Ornis Fennica 97: 89–100. 2020 Nest tree characteristics of Grey-headed Woodpeckers (Picus canus) in boreal forests Timo Pakkala*, Juha Tiainen, Heikki Pakkala, Markus Piha & Jari Kouki T. Pakkala, J. Tiainen, University of Helsinki, Lammi Biological Station, FI-16900 Lammi, Finland. * Corresponding author’s e-mail: [email protected] T. Pakkala, J. Kouki, University of Eastern Finland, School of Forest Sciences, P.O. Box 111, FI-80101 Joensuu, Finland J. Tiainen, Natural Resources Institute Finland, P.O. Box 2, FI-00791 Helsinki, Finland H. Pakkala, Käsityöläiskatu 47 A 2, FI-06100 Porvoo, Finland M. Piha, Finnish Museum of Natural History – LUOMUS, P.O.Box 17, FI-00014 Univer- sity of Helsinki, Finland Received 20 January 2020, accepted 10 September 2020 Woodpeckers are important species in forest ecosystems because they make tree cavities that are microhabitats for several other taxa. However, even in boreal areas where most tree cavities are made by woodpeckers, the properties of woodpeckers’nest trees and cav- ities are poorly known. We studied nest tree characteristics of the Grey-headed Wood- pecker (Picus canus) in a 170-km2 forest-dominated area in southern Finland during 1987–2019. The data included 76 nest trees with 80 nest cavities in five different tree spe- cies. During the study period, 44% of all nesting attempts were in old cavities. Nests were found in four forest types, but the proportions of nest tree species differed between them. In all, aspen (Populus tremula) with 70% of nest trees, and with 71% of nest cavities was the dominant cavity tree species. Most nest trees (85%) were dead or decaying, and most cavities (70%) were excavated at visible trunk injury spots. The mean diameter of a nest tree at breast height (DBH) was 37.2 cm and the mean height of a cavity hole was 7.8 m; these were significantly positively correlated. The results highlight the importance of large aspens as nest cavity sites for the species. Conservation and retention of groups of large aspens in main habitats, including clear-cuts, are important for continuous availabi- lity of nest trees. This applies particularly to managed boreal forest landscapes where scarcity of suitable trees may be a limiting factor for the species. 1. Introduction selves, especially to ensure the excavation of strong and safe cavities, but also for several other Woodpeckers are proposed as indicator or key- cavity-nesting animals like mammals, birds and stone species of forest structural complexity and invertebrates that use these cavities afterwards species diversity (Mikusiñski et al. 2001, Roberge (Jones et al. 1994, Drever et al. 2008, Cockle et al. et al. 2008, Pakkala et al. 2014). The properties of 2011, Siitonen & Jonsson 2012, Hardenbol et al. nest trees are important for woodpeckers them- 2019). Most tree cavities in the boreal zone are 90 ORNIS FENNICA Vol. 97, 2020 suggested to be made by woodpeckers (Aitken & 2.2. Grey-headed Woodpecker Martin 2007, Cockle et al. 2011, Andersson et al. nest tree surveys 2018), and thus are very important microhabitats in these forests. As part of an intensive population study of forest In this study, we investigated nest tree charac- bird species, especially woodpeckers, Grey- teristics of the Grey-headed Woodpecker (Picus headed Woodpecker nests and nest trees were canus) in a boreal forest landscape dominated by searched within the study area each year during the managed coniferous forests. The species has a period 1987–2019. The annual census typically wide Eurasian distribution, and it prefers decidu- lasted from early April to mid-July and included ous tree dominated or mixed forests with edges the mapping of woodpecker territories within the and openings (Dementiev & Gladkov 1966, Glutz study area with simultaneous efforts to locate po- von Blotzheim & Bauer 1980, Cramp 1985, Blu- tential nesting sites by observing the behaviour of me 1996, Saari & Südbeck 1997). Moreover, the the woodpeckers, and by searching for nests dur- Grey-headed Woodpeckers mostly use injured or ing the breeding season (described in detail in decaying deciduous trees for excavating their nest Pakkala 2012 and in Pakkala et al. 2014, 2017). cavities (Conrads & Herrmann 1963, Glutz von Annual territory locations and their approxi- Blotzheim & Bauer 1980, Cramp 1985, Südbeck mate borders were defined by information from 2009), but published information of the nest tree observed locations and movements of the wood- characteristics is scarce for boreal forests. peckers, and by the presence of the nest sites. The We explored and documented the main charac- centres of territory sites for the study period were teristics of the nest trees and forest types using a defined by locations of annual territories. The esti- large data set from southern Finland. As the spe- mated total number of territory sites during the cies number and composition of potential nest cav- study period was 29 with a mean of about 15 annu- ity trees greatly differ between boreal and more ally occupied territories within the whole study southern forest areas (Hågvar et al. 1980, Remm & area. Lõhmus 2011, Weso³owski & Martin 2018, Pak- Based on the annual occupancy rates of territo- kala et al. 2019), we anticipate that detailed ries and estimates of nesting success (T. Pakkala, knowledge of the characteristics of nest trees and unpublished data), the data in this study covered of their spatial variation in different types of fo- ca. 20% of all nesting attempts within the study rests and in different geographical locations is im- area during the study period. All surveys of the portant for explicit guidelines for forest manage- nest trees and cavities (see below) were carried out ment and conservation. by author TP. 2. Material and methods 2.3. Nest tree and cavity data 2.1. Study area All the trees with cavities where nesting of the Grey-headed Woodpecker was observed during The study area (170 km2) is located within the the study period were classified as nest trees. We southern boreal vegetation zone in southern Fin- included only those cavities where Grey-headed land (around 61°15’N; 25°03’E; see Pakkala et al. Woodpeckers reached at least the egg-laying 2017). It is dominated by managed coniferous fo- phase, e.g. we omitted cavities where nesting at- rests on mineral soils, with a mixture of stands of tempts were interrupted during excavation, al- different ages, and many small oligotrophic lakes. though they would have contained a seemingly Human settlements in the area are scarce. The fo- complete nest cavity. The observed nest trees and rest management in the study area aims for timber cavities were annually followed during the study production, and the prevailing harvesting method period to check their reuse by the Grey-headed is clear-cutting. Woodpecker. The species of all the detected nest trees was identified. At each nest tree location, the main fo- Pakkala et al.: Nest trees and cavities of the Grey-headed Woodpecker 91 rest type of the site was defined in the field, based 2.4. Statistical methods on the classifications of Finnish forest and peatland types (Cajander 1949, Laine et al. 2012). Forest type and condition of the nest tree between Additionally, the proportions of forest types within the respective groups were compared with good- a 700-m radius from the centre of each territory ness-of-fit tests. The distribution of DBH and the site, corresponding the approximate mean terri- height distribution of the cavity holes in various tory size during the breeding season (T. Pakkala, tree species deviated from normal distribution be- unpublished data), were calculated using land cause of skewness and/or kurtosis, and therefore, cover and forest classification data (Vuorela median-based Kruskal-Wallis or Mann-Whitney 1997), digital topographic maps of the area made tests were used in the comparisons of DBH and by the National Land Survey of Finland, aerial height of the cavity holes between groups. In post photographs and field information from the study hoc comparisons between pairs after a significant area (see Pakkala et al. 2014). result, either a Bonferroni-corrected level p <0.05 We used three classes of the condition of the in the comparisons of proportions (goodness-of-fit nest tree (see also Pakkala et al. 2018). 1) Healthy: tests) or Dunn’s test with Bonferroni correction mainly a vital tree with no signs of decay; small (Kruskal-Wallis tests) was used. Spearman’s rank wounds or damages by external factors possible. correlation coefficient was used in testing the de- 2) Decaying: tree alive, but clear signs of decay pendence between the DBH and height of the cav- visible, e.g. dead branches in the crown and/or top ity holes. All statistical analyses were performed defoliation or needle loss detected. 3) Dead: tree with IBM SPSS Statistics 25. not alive. At the tree level, we used the condition from the year when the first cavity with a nesting attempt was observed during the study period. At 3. Results the cavity level, the first nesting attempt year of each cavity was applied to describe the condition 3.1. Nest tree species and their forest types of the nest tree. The cavity type was divided to two classes A total of 76 nest trees of five deciduous tree spe- whether the cavity was in the main trunk, or in cies, 80 nest cavities, and 118 nesting attempts branch of the tree. We also assessed visually were found in the study (Table 1). Aspen (Populus whether the cavity opening was excavated to an in- tremula) was dominant (70–76%) in the tree, cav- jury spot or a healthy site in the bark layer of the ity, and nesting attempt numbers.