and non-gaps with at gaps than expected. No groups were more chi-squared goodness-of-fit tests. Blackcap (Syl- abundant than expected at non-gaps. More fly- via atricupilla) and Chiffchaff (Phylloscopus catchers (Muscicapidae), crown nesters, and collybitu) were strongly more abundant (xzl = long-distance migrants were counted at non- 16.2, P < 0.001 and xzl = 32.7, P < 0.001, gaps than gaps but the differences were not sig- respectively), and Dunnock (Prunella modular- nificant. is) weakly more abundant (x2, = 5.3, P < O.OS), at gaps than non-gaps. Several scarcer species VARIATION IN BIRD ASSEMBLAGES AT GAPS were recorded solely at gaps, but sample sizes AND NON-GAPS were too small to draw conclusions: Hazel Within gaps, numbers of species and individuals Grouse, Stock Dove ( oenas), Lesser could not be related to variation in vegetation or 272 ROBERT J. FULLER gap size. However, the main gradient in bird In terms of vegetation structure, natural tree- speciescomposition, as indexed by DCA axis-l falls typically differ from many young managed scores, was solely related to minimum gap di- standsin several respects.Treefalls usually con- mension (Table 2). Effect of gap size was ex- tain a substantial number of live standing trees, amined further by classifying gaps according to many of which may have damaged limbs which size and calculating frequency of occurrence rapidly become sources of dead wood and nest within each size class for the 12 speciesrecord- cavities. Young managed stands rarely retain ed most frequently at gaps. Exact sizes of gaps such high densities of standing trees. Dead wood were not obtained, so gaps were classified by is typically a scarceresource in managed forests minimum dimension as small (< 50 m, II = 17), at all stagesof growth (Kirby et al. 1991, 1998), medium (50-90 m, n = 18), and large (> 90 m, whereas natural treefalls offer concentrations of n = 15). Two species,Dunnock and Chiffchaff, standing and fallen dead wood. The floor of a appeared to avoid small gaps or to select large treefall is a dense tangle of fallen trees and ex- gaps, whereas five species appeared to avoid posed root plates and may even exhibit an “in- large gaps or to prefer small gaps: Winter Wren stant shrub layer” formed by the canopies of (Troglodytes troglodytes), Blackbird (Turdus fallen trees. Natural treefalls will tend, therefore, me&a), Wood Warbler, Great Tit (Parus ma- to offer more niches for forest bird speciesthan jor), and Nuthatch (Sitta europaea). There was will most managed young-growth. The differ- no evidence, however, that any of these species ence in bird assemblage structure between tree- occurred more or less frequently in particular fall gaps and adjacent closed old-growth forest gap size classes than one would expect by is, therefore, likely to be less marked than in the chance (all chi-square tests P > 0.10, except case of young and old stands in managed forest. Dunnock, Wood Warbler and Nuthatch where It appearsthat several speciesusing forest edg- 0.10 > P > 0.05). At non-gaps, the number of es or young-growth habitats within modem Eu- speciesrecorded was related to tree speciesrich- ropean forestsdo not occur in, or are rare in, the ness and number of individuals to tree species interior treefall gaps in Bialowieia National Park. richness and to proportion of broadleaves (Table All of the gap specialistsidentified in this study 2). Variation in bird speciescomposition at non- also used standsat the edge of the National Park gaps was related to proportion of broadleaves and man-made gaps in the surrounding managed and to understory density (Table 2). forest. Several species occur in edge habitats at DISCUSSION Bialowieia that I did not record in interior gaps including Redwing (Turdus iliacus), Icterine War- Previous multi-species studies of use of treefall bler (Hippolais icterina), Barred Warbler (Sylvia gaps in temperate forests have focused on mi- nisoria), and Rosefinch (Carpodacus erythrinus) grating birds and have found that some species, (TomiaIojC and Wesolowski 1990). Species oc- particularly foliage insectivores and frugivores, are more abundant in gaps (Blake and Hoppes curring in young plantations in the surrounding 1986, Martin and Karr 1986). To my knowledge, forest, but not in the natural treefall gaps, were this paper is the first study to provide evidence Whitethroat (Sylvia communis), Red-backed that treefall gaps also influence the spatial pat- Shrike (Lanius collurio), Linnet (Carduelis can- terning of bird communities during the breeding nabina), and Yellowhammer (Emberiza citrinel- season. Nonetheless, there was considerable la). Some other speciesare more abundant at edg- overlap in the bird species composition at gaps es or in managed stands than in treefall gaps. and non-gaps, with only a handful of species Chiffchaff, although strongly associated with showing strong preferences for gaps or for gaps within the natural forest, attains even higher closed-canopy areas. This contrasts with Euro- densities in the managed forests and appearsto pean managed forests where many species are have benefitted from forest management (Pio- confined to one particular stage of growth. Ex- trowska and Wesolowski 1989). Thrush Nightin- amples include the studies of Glowacinski gale (Luscinia Zuscinia)was recordedin just two (1975), Smith et al. (1985), and Ferry and Fro- treefall gaps but was far more abundant at the chat (1990) which show that hole-nesting spe- forest edge (pens. observ.). Willow Warblers cies can be largely absent from early stages of (Phylloscopus trochilus) were recorded in only growth in managed forests. two treefall gaps which was surprising given the POLISH TREEFALL GAPS 273 ubiquity of the species in young-growth forests invertebrates. The higher abundance of ground throughout central and western Europe. insectivores at gaps was probably a response to Previous work on the composition of Euro- the former resource. These species included pean forest passerine assemblages has shown ones that forage on the ground (e.g., Tree Pipit that tropical migrants contribute a higher pro- and Dunnock) and ones that forage in the field portion of individuals in young-growth habitats layer or low shrub layer (e.g., Garden Warbler, than in later stages of forest growth (Helle and Blackcap, Winter Wren). There was no evidence Fuller 1988). One would expect, therefore, that that bark insectivores, the group most likely to the proportion of tropical migrants should be benefit from an abundance of dead wood, were higher at gaps than non-gaps. This was not the more abundant at gaps. These species include case. The proportion of tropical migrant passer- the woodpeckers, Nuthatch, and Treecreeper ines in gaps was 0.29 compared with 0.47 at (Certhiu familiaris). Angelstam and Mikusinski closed-canopy points. This was largely due to (1994) have argued that the spatial dynamics of the high abundance of flycatchers (Muscicapi- several woodpecker species in natural forests dae) and Wood Warblers in closed-canopy for- will be closely linked with the availability of est. Previous studies of habitat distribution pat- dead wood created through natural disturbances, terns of tropical migrants in Europe have been both fire and blowdown. The failure to detect based mainly on managed plantations which sel- relationships between gaps and woodpecker dis- dom contain stands of old trees with large num- tribution in Bialowieia may be partly method- bers of suitable hole nesting sites for flycatchers. ological-studies of foraging ecology, including Fuller and Crick (1992) also reported that ma- ranging behavior are more appropriate for as- ture western oakwoodsin upland Britain support certaining the importance of gaps to these spe- high ratios of migrants to residents. I suggest cies. It also is likely that gaps will be more that the apparent associationof tropical migrants heavily used in winter than in the breeding sea- in Europe with young-growth habitats may be son by woodpeckers. During the breeding sea- partly a consequence of forest management son, several species feed mainly by surface which tends to remove habitats favored by hole- gleaning but in winter become more dependent nesting tropical migrants. on excavating insects from dead wood (Snow Treefall gaps provide concentrations of fruit and Perrins 1998). Another possibility that mer- and insects associated with dense low foliage its further investigation is that there may be rap- that are heavily exploited by migrant birds id successionof bark insect communities follow- (Blake and Hoppes 1986, Martin and Karr ing tree death and that gaps may offer a rather 1986). During the breeding season,gaps also are short-lived food resource for some bark insec- likely to provide important localized resources tivores. for birds. These resources may be food, espe- Gaps may be more significant to breeding cially invertebrates, or preferred habitat struc- birds than suggestedby the results of this paper. tures. The relatively dense understory foliage is Inevitably, breeding season counts in forest are probably an important factor influencing use of biased towards vocal individuals, especially gaps by Dunnock, Blackcap, Garden Warbler singing birds, and hence can reveal only the (Sylvia borin), and Chiffchaff. Selection of gaps broadest patterns of habitat use. Counts tend to by these insectivorous speciesis consistent with produce little information about nonvocal for- their habitat preferences in managed forests aging individuals. It is very likely, therefore, that where they are associated with early stages of important patterns of habitat use will have gone forest growth with low dense regeneration (Fer- undetected by the present study. For example, ry and Frochot 1990, Fuller and Henderson gaps could act as resource centers for insectiv- 1992). Tree Pipit (Anthus trivialis) is probably orous speciesnesting in the surrounding closed- the first breeding bird speciesto colonize newly canopy forest. If this is the case, territories close formed gaps, a pattern of habitat selection con- to gaps could be of higher quality than territories sistent with that in managed forest (Fuller and farther away with consequencesfor reproductive Moreton 1987). success,site fidelity, and spatial patterns of age Gaps probably offer concentrations of two structure. Smith and Dallman (1996) described main types of invertebrate food resources for a situation where breeding Black-throated Green birds: low-foliage invertebrates and dead-wood Warblers Dendroica virens used mosaics of gaps 274 ROBERT J. FULLER and closed-canopy forest but spent more time ties in the Niepolomice Forest (southern Poland). foraging within gaps than in contiguous forest. Ekol. Polska 23:231-264. HELLE,P, AND R. J. FULLER.1988. Migrant passerine Studies involving radio tracked individuals are birds in Europeanforest successionsin relation to highly desirable to gain a more realistic apprais- vegetation height and geographical position. J. al of the role of patchiness within forests in de- Anim. Ecol. 57:565-579. termining habitat quality for insectivorous birds. HUWO, R. L., S. M. PLETSCHET,AND F? HENDRICKS. 1986. A fixed-radius point count method for non- Such work is needed acrossa wider range of gap breeding and breeding seasonuse. Auk 103:593- sizes than examined in the present study. Even 602. small gaps, created by single treefalls, may cre- JONGMAN,R. H. G., C. J. E TER BRAAK,AND 0. E R. ate important feeding habitats in the context of VAN TONGEREN.1995. Data analysis in commu- nity and landscape ecology. Cambridge Univ. individual territories. Press, Cambridge. KIRBY, K. J., C. M. REID, R. C. THOMAS,AND E B. ACKNOWLEDGMENTS GOLDSMITH.1998. Preliminary estimatesof fallen dead wood and standing dead trees in managed I am indebtedto Ludwik TomiaIojC for giving me the and unmanaged forests in Britain. J. Appl. Ecol. opportunity to work in BiaIowieia National Park and 35:148-l%. for making all the arrangements for my stay in Poland. KIRBY, K. J., S. D. WEBSTER,AND A. ANCTZAK.1991. Both he and Tomasz Wesolowski provided much ad- Effects of forest management on stand structure vice and support during the study and made valuable and the quantity of fallen dead wood: some British comments on the manuscript, as did Dan Chamberlain and Polish examples. For. Ecol. Manage. 43: 167- and an anonymous referee. For further help in prepar- 174. ing this paper I thank Carolyne Ray, Steve Freeman, LEVEY,D. J. 1988. Tropical wet forest treefall gapsand Su Gough, and Nicki Read. I am grateful to the Na- distributionsof understorybirds and plants. Ecol- tional Park authority for permission to work freely ogy 69:1076-1089. within the Park. My-stay in Poland was supported fi- MARTIN,T. E., AND J. R. KARR. 1986. Patch utilization nanciallv bv the Universitv of Wroclaw. the British by migrating birds: resource oriented? Ornis Trust fo; Omithology, and ihe former Nature Conser- Stand. 17:165-174. vancy Council of the UK. PETERKEN,G. I? 1996. 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