Model assumptions

Under the HSI approach, the habitat conditions defined as optimal are those that allow the greatest carrying capacity for the target species (Schamberger and O´Neil 1986). The concepts of carrying capacity and habitat suitability underlying this present model reflect those factors of habitat structure that satisfy life-history needs, determine habitat preferences, and influence the distribution of Capercaillie. This narrow, operational definition does not include other factors that may impact the distribution and abundance of Capercaillie, e.g., predation or climate (Schamberger and O´Neil 1986). In general, such factors could be included in HSI models. However, for in the Alps, in general, and the study areas, in particular, data are lacking that would allow the inclusion of habitat factors other than vegetation.

The Capercaillie model uses information on habitat preferences to assess habitat suitability. The underlying assumption, which is commonly and perhaps uncritically made in Capercaillie conservation practice, is that habitat preferences reflect habitat quality (for definitions of habitat-related terms used, see Hall et al. 1997) and thus are related to reproductive success and/or survival, and finally to carrying capacity and population density (Van Horne and Wiens 1991). Although there is some preliminary evidence for a positive relationship between Capercaillie habitat preferences and survival (Gjerde and Wegge 1989, Storch 1994, 1997a), this assumption has not been formally tested. This study provides further evidence that habitats that provide small-scale structures preferred by Capercaillie indeed support greater numbers of birds.

Model development

I describe the processes of constructing and testing the HSI model using the terminology of Van Horne and Wiens (1991; see also Brooks 1997): model development (to identify, construct, and combine a set of habitat variables into a series of equations to calculate an overall HSI score), verification (to match the model operations and output to the data set used for model construction), and validation (to match the model output to an independent dataset that was not used in model development).

As is commonly done with HSI models, the model was constructed based on a combination of research data and expert opinion (Morrison et al. 1992). I identified a set of habitat variables that had been shown to significantly influence Capercaillie habitat use in a five-year (1988–1992) telemetry study of 24 male and 16 female Capercaillie in the 50-km² Teisenberg area of the Bavarian Alps (Storch 1993a, b, 1994). Because the model should be applicable throughout the Bavarian Alps, variables were included that were believed to influence Capercaillie elsewhere, but that had not been relevant on Teisenberg. For better applicability, variables were defined in order to match the measures used in stand descriptions by the Bavarian State Forest Service whenever possible. These are the kinds of data typically available for forest management decisions in the Alps and elsewhere in central Europe.

Based on the Teisenberg telemetry results (Storch 1993a, b, 1994) and experience from other areas in the Bavarian Alps (I. Storch, unpublished data, A. Zeitler, personal communication), a suitability index (SI) function was constructed for each model variable, describing the assumed relationship between the variable and Capercaillie habitat use by values between 1 (optimal) and 0 (unsuitable) (see Fig. A1). Some variables may reduce, but not exclude, habitat suitability; in such cases, minimum SI scores were set >0. SIs were then combined into simple equations to calculate overall HSI scores. Scores for winter (HSI_wi) and summer (HSI_su) habitat suitability were calculated separately and then combined in an annual HSI score (HSI_year).

Fig. A1. Suitability index (SI) functions for the variables of the HSI winter model (above) and the HSI summer model (below). A high SI score indicates preference, low scores avoidance of the habitat by Capercaillie. For successional stage, clearcuts < 1ha in size received greater SI succession scores that clearcuts > 1 ha; for canopy cover, SI canopy scores were greater if gaps existed, e.g., due to snowbreak or windthrow; for stand type, SI scores were greater if fir or pine trees existed, the preferred winter food sources of Capercaillie (dark vs. light bars).

The mathematical ways of combining the variables’ SIs were chosen to reflect their assumed role in Capercaillie habitat relationships (Van Horne and Wiens 1991). Multiplication of SI scores results in a large potential influence of each individual variable, and a zero score for any SI will lead to a zero overall HSI score; such variables function as limiting factors. The arithmetic mean is used if variables are assumed to be compensatory; their SIs contribute equally to the overall score. The geometric mean is used if the SIs are assumed to be partly compensatory, but the overall value is weighed by the smallest SI.

Model variables and suitability index functions

Capercaillie have seasonally distinct habitat needs. In winter, they feed on conifer needles and spend most of their time on trees, whereas in summer they prefer habitats with abundant ericaceous shrubs, particularly bilberry, Vaccinium myrtillis (Storch 1995b), for food and cover. For the purpose of the model, “winter” represents the time period with a snow layer, and “summer” represents the snow-free periods, although during spring and autumn, Capercaillie may show intermediate habitat preferences. Males, females, and broods show the same general patterns of habitat use (Storch et al. 1991, Storch 1993a, b, 1994, 1995a). Distinct winter and summer habitat suitability indices, HSI_wi and HSI_su, were constructed to reflect the seasonal habitat preferences of Capercaillie. Eight habitat variables were included in the model. Suitability index (SI) functions for the variables in the HSI winter model and the HSI summer model are shown in Fig. A1. Below, I sketch Capercaillie habitat preferences in the Bavarian Alps as a rationale for the development of suitability indices for each model variable, based on results from Teisenberg (Storch et al. 1991, Storch 1993a, b, 1994, 1995a, 1997a) and other work (A. Zeitler, unpublished data, I. Storch, unpublished data).

    1) Steepness of slope (SI_slo): Capercaillie rarely use steep terrain and prefer level ground and moderate slopes. This holds for both sexes, throughout the year, and independently of habitat. Because steepness of slope is not likely to completely exclude Capercaillie use, the minimum score is 0.4.

    2) Relative elevation (SI_ele): Capercaillie avoid the lower elevations, regardless of habitat structure. Elevation <300 m (score 0.75) and <100 m (score 0.5) above the farmland valley floor will reduce habitat suitability. (However, refer to the main text section Discussion: Speculation for a discussion of the relevance of elevation as a habitat variable.)

    3) Successional stage (SI_suc): Capercaillie are forest obligates. They largely avoid open areas such as alpine pastures or meadows (score 0.2). In even-aged managed forests, they prefer pole-stage and older stands (score 1); the canopy cover and ground vegetation of a stand, however, are more important than its age. Capercaillie rarely use thickets or large clearcuts (score 0.4), but use small clearcuts (<1 ha) more often (score 0.6).

    4) Canopy cover (SI_can): Moderate canopy cover is a prerequisite for a rich ground vegetation. Capercaillie prefer somewhat denser cover (±60%) in winter than in summer (±50%) (score 1), and may even use dense stands if a few gaps exist in the canopy, e.g., due to snowbreak or storm. Therefore, dense stands with gaps scored higher than those without. Stands with canopy cover <20% may be used in summer (score 0.6), but rarely in winter (score 0).

    5) Type of stand (SI_typ): The classification of stands followed the system used by the Bavarian State Forest Service, which only considers the major tree species; e.g., a stand with 95% spruce and 5% fir is classified as “spruce”. In winter, Capercaillie strongly prefer to feed on pine (Pinus sylvestris) or fir (Abies alba) needles. Availability of a few pine or fir trees in a stand is sufficient for preferred winter habitat. Therefore, sample plots with pine or fir received higher scores than plots without. Capercaillie avoid stands dominated by deciduous trees as winter habitat.

    6) Bilberry and other Vaccinium shrubs (SI_bil): In summer, Capercaillie show a strong affinity to a well-developed ground vegetation rich in ericaceous shrubs, especially bilberry. Bilberry is a major food plant of Capercaillie in the snow-free seasons. It is rich in insects for chicks, and it provides optimal hiding and thermal cover for adults and broods. Cover by bilberry and other Vaccinium species of >40% was considered optimal (score 1).

    7) Vegetation height (SI_veg): Capercaillie prefer a ground vegetation 30–50 cm high (score 1), tall enough to hide in but short enough to watch out of. Vegetation <10 cm and >70 cm was considered as unsuitable (score 0).

    8) Forest regeneration (SI_reg): If forest regeneration (young trees >0.5 m high) covers 25–50% of the forest floor, conditions for Capercaillie deteriorate (score 0.6); if forest regeneration covers >75% of the forest floor, conditions become unsuitable for the Capercaillie (score 0).

Combining the variables

The index for Capercaillie winter habitat suitability was calculated based on the variables successional stage, canopy cover, type of stand, slope, and elevation:

I used two existing data sets from the Teisenberg study area for model verification: the telemetry data from 1988 to1992 (Storch 1993a, b, 1994) on which the model was based, and data on indirect Capercaillie signs mapped between July and September 1992 (G. Schwab, unpublished data). Data on habitat structure for Teisenberg forest stands had been collected in 1989–1990 (see Storch 1993b for methods; see Table 2 for variables).

For 403 forest stands, I calculated HSI scores and the area-corrected number of radio locations (number per hectare) separately for winter (N = 3586 radio locations; Storch 1993a) and summer (N = 3656; Storch 1993b). Data were pooled for all radio-tagged birds because their habitat selection had not differed individually (Storch 1993a, b). I grouped forest stands into five HSI classes according to Table 3. For each HSI class, I calculated the mean HSI score for all stands, used the percentage of stands with at least one radio location per hectare as an index of the probability of Capercaillie use.

For 169 forest stands in the central part of Teisenberg, data on indirect Capercaillie signs had been sampled at 809 random plots (c. 1 plot/ha; >3 plots per stand). Plots were 5 m in radius, and the presence or absence of tracks, feathers, dustbaths, or feces had been recorded during a 15-min search (G. Schwab, unpublished data). As previously described, I grouped forest stands into five HSI classes, calculated mean HSI scores for all stands per class, and used the mean percentage of plots with sign per stand as an index of the probability of Capercaillie use.

If the HSI model adequately depicts Capercaillie habitat preferences, habitat use should increase from HSI class 5 (poor habitat) to class 1 (excellent habitat). This could be shown using simple Spearman rank correlations between mean HSI score and Capercaillie use: mean HSI scores within the five HSI classes were significantly related to the two indices of Capercaillie use calculated from telemetry data (winter and summer) and indirect signs (summer), respectively (Table A2). The greater the HSI score, the more proof of Capercaillie use had been found. Thus, one may conclude that, for the data set on which the model was based, the HSI classes adequately reflected the probability of Capercaillie use at the level of forest stands.

Table A2. Distribution of winter and summer telemetry relocations of Capercaillie (1988–1992; data reanalyzed from Storch 1993a, b) and indirect Capercaillie sign (1992), by HSI class among forest stands in the Teisenberg study area.

			Indices of habitat use by Capercaillie
			Stands with ≥ 1 telemetry location/ha (%)						Plots with indirect sign (%)
HSI			Winter			Summer			Summer
class	Score	Description	Mean	95% CI	n	Mean	95% CI	n	Mean	95% CI	n
1	[1.0–0.8]	Excellent	36.0	25–47	75	75.0	36–114	8	40.5	9–72	7
2	]0.8–0.6]	Good	27.1	15–39	59	44.4	4–85	9	30.0	0–86	5
3	]0.6–0.4]	Fair	13.2	5–21	76	42.2	30–55	64	17.8	7–28	32
4	]0.4–0.2]	Moderate	9.9	2–17	71	22.8	16–30	145	14.5	7–22	52
5	]0.2–0.0]	Poor	4.9	1–9	122	11.3	7–16	177	6.4	3–10	73

   Notes: For each of the use indices (telemetry in winter, HSI_wi, telemetry in summer, HSI_su, and indirect sign in summer, HSI_su), Spearman correlations were significant: all R_S = 1.0; all P≤ 0.001. Left-hand brackets preceding the score intervals indicate that the highest value is not included in the interval; i.e., ]0.8–0.6] is equivalent to an interval of 0.6 to <0.8. Sample size (n) is the number of stands.