Ornis Fennica 76:115-122. 1999 Suitable habitat distribution for the Long-tailed Tit (Aegithalos caudatus) as indicated by the frequency of occurrence - a long-term study Gunnar Jansson & Lennart Saari Jansson, G., Grimsd Wildlife Research Station, Department of Conservation Biology SLU, S-730 91 Riddarhyttan, Sweden. E-mail: [email protected] Lennart Saari, Vdrrid Subarctic Research Station, Department ofApplied Zoology, University ofHelsinki, P.O. Box 27 (Viiki C) FIN-00014, Finland Received 11 January 1999, accepted 23 April 1999 Data from 22 seasons were used to analyse the occurrence ofLong-tailed Tits (Aegithalos caudatus) in relation to the habitat distribution in 26 one km-squares on the island Aasla in the SW archipelago of Finland. The one km-squares most frequently used by Long-tailed Tits had considerably higher proportions of deciduous/mixed (all species) and alder forest, whereas the number of habitat patches in the km-squares were less correlated to bird occurrences. A threshold for the frequent presence of Long-tailed Tits showed at 15-20% deciduous/mixed forest in km-squares in a logistic regression model, which agrees with the level suggested from a study in a Swedish landscape where the total proportion of deciduous/mixed forest was much lower. Further, the occurrence pattern of Long-tailed Tits over time suggested a density dependent rela- tionship . The six km-squares occupied in low density seasons had a mean proportion of deciduous/mixed forest twice as high and a mean proportion of alder forest three times as high as the 20 others . Long-tailed Tits were also present in these six km- squares in all seasons with higher population densities, as predicted by a density dependent habitat selection. The importance of long-term monitoring, scale considera- tions and the use of population densities for habitat or landscape assessments are discussed. 1. Introduction heterogeneous landscapes (Morris 1995), or un- predictable rates of patch colonisation in land- Aspects oflandscape ecology are now commonly scapes with partly isolated populations (Hanski introduced into Scandinavian forest management et al. 1994, Edenhamn 1996). Further obstacles (Angelstam &Pettersson 1997). However, knowl- to the analyses of large study areas are the diffi- edge ofspecies-habitat relationships on large geo- culty ofdetermining relevant measures of habitat graphical scales applicable in landscape manage- quality (Morrison et al. 1992, Wiens 1995) and ment is scarce . This lack of general results may that long-term studies and experiments covering be due to, for example, different and complex pat- large spatial scales are rare. The relationships terns among species and for homogenous versus between population density and habitat quality ORNIS FENNICA Vol. 76, 1999 may also vary between, and within, species due for Long-tailed Tits on a scale larger than single to the scale of the study area, the season and ge- forest stands . We used the frequency ofbird pres- ography (Wiens et al. 1993, Jokimäki & Huhta ence to reflect the quality of sample plots (one 1996). km-squares) and thereby determine measures of In birds, long-term data are often available and suitable habitat distributions. are used for population surveillance, where The Long-tailed Tit is a resident species in changes in the relative density are analysed (Mar- Fennoscandia . Population densities are generally chant et al. 1990, Koskimies & Väisänen 1991). quite low but show great variations (Svensson However, these data are rarely used by managers 1996), where irruptive invasions and long-distance because, for example, they are often not spatially night flights occur (Lampolahti 1985). Pairs of explicit. It is also clear that the use of population the Long-tailed Tit defend territories only during density for assessment of habitat or landscape the breeding season, approximately three months quality involves scientific pitfalls . Habitat selec- from early to mid-summer (Gaston 1973). Most tion by individuals is suggested to be density de- of the year, however, they roam around in flocks pendent (Svärdsson 1949, Fretwell & Lucas searching for suitable habitat patches, with daily 1970). However, in territorial species with des- movements normally within 1 km2 (Nakamura potic distributions, population density may be in- 1969, Gaston 1973, Bleckert 1991). The preferred versely related to habitat quality or breeding suc- habitat of the Long-tailed Tit is mature deciduous cess (Van Horne 1983, Pulliam 1988, Vickery et forests (> 30 years), often mixed with some coni- al. 1992). In general, studies of the dynamics of fers, where they feed on insects in the canopy population density in relation to habitat quality (Gaston 1973, Rosenberg 1988, Harrap & Quinn show a great variation due to the species and scale 1996). Insects are most abundant on old decidu- under study (Whitham 1980, Holt 1985, O'Connor ous trees of, for example, alder (Alnus spp.) and 1986, Maurer 1986, Pulliam 1988). In spite ofthe birch (Betula spp.) (Ehnström & Waldén 1986). difficulties, the possible use of species occurrence These two generaare also known to be frequently 1991, frequency (if not density) for habitat assessments used by foraging Long-tailed Tits (Bleckert are also shown continues to interest ecologists (Furness &Green- L. Saari unpubl.). Long-tailedTits isolation, as they are com- wood 1993, Pollard & Yates 1993). For defini- to be sensitive to habitat areas with relatively dense aggrega- tion of habitat types plant species are often used mon only in of suitable habitats (Enoksson et al. 1995, (Bunce 1982, Hägglund & Lundmark 1984), but tion Hinsley etal. 1995). Jansson and Angelstam (1999) including animal species in such habitat evalua- presented a model, based on a five year study, for tions as well would further improve their value the occurrence of Long-tailed Tits prior to the (Morrison 1986, Angelstam 1997). breeding season (March-April) inhabitat patches However, to obtain reliable data regarding the in aboreal landscape. Their model showed thresh- relationships between population densities or dis- olds for the isolation ofpatches (< 300 m) and the tributions and habitat characteristics, long-term proportion of suitablehabitat (> 15% per km2) for the only tool studies are often a prerequisite. Also, the reliable presence of Long-tailed Tits. We re- to discriminate between good years andpooryears late our results to their model. and to define natural variations in population den- sities is long-term data on population trends (Kos- kimies & Väisiinen 1991, Morrison et al. 1992, 2. Methods Beshkarev et al. 1994). Unfortunately, useful and/ or comparable data on populations, spanning sev- 2.1. Study area eral seasons, are rare for most species and areas. In the present study we used data on Long- The census was conducted on the island of Aasla tailed Tit (Aegithalos caudatus) occurrences dur- (15.85 km2) in the south-western archipelago of ing 22 seasons in a landscape where suitable habi- Finland (60°17'N, 21°55'E) . The area is owned tats were fairly well represented. The aim of the by private landowners and is characterised by a study was to define a suitable habitat distribution mosaic of coniferous and deciduous forest, agri- Jansson & Saari: Suitable habitat distribution for the Long-tailed Tit cultural fields and lakes. Forestry and agriculture are fine-grained and the distribution offorest habi- tats has not changed notably during our study period. The forest is dominated by scots pine (Pi- nus sylvestris) and norway spruce (Picea abies), and the most common deciduous species are birches (Betula pendula and B. pubescens), black alder (Alnus glutinosa) and aspen (Populus tre- mula) (Saari 1984). Deciduous trees occur both as pure or mixed stands and as single trees dis- persed throughout most forest types. Deciduous and mixed habitats cover about 9% of the island. Based on field determinations, the position and shape of all deciduous/mixed (deciduous-conif- erous) and alderpatches largerthan approximately 0.25 ha were registered on a conventional topo- Fig. 1 . The number of one km-squares at Aasla with graphical map (scale 1 :20 000). These patches Long-tailed Tit presence per season from 1975/76 to were not always delineated by clear borders from 1996/97. The rectangle frames the three seasons selected as low density seasons (see Discussion) . the surrounding habitats, but still, since they were relativelylarge, we treated them as separate patches. A grid of 26 one kilometre squares, which often 2.3. Analyses included water (the Baltic Sea or lakes), covered all parts of the island . For each square we calcu- Observations of Long-tailed Tits, flocks or indi- lated the number of deciduous/mixed and alder viduals, were only used as presence/absence data patches, as well as the total proportion ofthe land per sample plot (one km-square) and season for area (%) covered by these habitats, and from that, the species-habitat analyses . Density was notused also the proportions of deciduous/mixed forest because flock size is often hard to determine and except alders were calculated. The habitat pro- usually decreases during the season and may not portions were measured from the map with aplan- be an accurate estimate of density. However, to imeter (Appendix) . what degree the frequency of observations re- flected the number ofindividuals seen was tested. The relationships between the number of sea- 2.2. Census sons with Long-tailed Tits presence per one km- square and the measured habitat variables were Long-tailed Tits were counted regularly on Aasla analysed by simple and multiple regressions. A during 22 seasons, from 1975/76 to 1996/97. Data logistic regression was used to analyse the rela- on the Long-tailed Tit occurrence were obtained tionshipbetween themostfrequently used one km- by bird counts along a route that went through all squares and a significant habitat variable . the 26 one km-squares ofthe island, approximately Further, the proportions of deciduous/mixed twice a month all the year around.