Appendix B. Habitat capability index for Western Bluebird (Sialia mexicana).
Developers: David Vesley, Brenda C. McComb, and Michael T. McGrath
Reviewers: Jennifer Weikel, Doug Runde, and Marie Morin
Western Bluebirds breed throughout western Oregon (Contreras 1998, Gilligan et al. 1994). They are rare in the Coast Range and Cascades in winter, except in Umpqua River valleys (Hunter et al. 1998). Balda (1967) reports western bluebirds defending territories that average 0.43 ha, and range in size from 0.29-0.61 ha, from other conspecifics and congeners. In Oregon Coast Range harvest units, nest snags averaged 71 cm dbh (range 25–137 cm) and 9.2 m in height (Schreiber 1987). Canopy cover surrounding western bluebird nests is generally <20% (Rosenstock 1996). Nest snags were most often decay class 3 or 4 (Schreiber and deCalesta 1992). Meslow and Wight (1975) reported western bluebirds nested primarily in shrub-sapling stages. Fix and Sawyer (1991) reported they prefer recent clearcuts having open areas and scattered dead trees for nesting. Schreiber and deCalesta (1992) found western bluebirds in 10 of 13 clearcuts (<4 yr old) with snags. McGarigal (1993) found western bluebirds most often in grass-forb stage and occasionally in shrub and sapling stages in the Coast Range of Oregon. Hansen et al. (1995) estimated mean bluebird densities were greatest at approximately 4 trees/ha and declined to zero at approximately 20 trees/ha (for all stems >10-cm dbh) in the western Cascades.
We assume that habitat capability for western bluebirds may be estimated by two forest characteristics: densities of snags and canopy closure (Figs. B1 and B2). Large diameter snags are weighted more heavily than small snags in rating habitat capability. We hypothesize that the greater volume of large snags increases potential availability of cavity nest sites. Estimates are pooled within an analytical window that approximates the home range area of a bluebird.
All metrics for this index are calculated for a focal pixel at the center of a 3×3 “moving window.” This moving window of pixels averages conditions for the 0.5625 ha surrounding the “focal” pixel (i.e., 3×3 pixels) and roughly corresponds with the territory size of the western bluebird (Balda 1967). The averaging is done to: (1) smooth inter-pixel variation and (2) provide a “patch” or “stand” level summary, which is consistent with the scale of the stand modeling and stand inventory data. The moving window is assumed to encompass ≥1 western bluebird territory(s).
where bluebirdHCI=habitat capability index for the western bluebird, NCI=nesting capability index, f=focal pixel, and i=pixel within window.
Nesting capability index
The nest index is calculated from the following equation:
where NCI=nesting capability index, S1=large snag availability index, S2=small snag availability index, S3 = canopy index, and i=pixel within window.
The snag index is calculated from the following equations:
|If S50Ti >5 Then S1i=1.0||
where S50T = number of snags (dbh >50-cm and >5-m tall) / ha, S1=large snag availability index, and i=pixel within window.
|If S2550T i > 11 Then S2i=1.0||
|Else S2i=S2550T i*0.09|
where S2550T = number of snags (25 < dbh ≤ 50-cm and >5-m tall) / ha, S2=small snag availability index, and i=pixel within window.
|FIG. B1. Relationship of snag index score to snag density and size.|
The canopy index is calculated from the following equation:
|If CCi ≤ 10% Then S3i=1.0||
|Else If 10% > CCi ≤ 20% Then S3i=[(20-CCi )/10]|
where CC = canopy closure (%), S3=canopy index, and i=pixel within window.
FIG. B2. Relationship of canopy closure index to percent canopy closure.
Balda, R. 1967. Ecological relationships of the breeding birds of the Chiricahua Mountains, Arizona. Dissertation. University of Illinois, Urbana, Illinois, USA.
Contreras, A. 1998. Birds of Coos County, Oregon. Special Publication 12. Oregon Field Ornithologists, Eugene, Oregon, USA.
Fix, D., and M. Sawyer. 1991. Checklist: the Douglas County western Cascades. Oregon Birds 17:312.
Gilligan, J., M. Smith, D. Rogers, and A. Conteras. 1994. Birds of Oregon: status and distribution. Cinclus Publications, McMinnville, Oregon, USA.
Hansen, A., W. McComb, R. Vega, M. Raphael, and M. Hunter. 1995. Bird habitat relationships in natural and managed forests in west Cascades of Oregon. Ecological Applications 5:555569.
Hunter, M. G., M. M. Sawyer, R. Maertz, B. Kruse, and K. Wilson. 1998. The hundred valleys of the Umpqua: birds of Douglas County, part 3. Oregon Birds 24:103117.
McGarigal, K. 1993. Relationship between landscape structure and avian abundance patterns in the Oregon Coast Range. Dissertation. Oregon State University, Corvallis, Oregon, USA.
Meslow, E. C., and H. M. Wight. 1975. Avifauna and succession in Douglas-fir forests of the Pacific Northwest. Pages 266271 in D. R. Smith. Proceedings of the symposium on management of forest and range habitats for non-game birds. USDA Forest Service General Technical Report WO-1.
Rosenstock, S. S. 1996. Habitat relationships of breeding birds in northern Arizona ponderosa pine and pine-oak forests. Technical Report 23. Arizona Game and Fish Department, Phoenix, Arizona, USA.
Schreiber, B. P. 1987. Diurnal bird use of snags on clearcuts in central coastal Oregon. Thesis. Oregon State University, Corvallis, Oregon, USA.
Schreiber, B., and D. S. deCalesta. 1992. The relationship between cavity-nesting birds and snags on clearcuts in western Oregon. Forest Ecology and Management 50:299316.