Grenier and Nelson 



Chapter 19 



Inland Habitat Associations in Oregon 



potential nest trees were located or where general activity 

 levels were high, and at previously unsurveyed sites. Eggshell 

 searches were conducted around all trees where birds were 

 observed landing or taking off, and around numerous other 

 trees that had potential nest platforms (platforms >18 cm in 

 diameter and >15 m above ground). When potential nest 

 trees were found, surveys were conducted on 1 to 3 successive 

 mornings to confirm the presence of an active nest and 

 identify its location. 



Nests located by tree climbing were found during an 

 intensive tree climbing study at a single site or while 

 reclimbing trees previously known to support nests (Nelson 

 and others 1994a). The tree climbing study consisted of 

 climbing all trees (Perry 1978) within a 40-m-radius plot and 

 examining all platforms for nests. In addition, seven trees 

 containing nests found between 1990 and 1992 were climbed 

 in 1993 to determine if the nests had changed over time and 

 to determine if nests or nest trees were reused. 



Characteristics of nests and nest trees were measured at 

 the 22 nest sites. Nest tree measurements included diameter 

 at breast height (d.b.h., cm), height (m), diameter at nest 

 limb (cm), and nest branch height (m), diameter at the trunk 

 and at the nest (cm), branch length (m), and position in 

 crown (percent tree height). Nest measurements included 

 distance from the trunk (cm) and cup dimensions (cm). In 

 addition, the moss depth adjacent to the nest (cm), nest 

 platform dimensions (cm), and canopy closure above the 

 nest (percent) were measured. 



Micro-site habitat features of nest sites and 2 to 3 adjacent 

 sites were measured at ten of the nests in 0.2-ha (25-m 

 radius) plots (as described previously). Plots for nest sites 

 were centered on the nest tree. Adjacent plots, centered 

 around a dominant canopy-forming tree, were a minimum of 

 75 m from nest trees, and were located the same distance 

 from forest edges as nest trees to minimize any edge biases. 

 Micro-site habitat characteristics were compared between 

 nest sites and adjacent sites within the same stand to determine 

 if the location of nests was associated with specific micro- 

 site characteristics. 



Data Analyses 



Occupied Sites and Habitat Associations 



We used the two-sample Kruskal-Wallis test (Zar 

 1984:138) to compare habitat characteristics of occupied 

 sites to a random sample of other sites (with unknown murrelet 

 status). The number of random sites selected equalled three 

 times the number of occupied sites (Breslow and Day 1980:27; 

 Ramsey, pers. comm.; Schafer. pers. comm.). The Chi-square 

 goodness-of-fit test (Zar 1984) and Bonferroni Z- statistic 

 (Byers and others 1984, Neu and others 1974) were used for 

 categorical data. 



Logistic regression was used to determine key habitat 

 components of occupied and random sites for each database 

 (Manly and others 1993, Ramsey and others 1994). The 

 following two steps were used in our analyses: (1) habitat 

 variables (continuous and categorical) were divided into 



groups of related variables that described o'ae or two 

 biological aspects of the site. Logistic regressi ion was used 

 to test statistical significance of each variab le within the 

 group. (2) habitat variables that were statistica lly significant 

 within the groups (P < 0.05) were then used ii 3 the stepwise 

 procedure to determine the final model. V ariables were 

 excluded if P> 0.05. 



Logistic regression helps select a set of key habitat 

 variables that represent the probability of s ite occupancy. 

 We chose to use logistic regression as a tool to identify key 

 components of murrelet habitat, rather than < determining the 

 predictive probabilities of the occupancy rat s of muirelets. 

 This was due to: ( 1 ) the use of retrospective sa mpling (Ramsey 

 and others 1994); (2) the limitations of the; databases (i.e., 

 data were collected over many years so sa mpling methods 

 and data collectors may have changed yearly and. the data 

 were not collected with the murrelet in m ind); aiad (3) the 



murrelet status of random sites was unkno wn. 



. 



Nest Sites ! 



Habitat characteristics within nest plots were compared 

 to average values for adjacent plots usir ig a Wilcoxon test 

 (paired-sample signed-rank; Snedecor and Cocbran 1980:140). 

 Each nest site was treated as a block, ;and t'ne overall test 

 statistic was based upon the cumulative: differences among 

 plots. A Chi-square test using a BonfeKToni, Z- statistic was 

 used to compare snag decay class. 



Results 



Occupied Site Characteristics 



State Lands Database 



Tree Species Douglas-fir was 'die dominant tree 

 species (SPECIES 1) in 67 percent of occupied sites (n = 

 72) and 83 percent of random sites (n = = 2. 1 6). The codominant 

 trees (SPECIES2) were generally a cc >D.ibination of Douglas- 

 fir, western hemlock (Tsuga heter'Ophylla), Sitka spruce 

 (Picea sitchensis), and red alder (Aln us oregond) in occupied 

 and random sites. 



Age, Tree Diameter, and Tre* Density Twenty-two 

 percent (16 of 72) of occupied sit es were <80 years of age 

 (AGE 1 993) whereas 60 percent ( 1 30 of 2 1 6) of random sites 

 were <80 years old. Ninety four f >ercent (15 of 16) of these 

 young occupied sites had remna nt trees (TPH66, >66 cm 

 d.b.h.), averaging 19.5 trees/ha (range: 2.5-75.0 trees/ha; 

 s.e. = 5.0), and 15 of these sites contained mature trees 

 (TPH46, >46 cm d.b.h.; x = 73.' 0; s.e. = 1 1.6; range = 12.3- 

 140.8). Random sites <80 year s of age averaged only 10 

 remnant trees/ha (s.e. = 1.4; range = 2.5-59.3) (table 1, 

 appendix 1). All occupied sites >81 years old had remnant 

 trees, averaging 46.9 trees/ha (range = 2.5-116.1 trees/ha; 

 s.e. = 2.8). Similarly, 97 perce nt (83 of 86) of random sites 

 >8 1 years old had remnants av< iraging 45.9 trees/ha (range = 

 2.4-79.0 trees/ha; s.e. = 2.3). 



Mistletoe, Platforms, and Moss Abundance Variables 

 in the database did not include information on nest platforms. 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



193 



