Spring Bird Communities in the Southern

Washington Cascade Range

David A. Manuwal

Author

DAVID A. MANUWAL is a professor, Wildlife Science

Group, College of Forest Resources, University of Washington, Seattle, Washington 98195.

Abstract

Spring bird populations were studied in 1984 and 1985 in

46 Douglas-fir forest stands in the southern Washington Cascade Range. Stands were classified into young (55-80 years),

mature (95-190), and old-growth (250-700) age-classes and

varied in elevation and latitude. Birds were surveyed by the

point-count method, and detection rate (birds/visit) was used

as an index of abundance. All birds detected within 75 m of

a count station were included in the analysis. Thirty-four species were detected in the study areas. Mean bird species richness varied from 13.6 to 16.4 per stand and differed significantly among age-classes in 1985 but not in 1984, although

mean richness was not significantly different between years.

Mean richness showed a weak negative relation to increasing

elevation and latitude. The nesting avifauna typically consisted of a core group of nine common species and about five

uncommon species. Highest pooled detection rates occurred

in old-growth wet and old-growth dry sites; lowest rates were

in mature and young stands. The five most abundant species

in decreasing order of abundance were winter wren, goldencrowned kinglet, hermit/Townsend’s warbler, chestnut-backed

chickadee, and western flycatcher. Total avian abundance

showed a weak negative relation to gain in elevation and

latitude. A correlational analysis was made of vegetation

variables associated with bird abundance. Cluster analysis

revealed two major groups of forest stands that differed in

elevation and latitude. Among birds in which significant

relation to vegetation variables were found, live trees and

snags typical of late-successional forests were negatively

correlated with abundance of the blue grouse and evening

grosbeak. These same factors were positively correlated with

abundance of the varied thrush, winter wren, and western

flycatcher. All but 3 of 17 species analyzed were most abundant in old-growth compared to younger forests. Although

many species were most abundant in old growth, the Vaux’s

swift appears to be the only diurnal bird that may depend on

old growth for its continued survival. Cavity nesters and birds

that feed on the forest floor may suffer drastic declines in

intensively managed monocultures. Bird populations may be

most affected by changes in forest structure and fragmentation in the nesting range and by winter habitat availability.

The relatively large percentage of permanent residents in

Douglas-fir forests makes winter habitat particularly important to their survival.

Introduction

Until recently, few accounts have been published on bird

community studies in naturally occurring, coniferous forests

of the Washington Cascade Range. Manuwal and Huff (1987)

described spring and winter bird populations in a subset of

the stands reported in this paper. Manuwal and others (1987)

161
discussed the characteristics of subalpine bird communities

in the Washington Cascades. The widespread conversion of

remaining old-growth coniferous forests of the Pacific Northwest to younger, more intensively managed forests may have

an impact on forest bird communities. Sound management of

forests to protect bird species richness and rare species is

possible only with an understanding of bird communities in

natural systems as a baseline.

This report summarizes a study of spring bird populations in

different age- and moisture-classes of unmanaged Douglasfir forests on the west slope of the southern Washington

Cascade Range during 1984 and 1985. Objectives of the

study were to determine the abundance and species composition of bird populations over a range of forest stand-age

and site conditions in the Douglas-fir forests of the southern

Washington Cascades; to examine the relation of bird presence to vegetative characteristics at bird-survey locations;

and to identify bird species that may depend on old-growth

forests for their continued existence.

Methods

Field

We surveyed birds by the point-count method (Verner 1985):

observers counted all birds detected during an 8-minute count

period beginning within 30 minutes of official dawn at each

of 12 stations (points) spaced 150 m apart We did this in

46 stands located from Mount Rainier to the Columbia River

(see frontispiece and table 1). Birds were not surveyed during

periods of rain, snow, or strong winds. We visited each stand

six times from 15 April to 30 June in 1984 and 1985. The

abilities of observers to identify birds by both sight and sound

and to estimate horizontal distances to within 10 percent of

the actual distance were tested. During a given year, most

stands were surveyed by more than one observer. Most stands

were surveyed in 1985 by different observers from those of

the previous year to avoid as much bias as possible. Because

of logistical constraints, some stands were surveyed by only

one observer during a particular year, so some interstand

variability may have resulted from observer differences.

All vegetation sampling was done at all 12 bird-count stations located in each stand. At each station, trees or snags

were counted, depending on diameter at breast height (d.b.h.),

in either a 13-m-radius (0.05 ha) or a 25-m-radius circular

plot (0.2 ha). The total area sampled in a stand was 0.6 ha for

13-m-radius plots and 2.4 ha for 25-m-radius circular plots.

In the 13-m plots, small trees (l-10 cm d.b.h.), medium trees

(11-50 cm d.b.h.), large trees (51-100 cm d.b.h.), and small

snags (l0-19 cm d.b.h.) were counted. In the 25-m-radiusplots, very large (>l00 cm d.b.h.) live trees, and medium (20-

49 cm d.b.h.) and large (>50 cm d.b.h.) snags were counted.

Density estimates were then calculated for each species by

Table l-Mean elevation, latitude, and age of 46 Douglas-fir

stands studied in 1984 and 1985 in the southern Washington

Cascades

Old growth

Young Mature wet Mesic Dry

Number of stands 10 8 9 12 7

Mean elevation

(meters) 757 784 762 736 838

Mean latitude

(degrees) 43.39 46.23 46.76 46.35 46.41

Mean (years) age 68 133 520 439 336

Range of age (years) 55-80 95-190 300-730 250-700 210-440

diameter-class. Snags were classified into five decay-classes;

class 1 was a hard snag and class 5 a soft, highly decayed

snag (Cline and others 1980).

Data Analysis

Detection rate (birds per visit) was used as an index of abundance. A visit represented the mean number of birds detected

at 12 counting stations on a particular day. Detections were

used to compare relative abundance of species among ageclasses and environmental gradients, assuming that detectability of a given species did not differ among stands and ageclasses, observers were equivalent in their ability to detect

birds, and all species had the same probability of detection.

Several stands were surveyed by two observers instead of

one during a particular spring. Furthermore, observers were

trained to estimate distances to detected birds to within

10 percent of actual distances and periodically retested themselves to remain accurate. In the gradient analyses, I used

bird detection rates and species richness data from all detections within 75 m of a counting station. For the Vaux’s swift,

all detections of flying birds (within 150 m) judged by the

observer to be associated with the stand being surveyed were

used. Furthermore, all species detected less than twice during

a season were omitted from the analysis because they were

probably associated with other habitats. In assessing the

relation of birds to vegetation, birds detected <l50 m from

the counting stations were used.

I used Student’s t test to compare differences in mean species richness and abundance between years, and the MannWhitney U Test (Siegel 1956) to compare differences between two means of small samples. For comparing mean

richness and total detection rates among age-classes, I used

one-way analysis of variance. Differences in abundance

among age-classes for individual bird species were tested

by using either analysis of variance or the nonparametric

Kruskal-Wallis test (Siegel 1956), depending on whether

or not the data were normally distributed. I calculated the

162
Pearson correlation coefficient (r) or Spearman’s rank correlation coefficient (rs

) to evaluate responses of birds to latitude

factors with a minimum eigenvalue of one (Battacharyya

1981) by examining the relative sizes of correlations (“loadand elevation and preliminary bird-vegetation relationships. ings”) between the factors and original variables.

Associations between habitat variables and counts of each

bird species were examined in two different, but related ways.

First, Spearman rank correlation coefficients (SPSS, Inc.

1986) were computed to look for significant correlations between tree and snag densities and counts of each bird species.

For this analysis, vegetation data were first summarized into

the seven most common tree species (or groupings), with

four diameter-classes of live trees (l-10 cm, 11-50 cm,

51-100 cm, >l00 cm) and three diameterclasses (l0-19 cm,

20-49 cm, >50 cm) and decay-classes (hard, composed of

classes 1 and 2; and soft, composed of classes 3,4, and 5) of

snags.

The large number of variables was reduced to a subset of 22

(appendix table 12) by examining highly significant correlations among vegetation variables. Any pair of variables with

correlations of >0.50 (thus explaining >25 percent of each

other’s variances) was either combined into one, or the more

ecologically interpretable variable was retained. Thus variables such as all live trees >l00 cm d.b.h., or live western

hemlock < 50 cm d.b.h., were created.

Bird counts and vegetation variables were first logtransformed [log10 (x+10)] for regression analyses to more

closely meet underlying distributional assumptions. Transformed bird counts were regressed on vegetation factor

scores separately for 1984 and 1985 data. Because of the

large number of equations generated and time limitations, we

were unable to search for better fits with polynomial and

interaction terms, which may explain some species’

distributions more fully (Meents and others 1983). The

accuracy of each regression was evaluated by the adjusted

coefficient of determination (adjusted R2), an unbiased

measure of the percentage of total variation in bird

abundance explained by the equation (Zar 1984:336).

In computing a large number of correlations, Type I errors

may cause some to be judged statistically significant (that is,

P < 0.05) simply because of random sampling error. Therefore, we focused on those correlations that ‘occurred with the

the same sign (positive or negative) in both years and emphasized those that were significant at the P < 0.01 level. Conflicting results, such as correlations that were positive one

year and negative the next, were considered as possible evidence of Type I errors. (Marcot 1984, Meslow and Keith

1971). Even where correlations are consistently significant,

however, they demonstrate only numerical association and

do not necessarily imply some biologically meaningful cause.

Second, associations between bird abundance and tree and

snag densities were investigated by stepwise multiple linear

regression (Kleinbaum and Kupper 1978) with SPSSX (SPSS,

Inc. 1986). This approach generated equations explaining

each bird species’ abundance from vegetation variables and

was used for descriptive, rather than predictive purposes. The

stepwise procedure evaluates each variable to be entered

while controlling for variables already in the equation. Because of the sample size (n = 46 stands) and some remaining

intercorrelations, a new, smaller set of uncorrelated vegetation variables was generated using factor analysis (Afifi and

Clark 1984). The initial factors, each of which was a linear

combination of all 22 original variables and explained a

known amount of the total variance in vegetation data, were

extracted by principal components analysis and rotated with

Varimax rotation to ease interpretation. We interpreted the

Cluster analysis (Afifi and Clark 1984) was used to describe

assemblage-level patterns of the similarities of bird species’

relative abundances (or simple occurrence). Bird communities

were classified with total count data for the spring of 1985

by means of hierarchical, agglomerative cluster analysis.

This classification groups species into clusters based on their

stand-specific relative abundances and distributions. The properties common to a group of species in a cluster are then

used to describe that set of species as a whole. More detailed

descriptions of cluster analysis and its applications can be

found in Afifi and Clark (1984), Cormack (1971), Gauch

(1982) and Sneath and Sokal(l973). Because each distance

or similarity measure, as well as each clustering technique,

imposes a “structure” on the data, I searched for obvious

clusters in the data by comparing results from several of

them. Two separate sets of analyses were run. For the first, I

converted total counts of the 28 most commonly occurring

bird species to presence-absence data and compared

sets of clusters formed by several similarity or dissimilarity

measures and clustering techniques. I did cluster analyses

using similarity measures of presence-absence bird data

described by Mueller-Dombois and Ellenberg (1974) and

Dice (1945), and the simple-matching equation of Sokal and

Michener (1958). The second set of analyses was performed

on total bird counts by using squared euclidean distance,

After reviewing all the analyses, I decided to present only

results from the measures that gave the most definite

classification patterns--the simple-matching and squared

euclidean measures. They are discussed briefly below:

Simple-matching (Sokal and Michener 1958)

where

S (x,y) = a + b / a + b + c + d,

a = number of stands in which species x occurred,

b = number of stands in which species y occurred;

163
c = number of stands in which both species occurred; and

d = number of stands in which neither species occurred.

Squared euclidean:

The second set of analyses was performed on stand-specific

total counts of birds. Only birds detected during 8-minute

station counts, at distances less than 150 m, and associated

with a given stand were counted for these analyses. Bird

counts were first log-transformed to reduce the controlling

influence of the more abundant species on classification

patterns. The formula for the squared euclidean distance is:

80

60

0

l-10 11-50 51-100 >lOO

Diameter-class (cm)

II Young

H Mature

Old growth

D (x,y) =!k (Xik - yjk)2 ;

k=l

Figure. l-Density of live trees in three diameter-classes among young,

mature, and old-growth Douglas-fir forests in the southern Washington

Cascades.

I clustered stands by means of average (between groups) and

complete linkage techniques for both distance measures and

also by Ward’s method on the squared euclidean distance. As

with the presence-absence data, I compared clusters obtained

in all these runs to elucidate apparent clusters.

Results

Stand and Age-Class Characteristics

Live trees--Highest mean densities of live trees occurred in

young stands with very high numbers of western hemlock

saplings (l-10 cm d.b.h.) and ll- to 50-cm-d.b.h. Douglasfirs (fig. 1). Not surprisingly, highest densities of the largest

diameter trees (>lOO cm d.b.h.) were found in old-growth

stands, but nearly equal densities of 51- to lOO-cm-d.b.h.

Douglas-fir were found in young stands. Also, many young

stands had some very large (~100 cm) Douglas-fir, hemlock,

and western redcedar that had survived previous fires. These

trees apparently made the stands more old-growth-like in

structure than if the stands were even aged. Although stands

varied considerably, old-growth stands had higher densities

of hardwood trees than other age-classes.

Among old-growth stands, wet sites had the highest densities of western hemlock and Pacific silver fir, but mesic

sites had the highest densities of vine maple and miscellaneous conifers. Highest densities of the largest live treesparticularly western redcedar-occurred on old-growth wet

sites.

Deadwood-In general, snag composition reflected past

successional dynamics of the stands. Stem competition in the

pioneer species Douglas-fir resulted in early mortality after

canopy closure. These trees resulted in relatively high snag

densities among boles less than 49 cm d.b.h. in young and

mature stands (fig. 2). As western hemlock became more

common in the understory and later in the canopy, the

number of hemlock snags also increased. Pacific silver fir

80,

n Young

EZI Mature

El Old growth

10-19 20-49 50+

Diameter-class (cm)

Figure 2-Density of snags in three diameterclasses among young, mature,

and old-growth Douglas-fir forests in the southern Washington Cascades.

snags also become more prevalent in later stages of succession. The opposite trend appeared for western white pine,

which was present in low densities in all age-classes but did

not reach maturity because of blister rust. Highest densities

of western white pine snags were in young stands and continued to decrease with stand age. The highest density of snags

occurred in young stands where 70 percent of all snags were

Douglas-fir and over 40 percent of them were in the small

diameter-class (lo- 19 cm). The 6.2 large snags per ha in

young stands reflected the fire history of the stands and

probably represented a significant biological resource for

cavity-nesting birds and mammals. Predictably, old-growth

stands had the highest densities of very large snags. Those of

Douglas-fir and western hemlock were most common. Large

hardwood snags, primarily bigleaf maple and cottonwood,

were most abundant in mature stands.

Snag abundance varied considerably among stands, presumably reflecting fire history and vagaries of successional

patterns. Nevertheless, on an age-class basis, the outcome of

tree competition followed a predictable pattern (see Cline and

164
Table 2Stand variation in bird species richness in Douglas-fir forests of the southern Washington 

Cascades 

Total species 

Mean richness 

SD 

27.0 28.0 

15.2 17.2 

2.x 3.8 

Table 3-Number of species of flycatchers, thrushes, and 

warblers detected in the 7 lowest and the 7 highest elevation 

Douglas-fir stands in the southern Washington Cascades in 

1985 

Bird group 

Mean 351 584 46.39 2.6 3.6 2.4 

418 550 1037 46.9, 2 3 2 

332 6W 1049 46.67 2 4 3 

419 320 ,072 46.93 1 3 2 

346 190 1124 46.40 2 3 1 

468 65 1164 46.98 0 2 1 

303 6W ,168 46.69 2 2 3 

432 3x ,217 46.94 1 2 1 

Mean 375 1119 46.79 1.4 2.7 1.9 

others 1980, Franklin and Dymess 1973) in Douglas-fir forests. Most stands had five tc seven conifer species and two 

to four hardwood species The large snags (>50 cm d.b.h.) 

were primarily Douglas-fir, western hemlock, western redcedar, Pacific silver fir, and western white pine. All other 

species combined usually comprised less than 5 percent of 

the total snag population. 

Avian Species Composition and Richness 

General-In this analysis, I used 27,156 bird detections 

of 34 species in the study stands. Bird groups in natural 

Douglas-fn forests (between the ages of 55 and 700 years 

old) with the largest number of species were the woodpeckers, thrushes, flycatchers, corvids, warblers, and finches. 

Mean bird species richness was not significantly different 

between years (in 1984, 14.5, SD = 2.8: in 1985, 15.3, 

SD = 3.4 (I = 1.17, P = 0.24). 

23.0 

17.7 

1.4 

- 

Effect of forest age and moisture-Mean species richness 

was 16.2 species per visit in young stands, 13.6 in mature 

stands, and 15.0 in old-growth stands. It was not significantly 

different among age-classes in 1984 (F = 0.25, P = 0.91) but 

it was significantly different in 1985 (F = 2.63, P < 0.05). 

Along the moisture gradient, mean species richness was 

14.1 in old-growth wet, 14.8 in old-growh mesic, and 16.4 

in old-growth dry (table 2). Mean richness was not significantly different along the old-growth moisture gradient in 

1984 (F = 0.01, P = 0.98), but it was significantly different 

in 1985 (F = 5.67, P < 0.01). 

Effect of elevation-Bird species richness declined witb 

increasing elevation of study sites in both years (r = 4.32; 

P > 0.05). The lowest seven stands had a mean of 15.4 species, and the highest seven stands, 12.9 species. The diffcrence was not significantly different. Among groups, significantly fewer species of flycatchers (f = -2.41, P = 0.03) and 

thrushes (t = 2.57, P = 0.02) but not warblers (t = 2.07, P = 

0.06) were found in the high-elevation stands compared with 

low-elevation stands (table 3). 

Effect of latitude-Richness showed a weak negative relation to latitude (1984: r = -0.36, P = 0.013; 1985: I = 4.41, 

P = 0.005). Species richness was highest in the Wind River 

area and lowest near Mount Rainier. When all 46 stands are 

considered, however, stand elevation increased with latitude 

(r = 0.63, P < 0.001). so the two factors were interrelated. 

Nevertheless, the latitudinal effect was probably a real one. 

Avian Abundance 

General-A core group of species nesting in natural 

Douglas-fir forests between the ages of 55 and 700 years 

old typically consisted of about ten common and widely 

distributed species, with frequencies of occurrence of at least 

85 percent, and four less-common species, with frequencies 

of Occurrence of 46 to 54 percent (mble 4). A third group 

contained an additional 19 relatively rare species. The Vaux’s 

swift was not included in this analysis because of its wideranging foraging behavior. In addition to these 34 species 

detected within 75 m of the counting points, other species, 
Table 4-Frequency of recurrence of birds in 46 Douglas-fir 

stands in the southern Wasbbqton Cascades, 1984 and 1985 

Species 1984 1985 Mea2 

loo 

100 

IW 

103 

93 

93 

91 

91 

89 

87 

40 

41 

52 

48 

36 

30 

41 

33 

15 

2.5 

15 

7.0 

13 

11 

7 

4 

11 

7 

0 

4 

2 

4 

II 

103 

loo 

loo 

96 

98 

89 

89 

89 

89 

83 

67 

61 

48 

43 

35 

39 

26 

26 

35 

17 

22 

9 

; 

11 

11 

0 

2 

7 

4 

4 

0 

2 

loo 

100 

IW 

98 

96 

91 

90 

90 

89 

85 

54 

51 

50 

46 

36 

35 

34 

3u 

25 

22 

19 

15 

11 

10 

9 

8 

t 

4 

4 

3 

2 

1 

such as raptors and grouse with large territories, were rarely 

detected within 75 m but were nevertheless members of the 

avifauna associated with the study stand. 

Effect of forest age and moisture-Significant differences 

occurred in mean counts/visit among forest age-classes (1984: 

F = 13.2, P < 0.01; 1985: F = 2.50, P = 0.06) and along the 

moisture gradient (1984: F = 18.3, P < 0.01; 1985: F = 3.30, 

P = 0.05) (table 5, fig. 3). Highest detection rates were in oldgrowth wet and old-growth dry sites, and lowest detection 

rates were in mature and young sites. The higher numbers in 

Table S-Mean number of birds detected per visit in 

Douglas-fir forests in the southern Washington Cascades 

in 1984 and 1985 (feeding guil& are shown in parenthas.) 

.m .ll 

.O3 

.M 25 

4.49 3.65 

.94 .83 

.39 .14 

3.31 3.47 

2.39 1.53 

1.49 2.88 

6.98 8.cu 

6.20 7-n 

.18 .I5 

.9O 28 

24 .17 

2.72 2.48 

.03 

.04 .J2 

.I9 .sl2 

7.90 5.61 

.c6 

.19 .I3 

.25 22 

3.71 2.20 

.06 .ll 

1.29 .54 

.47 .49 

42.09 

1.73 

.35 

0.52 

.19 

.04 

.51 

.O4 

.trl 

.36 

34 

.03 

.22 

.I3 

5.28 

1.21 

.87 

02 

.27 .43 

5.26 3.75 

1.46 1.31 

.34 .I9 

6.51 5.43 3.84 

1.67 3.48 4.71 

2.12 3.23 2.1” 

9.45 10.94 8.24 

8.86 6.52 9.M 

22 .43 .36 

.27 53 .76 

.48 .16 .28 

5.71 4.1” 4.68 

.c6 

1.59 

.m 

.49 

.c6 

1.59 

.lO 

55 

0.97 

23 

.06 

211 

.a 

22 

.M 

- 

6.23 

.M 

.34 

.13 

2.30 5.30 

60 .03 

.78 2.03 

-07 .O4 

51.10 54.92 

166 
80

-5 60

.-*

5 40 a 1995

Ei 1994

zi

.fj 20

Young Mature Old Old Old

wet mesic dry

Age- and moisture-class

Figure 3-Mean number of bids per visit in young, mature, and old-growth

Douglas-fir stands in the southern Washington Cascades.

Table f5-The 5 most abundant bird species in Douglas-fir

forests in the southern Washington Cascades in 1984 and

1985

Birds per visit

Golden-crowned kinglet

Hermit/Townsend’s warbler 19.2 17.5 90.9 37.3 34.8 93.1

Western flycatcher 23.3 15.2 65.1 32.5 21.4 65.9

Winter wren 55.3 29.9 54.1 77.0 45.1 58.6

a Standard deviation.

b Percent coefficient of variation (100 SD/y).

old-growth wet sites were due to large counts of Vaux’s

swifts, winter wrens, chestnut-backed chickadees, and

western flycatchers.

Effect of elevation and latitude-Although total avian abundance declined with gain in elevation, the relation was not

significant and not all species showed a decline. Among the

five most abundant species, only the western flycatcher

showed a strong relation to elevation (r = -0.58, P = 0.0001).

The response of other species was less obvious: goldencrowned kinglet (r = 0.42, P = 0.004); chestnut-backed chickadee (T = 0.35, P = 0.019); hermit/Townsend’s warbler (r =

0.13, P = 0.951); winter wren (T = -0.03, P = 0.344). Pooled

counts of all species did not change with increase in latitude.

Between-year variation-The mean detection rate was significantly lower in 1984 than 1985 (t = -6.65, P c 0.01). The

largest differences occurred in young and old-growth dry

stands (fig. 3). Of the five most abundant species, only the

golden-crowned kinglet was not significantly more abundant

Young Mature Old Old Old

wet mesic dry

Age- and moisture-class

Figure 4-Mean detection rates of the Vaux’s swift in young, mature, and

old-growth Douglas-fir stands in the southern Washington Cascades, 1984

and 1985 combined.

in 1985 then in 1984 (table 6). The winter wren and hermit/

Townsend’s warbler showed the largest differences between

years.

Species and species-group abundance patterns-The data

indicate that the Vaux’s swift is the only diurnal bird species

that may depend on old-growth forests. This species was

significantly more abundant in old-growth compared with

other age-classes (that is, 7.0 and 4.4 times more abundant in

old-growth than in young or mature forests, respectively)

(fig. 4). Among the picids, the hairy woodpecker was the

most abundant and was found in virtually every stand. It did

not show a strong trend in abundance along the age gradient.

The northern flicker was missing or uncommon in all stand

types. This species uses edges primarily, and apparently

found the Douglas-fir forests to be marginal habitat. The redbreasted sapsucker was uncommon and was recorded only in

mature and old-growth wet stands, but the sample size was

very small. We detected both the red-breasted and yellowbreasted color morphs, the red being by far the most common. About the same number of pileated woodpeckers were

found in all age-classes. The rufous hummingbird was substantially more abundant in old-growth mesic and dry stands.

Of the three species of flycatchers, the western was overwhelmingly the dominant one. It was found in all stands and

comprised 93 percent of all flycatcher observations. It was

mcst common in old-growth wet sites. The Hammond’s flycatcher, the second most commonly detected flycatcher, was

found primarily in old-growth mesic and old-growth dry

stands.

167
N

cs

TS

Al

BI

TFI

n LUHI

Young Mature Old Old Old

wet mesic dry

Age- and moisture-class

Figure 5-Avian feeding-guild structure based on mean detection rate for

1984 and 1985 in young, mature, and old-growth Douglas-fir forests in the

southern Washington Cascades. Guild codes: AI, air-insect; BI. bark insectivore; LUHI, low understory herbivore-insectivore; N, nectarivore; OS,

omnivore-scavenger. TFI, timber-foliage insectivore; and TS, timber-seed

eater.

Among the corvids, the gray jay was slightly more common

than the Steller’s. Both species were detected less in mature

stands than in either young or old-growth types. Steller’s jays

were significantly less common at higher than lower elevations. No such change was found for the gray jay. No trends

in abundance with environmental gradients were detected for

the common raven.

The only parid found in Douglas-fir forests was the chestnutbacked chickadee. This species was significantly more abundant in old-growth sites, particularly old-growth wet, than in

younger stands.

The hermit warbler (and hermiflownsend’s hybrids) was by

far the most abundant of the six species of warblers. It was

the predominant upper canopy warbler in all stands where it

occurred. Other upper canopy warblers included the Townsend’s and yellow-rumped, but they were far less abundant

and had relatively low frequencies of occurrence. The second

most abundant warbler was the Wilson’s, which was the dominant understory species. The black-throated gray warbler was

uncommon but was most often found in young stands with a

large broadleaved tree component. MacGillivray’s warbler

seemed to be associated with edges or patches of broadleaved shrubs growing in tree-fall gaps. The most abundant

finches were the pine siskin, evening grosbeak, and red

crossbill. The siskin and grosbeak tended to be more abundant in young and mature stands, but the crossbill was more

often detected in old-growth stands. The siskin and grosbeak,

however, were much commoner than the crossbill.

Foraging guild structure---The two dominant foraging

guilds in all stand types were the low understory herbivoreinsectivore and the timber-foliage insectivore (fig. 5). The

principal species in the low understory guild were the winter

wren, varied thrush, and dark-eyed junco. Birds in this guild

were most abundant in old-growth and least abundant in mature stands, except for the junco, which was most common in

young and old-growth dry stands. The primary birds in the

timber-foliage guild were the chestnut-backed chickadee,

golden-crowned kinglet, and hermit warbler. The chickadee

was most common in old-growth stands. The golden-crowned

kinglet did not show a strong trend among age-classes, but

the winter wren was clearly most abundant in old-growth.

Hermit/Townsend’s warblers clearly preferred younger forests

and responded negatively to moisture. Another obvious difference in guild structure across the age and moisture gradients

was that the aerial insectivores were more abundant in oldgrowth wet and mesic sites than in other stand types because

of the high densities of the western flycatcher and Vaux’s

swift.

Habitat Characteristics Associated With Avian

Abundance

Correlational analysis between primary live-tree and snag

variables (tables 7,8; appendix table 12) and six non-cavitynesting species showed a significant response to at least one

of the six vegetation factors both years of the study. A similar

analysis of cavity-nesting species is presented elsewhere in

these proceedings (Lundquist and Mariani, this volume).

The six factor scores were correlated with stand-age and

moisture values. Factors 1 (rs = 0.55) and 4 (rs = 0.55)

were positively correlated, and factor 6 (rs = -0.30) was

negatively correlated with stand age (P I 0.05). Factor 1

(r = 0.50) was positively correlated with moisture, but factor

4 (r = -0.68) was negatively correlated.

For the blue grouse, the most significant positive relationships

(obtained from initial regression analysis) were with vine

maple and other hardwoods, and density of small live

Douglas-firs. Negative responses were to all very large live

trees (>lOO cm d.b.h.) and density of true firs (l-10 cm

d.b.h.). Blue grouse tended to be negatively correlated with

most true fir characteristics. Blue grouse were negatively

correlated with factor 1. The evening grosbeak was also

negatively correlated with factor 1, but the varied thrush and

winter wren were positively correlated. Factor 2 was significantly correlated with the varied thrush, western flycatcher,

and western tanager.

Bird Habitat and Community Classification

Analysis of stands-Two distinct clusters of stands were

produced by the simple matching clustering technique using

the 1985 presence-absence data (fig. 6). Two other but similar clusters were produced by using 1985 bird-detection data

(squared euclidean distance) (table 9). No distinct clustering

168
Table PResults of stepwise multiple liqear regression of mean

total bird count8 with vegetation factor analysis scores

Tahle 7-Descrfption of vegetation variables comprising

6 factors derived by factor analysis

Factor Vegetation variable

Percent

of

Eigenvalue variation Species and year

Multiple Adjusted

Step Factor R R2 Significance

Blue grouse

1984

1985

Evening grosbeak

1984

1985

Varied thrush

1984

1985

1984

1985

Western flycatcher

1984

1985

1984

1985

1984

1985

Western tanager

1984

1985

Winter wren

1984

1985

Factor 1 Live Douglas-fir <lOO cm d.b.h.

Douglas-fir snags 20-49 cm d.b.h.

AU live trees >I00 cm d.b.h.

Live tme firs <IO0 cm d.b.h.

All soft snags

6.65 31.7

Factor 2 Hardwood snags l-49 cm d.b.h.

Western hemlock snags 20-49 cm

d.b.h.

3.06 14.6

Western hemlock snags 250 cm

d.b.h.

True firs snags 20-49 cm d.b.h.

Live western hemlock 5 l-100 cm d.b.h.

Factor 3 2.68 12.8

Live western redcedar l-50 cm d.b.h.

Live western redcedar 51-100 cm d.b.h.

Western redcedar snags 20-49 cm d.b.h.

Western redcedar snags 250 cm d.b.h.

Factor 4 Other live conifers <IO0 cm d.b.h.

Western white pine snags 20-49 em

d.b.h.

1.56 7.4

Western white pine snags 250 cm d.b.h.

Hard Douglas-fit snags 250 cm d.b.h.

Factor 5 Soft Douglas-fir snags 250 em d.b.h.

Factor 6 Live western hemlock GO cm d.b.h.

Stumps and stubs

1.30 6.2

1.15 5.5

1

1

1

1

1

1

3

2

2

2

3

1

1

3

1

4

1

1

1 -0.46 0.19 0.001

1 .36 .ll .013

1 -.39 .13 .007

1 .30 .07 .046

1 .31 .08 .034

.019

.040

.002

1 .35 .lO

2 .52 .22

2 .50 .21

2 -.53 .25 .OOl

2 -.52 .24 .OOl

5 .62 .35 .OOO

5 .40 .14 .006

6 -.38 .13 ,008

6 -.60 .32 .ooo

2 .35 .ll .015

2 .72 .47 BOO

1 .47 .20 .OOl

1 .40 .40 .006

a Bird counts and tree and snag densities were. log-transformed.

b Vegetation variables ccmprising the vegetation factors are in table 7.

Table 9Some characteristics of stand clusters

created by the squared euclidean technique using

1985 bird detections

occurred with the 1984 data. Cluster analysis verified the

previous conclusions on the importance of elevation, latitude,

and forest structural characteristics. Stands that were lower in

elevation and more southerly had higher bird species richness

and abundance than lower elevation stands in northern sites

of the study area (table 9). Cluster 1 Cluster 2

Although I did not analyze all bird species in each cluster, Number of stands 26 17

differences in bird abundance between clusters were apparent. For example, the flycatchers (U = 0, P = 0.0001) were

significantly more abundant in low-elevation, lower latitude

stands than elsewhere (table 9), but the winter wren was not

(U = 32.5, P = 0.45). Old-growth wet stands tended to be

clustered together in one of the two larger clusters (fig. 6).

Stands 401,403,410,411, and 302 were strongly associated

with each other. In other words, the bird assemblage associated with each-of those stands was similar. This pattern presumably indicates that the stands themselves were similar

and provided nearly equivalent habitat. Stand 402, despite

being adjacent to stands 401 and 403, was consistently classified in a different cluster; no reasons for this difference are

apparent, although 402 had fewer birds detected. It had hermit warblers, and the others did not, it did not have Wilson’s

warblers and it had only the western flycatcher, but 401 and

Stand classification

Young

Mature

Old-growth wet

Old-growth mesic

Old-growth dry

Mean elevation 885.5 600.1

Mean latitude 46.6 46.2

Mean age 261.5 350.0

Total detections 454.9 653.4

Total warbler detections 60.0 50.9

Total flycatcher detections 27.1 72.4

Total winter wren detections 71.9 121.6

169

Discussion

Because this was only a 2-year study, drawing conclusions

on some aspects is difficult. For example, species richness

was significantly different among forest age-classes in 1985

but not in 1984. The same was true along the moisture gradient. The 1984 spring was colder and wetter than average

(data from U.S. Department of Commerce), and most

observers that contributed to this study during both years

believed that the cold wet weather depressed bird singing

and activity, thus preventing detection. The large difference

in abundance between 1984 and 1985 seems unlikely to be

because of differences in actual populations. The pilot study

year of 1983 was similar to 1985. The spring of 1985 was

typical for the southern Washington Cascades, so the

relationships exhibited in 1985 probably more accurately

reflected ecological relationships than those in 1984.

Abundance patterns were consistent between years even

though total bird abundance was significantly lower in 1984

than in 1985. Bird abundance was significantly higher in latesuccessional forests both years, as was the response of birds

to the moisture gradient. The primary reason was the consistently higher numbers of Vaux’s swifts, winter wrens, chickadees, and western flycatchers in old-growth wet stands. The

high numbers of wrens and chickadees might have been due

to the large amounts of standing and forest-floor woody

debris common in most wet stands. Those stands were the

oldest and more likely to have snags, logs, and canopy openings (tree-fall gaps) that could also have been more conducive

to flycatcher foraging.

Elevation gain is ecologically similar to gain in latitude, so

presumably similar ecological forces are at work on mountain

slopes. The finding that species richness declines with increasing elevation agrees with the study by Able and Noon

(1977) in the northeastern United States. Although the reasons for this decline are unclear, some combination of climate, habitat structure, and competition probably shape the

response of birds to these gradients. Emlen and others (1986)

showed that the abundance of 12 of 41 bird species in the

northern Midwest responded significantly to latitude. The

remaining species were affected by habitat factors or competition effects. They attributed the latitudinal effect to breeding

season length and hours available for adult feeding and feeding of young. I found that some bird species increased with

elevation, but others decreased or showed little change, a

phenomenon similar to that found by Emlen and others

(1986) for latitude.

The analysis of vegetation characteristics associated with bird

abundance was inconclusive for all but six species, probably

because of inadequate sample size for some uncommon species, complex interactions among habitat variables, and bird

response to forest characteristics that were not measured. Significant bird responses were related only to variables normally associated with late-successional stages (factors 1,2, and

5) or with earlier succession (factor 6). Of particular significance may be the large leaf surface area associated with

late-successional forests. The most significant variables in

factor 1 included all live Douglas-fir, live true firs ~100 cm

d.b.h., and all live trees >100 cm d.b.h. This association

implies that birds may have responded to large amounts of

foliage, which attract insects. Net primary production tends

to increase with leaf surface area (Gholz 1982). Studies by

Grier and Logan (1977) indicate that greater leaf biomass

may allow an increase in insect abundance. Because virtually

all songbirds are insectivorous in the spring, they would

presumably be more abundant in forests with high insect

populations.

I believe that the fact that the mean stand age was not significantly different between groups of stands derived from

cluster analysis verified our earlier observations that many

young and mature stands had remnant very large live trees

and snags that apparently made stand characteristics similar

to old-growth sites. Further analysis of the vegetation in

these stands might give clues to some of the important habitat features to which some birds responded.

Birds Finding Optimal Habitat in Old-Growth

Douglas-Fir Forests

This study was largely based on conventional correlational

analyses. Such analyses do not necessarily show causal relationships between avian abundance or occupancy and habitat

types or features (Wiens and Rotenberry 1981). Nevertheless,

the extensive data set enables us to describe the general relationships between bird abundance and habitat types. Ideally,

reproductive success and subsequent information on survival

of offspring would be necessary to ascertain whether a particular habitat was optimal for breeding species (Van Home

1983). Such data were unavailable, so avian abundance was

used as an indicator of habitat suitability with the knowledge

that some stand types may have acted as either source or

sink segments of some populations (Wiens and Rotenberry

1981).

Old growth was optimal habitat for all but 3 of 17 species

(table 10). These species reached maximum abundance in old

growth but were also relatively common in other age-classes.

I was unable to assign optimum habitat for other species

because the data were either inadequate or no peak in abundance in any stand type was recognizable. Old-growth, particularly old-growth wet stands, appeared important to many

species of birds, however.

171
Table M-Optimum habitat types for selected

bird species nesting in Douglas-fir forests of the

southern Washington Cascades

Bird species Standtype

Vaux’s swift Old growth’

Hairy woodpecker Old growth (wet)

Pileated woodpecker Old growth

Hammond’s flycatcher Old growth

Western flycatcher Old growth (wet)

Gray jay Old growth

Steller’s jay Old growth (wet)

Chestnut-backed chickadee Old growth (wet)

Red-breasted nuthatch Old growth (dry)

Winter wren Old growth (wet)

American robin Old growth (wet)

Varied thrush Old growth (wet)

Hutton’s vireo Young

Black-throated pray warbler Young

Hermit warbler Young

Wilson’s warbler Old growth (wet)

Red crossbill Old growth

a

 Old-growth category means that no trend was evident

among the 3 moisture categories.

b

 Includes Townsend’s warbler and hybrids between them.

Birds Potentially Dependent on Old-Growth Douglas-Fir

Forests

Birds potentially dependent on old-growth Douglas-fir forests are those found almost exclusively in old-growth and

that rely on habitat features found primarily in old-growth

forests. The Vaux’s swift nests colonially in very large,

hollow snags-a habitat feature occurring mainly in oldgrowth forests. Because swifts are aerial insectivores, their

food or forest structure are unlikely to have an important

bearing on survival-unless, of course, the insects on which

they feed depend on old-growth forests. Even though the

feeding ecology of this species is poorly known, the nest site

is likely to be the critical limiting resource for this species

and the species may depend on old-growth for its continued

existence.

Other researchers (see Gutierrez and Carey 1985) have tentatively concluded that the northern race of the spotted owl is

potentially dependent on old-growth forests for continued

survival. Our data, collected opportunistically along with regular bird survey work, also suggest the close association between old growth and spotted owls. We detected five spotted

owls in mature and seven in old-growth stands over the 3

years (1983-85). We also detected one barred owl in a young

stand and four in old-growth stands. The potential impact of

the barred owl on the spotted owl is a concern to many

biologists.

Ramifications of Changes in Forest Structure and

Fragmentation

Changes in forest structure-Many habitat variables such

as density of trees or shrubs of various species are correlated

with the presence or abundance of some bird species. A more

extensive analysis might uncover yet more significant relations. These results support the basic premise that avian

diversity, abundance, and species composition are strongly

influenced by habitat patchiness. Recent research by Blake

and Hoppes (1986), Brokaw (1985), Hartshome (1978),

Martin and Karr (1986), Runkle (1981), Whitmore (1978),

Willson and others (1982) indicates that canopy gaps are

important patches for forest birds. Most research on this

topic has been done in tropical forests or in eastern North

American hardwood forests, but similar phenomena are

likely to exist in Douglas-fir forests. Old-growth Douglas-fir

forests often have tree-fall gaps similar to those described by

the cited authors. The general conclusions they draw are that

these canopy openings provide food or food-getting resources

in greater abundance than the forest interior in general. Blake

and Hoppes (1986), in particular, have documented the increase in fruit and insect abundance in tree-fall gaps in an

Eastern deciduous forest, coincident with an increase in birds

and a change in species composition in those areas.

The fundamental differences in canopy structure between

young and old-growth Douglas-fir forests undoubtedly

account for some of the differences in the avifauna between

these age-classes. The vegetation variables in this study were

not specifically designed to measure canopy structure, but

those that were measured probably gave an indirect measure

of some dimensions of the canopy. The different tree species

have different geometric shapes and leaf types. These attributes probably affect both resource abundance (mostly insects) and the ability of birds to exploit those resources. Forests composed of different tree species and tree age-classes

would presumably differ somewhat in resource availability.

If these resources were limiting, differences in avifaunal

response would be expected. In fact, such differences apparently existed in Douglas-fir forests, but they appeared to have

more impact on winter birds than summer birds (Manuwal

and Huff 1987). Even-aged management of Douglas-fir reduces canopy and understory heterogeneity, the same heterogeneity characteristic of natural stands of older Douglas-fir.

The primary aspects of heterogeneity reduced by even-aged

management will be canopy and understory patchiness and

standing and forest-floor dead wood. Bird species affected by

these changes are cavity-nesters and birds associated with the

forest floor, where coarse woody debris provides habitat for

insect prey. The magnitude of these effects in Douglas-fir

forests is unknown but may be strongly influenced by effects

of fragmentation.

172
Table Ii-Comparison of the major wintering areas for birds of the Washington Cascades and those 

from sites in Eastern hardwwd forests” 

3 

Fragmentation of nesting habitat-The effects of forest 

fragmentation result from many factors, such as the size, dispersion, and structure of the patches, and the natural history 

characteristics of the birds. The intent here is merely to mention the problem that fragmentation may have on birds in 

Douglas-fir forests. Birds that have large home ranges or 

those rhat are found only in the forest interior are those most 

affected by fragmentation (Martin 1981, Robbins 1980, 

Robbins and others 1989, Rosenberg and Raphael 1986, 

Whitcomb and others 1981). The Douglas-fir forests of the 

southern Washington Cascades are becoming increasingly 

fragmented (Morrison 1988). New studies similar to that 

conducted by Robbins and others (1989) are now needed to 

understand the relation of patch size and shape to the population size and persistence of birds. 

The combined effects of forest fragmentation and simplification of forest structure through even-aged management will 

probably result in substantial declines in some bird species. 

Species or groups likely to show declines are all cavitynesting birds (woodpeckers, nuthatches, chickadees, brown 

creeper, small owls) and species that are closely associated 

with the forest floor (winter wren, thrushes) or vertical 

habitat layering (warblers, chestnut-backed chickadee). 

Fragmentation and changes in winter habitat-Reductions 

in breeding bird populations may not result from changes in 

the nesting habitat. Factors operating outside the nesting 

season may have consequences during later nesting seasons. 

Recently, ecologists have become alarmed at the amount of 

habitat being lost through harvesting of neotropical forests 

(Powell and Rappole 1986), although many temperate longdistance migrants reach maximum abundance in tropical 

second growth (Hutto 1989). A substantial percentage (38-44 

percent) of North American nesting birds spend the winter in 

neotropical forests (table 11). Typical neotropical migrants 

include the flycatchers, swallows, thrushes, tanagers, videos, 

and many species of warblers. The percentage of neolropical 

migrants in Douglas-fir forests is relatively low compared 

with other regions of the United States, and a relatively high 

percentage of permanent residents live in the Cascades 

Douglas-fir forests (table 11). 

One obvious consequence of the large percentage of pcrmanent residents in the Pacific Northwest is the importance of 

winter habitat. The effects of fragmentation and simplification of forest structure will not only affect breeding birds but 

also the permanent residents. The ecological bottleneck that 

so often affects northern temperate wildlife may be more 

severe if the forest lacks suitable resources to allow birds to 

survive the winter. Changes in forest stmcture and reduction 

in patch (stand) size would be expected to reduce the winter 

carrying capacity of forests. Previous studies (Manuwal and 

Huff 1987; Huff and others, lhis volume) suggest that oldgrowth forests, with their complex canopy structure and rich 

tree-species diversity, provide substantially better winter habitat for many bird species than do younger stands. Old growth 

may actually provide habitat in which a much larger percentage of birds survive the winter than survive in younger stands 

and therefore are able to replenish stands unable to support 

winter populations at numbers adequate. to maintain the resident population. Studies of the response of birds to intensively managed Douglas-fir forests arc now urgently needed, 

so that new silvicultural prescriptions can be developed to 

reduce the loss of birds from massive changes in forest 

structure. More information is particularly needed on habitat 

needs of permanent residents in the Pacific Northwest. 

173 
Acknowledgments Appendix

The study was funded through the Old-Growth Forest Wildlife Habitat Program of the USDA Forest Service, Olympia,

Washington, I thank Len Ruggiero, Andrew Carey, Fred

Samson, Mark Huff, and Keith Aubry for their assistance in

various stages of the study. Many able field ornithologists

contributed to this investigation: M.Q. Affolter, B. Booth,

J.B. Buchanan, M. Donahue, C.B. Chappell, M. Emers, S.

Gaughan, M. Harris, K.W. Hegstad, A. Hetherington, M. H.

Huff, A.B. Humphrey, D.A. Leversee, R.W. Lundquist, J.M.

Mariani, B.R. North, M.J. Reed, B.A. Schrader, and L.A.

Willimont. I thank the personnel of the Wind River, Randle,

and Packwood Ranger Districts and the staff of Mount

Rainier National Park for their cooperation. I especially

thank Mark Huff, Richard Lundquist, and Jina Mariani, who

were an integral part of the fieldwork, data processing, and

data analysis phases of the study. Richard Lundquist deserves

special mention for his help with the final regression analyses

on bird-habitat relationships. Jared Verner, William Mannan,

Naomi Manuwal, and an anonymous reviewer offered helpful

suggestions for improving the manuscript.

Table 12-Stem count variables for bird and vegetation

correlation and regression analyses

Code

Coarse variables

LT4

TSTP

ss

This paper is contribution 112 of the Wildlife Habitat

Relationships in Western Washington and Oregon Research

Project, Pacific Northwest Research Station, USDA Forest

Service. 0

Species-specific variables

DFD13

WHD12

WHD3

RCD12

RCD3

ABD13

CPD13

VMD12

HDD13

DFS34

DFS5

DFS6

WHS34

WHS56

RCS34

Description

Live trees >l00 cm d.b.h.

Stumps and stubs (snags <5 m tall,

>l0 cm d.b.h.)

AU small snags l0-19 cm d.b.h.

Live Douglas-fir l-100 cm d.b.h.

Live western hemlock l-50 cm d.b.h.

Live western hemlock 51-100 cm d.b.h.

RCS56

ABS34

WPS34

WPS56

Live western redcedar l-50 cm d.b.h.

Live western redcedar 51-100 cm d.b.h.

Live true firs l-100 cm d.b.h.

Live other conifers l-100 cm d.b.h.

Live vine maple l-50 cm d.b.h.

Live other hardwoods l-100 cm d.b.h.

All Douglas-fir snags 20-49 cm d.b.h.

Hard Douglas-fir snags > 50 cm d.b.h.

Soft Douglas-fir snags > 50 cm d.b.h.

All western hemlock snags 20-49 cm

d.b.h.

All western hemlock snags > 50 cm

d.b.h.

All western redcedar snags 20-49 cm

d.b.h.

All western redcedar snags > 50 cm

d.b.h.

All true fir snags 20-49 cm d.b.h.

All western white pine snags 20-49 cm

d.b.h.

All western white pine snags 550 cm

d.b.h.

174
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Regional Patterns of Diurnal Breeding Bird

Communities in Oregon and Washington

Mark H. Huff and. Catherine M. Raley

were hole or bark nesters: red-breasted sapsucker, brown

creeper, chestnut-backed chickadee, red-breasted nuthatch,

and Vaux’s swift.

Authors

MARK H. HUFF is a research wildlife biologist, and

CATHERINE M. RALEY is a wildlife biologist, USDA

Forest Service, Pacific Northwest Research Station,

Olympia, Washington 98502.

Abstract

Breeding bird communities surveyed in 132 natural fireregenerated Douglas-fir forest stands were examined for

differences in species richness and abundance among forest

stand age-classes: young, 35 to 79 years old, n = 27; mature,

80 to 190 years old, n = 37; and old-growth, 200 to 730 years

old, n = 68, and among physiographic provinces: Oregon

Coast Range (1985 and 1986, n = 45); Oregon Cascade

Range, 1984 and 1985, n = 48 and 15, respectively; and

southern Washington Cascade Range, 1984 and 1985, n = 39.

Stands were surveyed six times each spring using 8-minute

counts at 12 counting stations spaced 150 or 100 m apart.

More than 115,000 birds were detected. We counted 41 species that regularly used Douglas-fir forests >40 years old,

after the exclusion of raptorial, nocturnal, and poorly detected

species. The most widespread and abundant species were

chestnut-backed chickadee, western flycatcher, winter wren,

hermit and Townsend’s warblers, and golden-crowned kinglet. All bird species significantly more abundant in forests

>80 years old in at least two of three regions at P < 0.10

Results suggested that patterns of bird abundance were regionally distinct, even though the study design differed between regions. Total bird abundance, species richness, and

bird abundance in four spatial-foraging guilds (aerial, bark,

canopy, and understory) were significantly higher in the

Oregon Coast Ranges than in the southern Washington or

Oregon Cascade Range. Aerial and bark guilds showed

strong differences among ageclasses: the aerial guild reached

its highest abundance in young and old growth and the bark

guild was least abundant in young and highest in old growth.

Vegetation characteristics used for multiple linear regression

models predicted only a small proportion of the variance in

bird abundance.

Introduction

Douglas-fir is the primary tree species of the densely forested slopes of western Oregon and Washington. Old-growth

Douglas-fir forests (>200 years old), which once covered

large land areas throughout the region (for example, Dodwell

and Rixon 1902), are rapidly becoming scarce (Marcot and

others, this volume). Most old-growth Douglas-fir forests

have been cut on private and industrial holdings; consequently, those that remain are almost exclusively on Federal

lands (DeBell and Franklin 1987, Franklin and others 1981).

177
To meet Federal laws, land managers must decide which oldgrowth forests to cut and which to preserve, yet the amount

of information available to make these decisions as they relate to wildlife populations is often inadequate (Carey 1984).

Converting large forested regions from older to younger ageclasses poses specific problems for interpreting and predicting the relation of birds to their habitats. Difficulties arise

from, for example, high variability in interregional habitat

characteristics (Hamel and others 1986) and scale of measurement, which influences how habitat occupancy by birds is

interpreted (Wiens 1981a, Wiens and Rotenberry 1981b).

Bird movements resulting from rapid liquidation of oldgrowth forests, exceeding 20,000 ha per year on Federal land

in the Pacific Northwest (Torrence 1986), are poorly understood and could be misleading if bird dispersal from potentially optimal into suboptimal habitat (sensu Pulliam 1988) is

widespread.

In Finland, where bird populations have been surveyed relative to forestry practices over several decades, researchers

found that, in general, species associated with edge and

brushy habitats have increased proportionally with the creation of forest openings from cutting (see Stjernberg 1979),

but other species closely associated with older coniferous forests have decreased (see Jarvinen and Vaisanen 1978).

Similarly, birds associated with mature and old-growth

forests in northwestern California have likely declined from

the presettlement period to the present because of extensive

logging, and especially over the last 20 to 30 years, but

ground-brush-foraging birds have increased substantially

during the same period (Raphael and others 1988). Species

projected to be most significantly affected by rapid and

widespread changes in forest composition and structure are

those with relatively restricted ranges and habitat requirements (Raphael and others 1988).

In Pacific Northwest forests, 58 percent of the vertebrate

fauna consists of bird species (Harris 1984), but, only a few

of them regularly use the interior of coniferous forests (Carey

1988, Wiens 1975). Even though birds strongly associated

with old-growth Douglas-fir forests seem to be many (sensu

Brown 1985), the spotted owl and Vaux’s swift are suggested

as being the most closely associated (Carey 1985, Lundquist

1988, Manuwal and Huff 1987, Meslow and others 1981).

We compared bird species richness and abundance along a

forest sere within a large geographic area, throughout the

Oregon Coast Ranges and the Oregon and Washington Cascade Ranges, to determine which species consistently are

most abundant in old-growth Douglas-fir forests; if oldgrowth forests have distinct faunal associations; which abundance trends are consistently related to forest age between and

within the study regions; and which vegetative characteristics

help to explain the patterns of bird abundance among the

study regions. We examined only bird communities of unmanaged, fire-regenerated stands. Wherever plausible, we

speculated on how our results might be applied to forested

areas intensively managed for timber production.

Methods

Study Area

Breeding bird communities were surveyed in Douglas-fir

stands of three physiographic provinces, the southern Washington Cascade Range, Oregon Cascade Range, and Oregon

Coast Ranges (see frontispiece). The three provinces were

divided into subprovinces: three each in the southern Washington and Oregon Cascade Range and two in the Oregon

Coast Ranges. A subprovince consisted of study stands within about 60 km of one another and isolated from other subprovinces by varying latitude (see frontispiece). Stands were

selected to encompass the range of variation within Douglasfir forests of each subprovince (table 1).

A study stand was defined as an area relatively homogeneous

in structure, disturbance history, vegetational composition,

physiography, and soil. Unmanaged, naturally regenerated

stands were selected to represent a chronosequence of forest

development (young, 35 to 79 years; mature, 80 to 190; and

old-growth, 200 to about 730 years). Stand elevation was

restricted to the range in which Douglas-fir was a dominant

overstory species (see Franklin and Dyrness 1973) and varied

substantially within and among provinces (see frontispiece).

Although the experimental design called for stands of. at least

40 ha to avoid edge effects, not all stands in the Oregon

Coast Ranges met this criterion because few large tracts of

unmanaged forest remained. Median size of study stands in

the Oregon Coast Ranges was 29 ha, ranging from 14 to 62

ha (Carey and others, this volume).

Avian surveys were done in 151 Douglas-fir stands; 19 stands

were excluded from our analysis because they exhibited

moisture conditions (either extremely wet or extremely dry)

that were outside the range of the young and mature stands

sampled. Of the 132 stands suitable for bird community analysis, 39 were in the southern Washington Cascades, 48 in

the Oregon Cascades, and 45 in the Oregon Coast Ranges.

Because no stand vegetational data were collected from two

mature stands in the Oregon Cascades, only 130 stands were

analyzed.

Rugged mountains, steep valleys, and high annual precipitation (800 to 3000 mm), which increases rapidly from low to

high elevations, characterize the three provinces (Franklin and

Dyrness 1973; Spies and Franklin, this volume). Although

178
Table l-Means and range (in pareotbeses) of envIronmenta variables for young, mature, and old-growth stands 

sampled io 3 phgslogrsphic provinces (n = sample size) 

Variable 

Oregon Coast Ranges: 

Latitude 

Longitude 

Elevation (m) 

Percent slope 

AspecP 

Stand age (ye) 

Oregon Cascades: 

JAitude 

Longitude 

Elevation (m) 

Percent slope 

AspecP 

Smd age W 

southem Washington 

cascades: 

Latitude 

Longitude 

Elevation (m) 

Percent slope 

AspecP 

Stand age Cyr) 

123.7 (123.5~124.0) 

307.0(86-473) 

48.0(16-73) 

.96 (.OS-1.67) 

59.0(40-72) 

n=9 

44.2 (42.8-45.5) 

1'22.3 (121.8-122.8) 

920.0(520-1428) 

42.0(16-71) 

1.18 (.07-1.91) 

65.0(35-79) 

n=lO 

46.4 (45.8-47.0) 

121.9 i121.5.12i.O) 

728.0 (476.1168) 

41.oi13.66) 

1.27 (29-1.97) 

67.0 (55-75) 

L 

46.3 (45.9-46.8) 46.3‘(45.8-47.0) 

121.9 (121.7~122.0) 121.8 (121.5-122.0) 

749.0 (483.1124) 759.0(420-1049) 

46.0 (19.73) 35.0 (9.80) 

1.02(.08-1.96) .89 (.02-1.99) 

127.0 (80-190) 418.0 (210.700) 

Douglas-fir dominates the study region, the climax species 

are western hemlock and Pacific silver fir (Spies, this volume). Forests begin displaying old-growth characteristics at 

about 200 years, yet rarely approach climax because catastrophic fires occur before shadetolerant species completely 

replace the long-lived Douglas-firs (Franklin and Spies 1984, 

Franklin and others 1981). 

Sampling Methods 

Birds-Birds were surveyed in each province for two consecutive breeding seasons: 1984 and 1985 in the southern Washington Cascades (Manuwal, this volume), 1984 and 1985 

(only 32 percent of the 1984 stands in 1985) in the Oregon 

Cascades (Gilbert and Allwine, this volume b), and 1985 and 

1986 in the Oregon Coast Ranges (Carey and others, this 

volume). Birds were sampled by using the variable circular 

plot technique (Reynolds and others 1980). Twelve counting 

stations were spaced at 150-m intervals in a rectangular pattern placed no closer than 75 m from the edge of the stand. 

, 

43.8 (43.0-44.5) 

123.6(123.3-124. 

403.0(86-1022) 

48.0 (16.82) 

.95 (.04-1.89) 

213.0(40-525) 

lx=48 

43.9 (42.8-45.6) 

122.4 (121.8-122.' 

849.0(326-1278) 

43.0(3-90) 

.90 (.02-1.95) 

213.0 (35.500) 

n=39 

Stands were surveyed six times from April 26 to July 7. 

Because the Coast Ranges stands were small, the spacing 

between stations was reduced to 1CO tn. Six stands in the 

Oregon Coast Ranges had fewer than 12 stations: one stand 

with 7 stations, one stand with 10 stations, and three stands 

with 11 stations. Additional surveys were done to make the 

total sample time at these six stands comparable to all other 

stands. All surveys began no later than 15 minutes after 

sunrise and were completed within 4 hours. All birds seen or 

heard during each 8.minute count at each counting station 

vwe recorded. Field observers, trained to estimate horizontal 

distances, recorded horizontal distance for each detection. 

Vegetation--Vegetation data on snags, logs, live trees, 

shrubs, and herbaceous vegetation were collected in each 

stand in five nested, systematically established plots (Spies 

and Franklin, this volume; Spies and others 1988). Our 

objective in using this data set was to quantitatively describe 

the structure and composition of the stand as a whole, rather 

179 
than specific points associated with the bird counts as

reported in this volume by Manuwal, Gilbert and Allwine,

and Carey and others.

Sampling Biases

Verner (1985) describes five sources of bias in bird counts:

observer, habitat, birds, study design, and weather. During

this study, attempts to minimize some of these biases included using trained observers, avoiding weather conditions

that would interfere with locating and identifying birds, spacing counts throughout the breeding season, and, where possible, rotating observers in each stand. Robbins and Stallcup

(1981) suggested rotating observers among plots to help compensate for observer variability. Observers were rotated in all

stands surveyed in the Oregon Cascade and Coast Ranges,

but logistical problems prevented rotation in more than half

of the stands in the southern Washington Cascades. Over all

provinces, observers were rotated in 77 percent of the stands

surveyed. In general, different observers were used between

survey years.

Only birds detected within 50 m of the counting stationrepresenting half of the shortest distance between the

stations-were used in the main analysis. Detection distances

were truncated so that the ares searched would be comparable

among stands, and to minimize biases associated with counting the same birds twice. Additionally, we believe that truncating detection at 50 m should help to reduce other biases

inherent in bird surveys. Richards (1981) describes three environmental acoustic factors that are distance- and frequencydependent, and affect the ability of an observer to detect and

accurately identify a singing bird in forested habitats: attenuation, the decrease in intensity of sound with distance;

random fluctuations in the received level of sound resulting

from wind and thermal conditions; and scattering and reverberation of sound by trees, foliage, and the ground. Selecting

trained observers can minimize these factors (Richards 1981),

but reducing the distance searched further minimizes them as

well as differences among observers.

To evaluate whether detectability of bird species changed

with location, we compared mean detection distance (60 m

radius of the counting station) within each forest age-class

among the three physiographic provinces by the KruskalWallis procedure (Zar 1984). We selected 10 common species

to evaluate the relative magnitude of differences among the

three provinces (table 2). Within young stands, few strong

differences were found among the provinces. More strong

differences among provinces were found within mature and

old-growth stands, as indicated by the P-values, but the mean

values were well within the 50-m cut distance and differences among mean values were generally small (<5 m).

Table 2-For each forest age-class, mean detection distance for

10 common bird species and significance level of Kruskal-Wallis

comparisons among 3 provinces

Mean detection distance (m)

Bird species

Oregon Southern

Coast Oregon Washington

Ranges Cascades Cascades P-value

Young forests:

Brown creeper

Chestnut-backed chickadee

Dark-eyed junco

Golden-crowned kinglet

Hermit thrush

Hermit/Townsend’s warbler

Red-breasted nuthatch

Varied thrush

Western flycatcher

Winter wren

Mature forests:

Brown creeper

Chestnut-backed chickadee

Dark-eyed junco

Golden-crowned kinglet

Hermit thrush

Hermit/Townsend’s warbler

Red-breasted nuthatch

Varied thrush

Western flycatcher

Winter wren

Old-growth forests:

Brown creeper

Chesmut-backed chickadee

Dark-eyed junco

Golden-crowned kinglet

Hermit thrush

Hermit/Townsend’s warbler

Red-breasted nuthatch

Varied thrush

Western flycatcher

Winter wren

24.0

22.9

21.4

22.0

32.6

34.2

35.0

34.8

30.1

29.7

25.4

23.0

30.6

22.6

36.4

35.7

35.6

34.0

30.4

30.3

24.3

22.0

29.4

21.4

31.9

35.5

37.7

35.7

28.5

29.4

20.8 24.1 0.5541

24.0 24.1 .5619

27.6 27.2 .7819

21.0 22.7 .7143

31.8 32.0 .8135

32.5 34.1 .3142

34.3 39.6 .0067

35.7 31.7 .2018

32.1 31.5 .6390

29.5 30.8 .4933

24.4 29.0 .1050

23.6 29.2 .0039

28.8 26.2 .4058

21.5 27.4 .oolS

27.8 33.0 .0718

33.8 35.1 .1938

32.0 39.5 .ooo7

33.6 35.7 ho07

32.3 32.8 .4485

30.8 35.4 a059

23.1 28.7 .0011

23.4 27.3 mo3

27.8 26.7 .1880

21.7 25.6 .OO27

34.6 31.5 .4478

33.0 36.4 .ooo2

32.3 36.3 .oooo

32.8 34.1 .1099

29.4 32.8 .OOOO

30.9 32.5 .oot?2

Major biases in the sample design and data collection occurred during this study. First, elevation was not controlled

for when study stands were selected; consequently, the degree

and magnitude of variation is different within and among

provinces. The three provinces were not surveyed during the

same two sample years, and in no year were all stands in all

provinces surveyed. Also, some stands were surveyed in only

one sample year instead of two. The data from these stands

represent a mean based on one sample year, whereas data

from the remaining stands represent a mean of two sample

years. Five stands in the Oregon Coast Ranges had fewer than

12 counting stations (one stand had 7, one had 10, and three

had 11 stations). Analyses were conducted to evaluate the

effect of fewer counting stations, and differences with and

without the five stands were negligible. Finally, observers

were different among the three provinces and generally