RESOURCE PARTITIOΝING

BY AΝ AVIAN GUILD

IN AN OAK WOODLAND

BY

Patricia N. Manley

A Thesis

Presented to

The Faculty of Humboldt State University

In Partial Fulfillment

of the Requirement for the Degree

Master of Science

Νovember, 1988 RESOURCE PARTITIONING

BY AN AVIAN GUILD

IN AN OAK WOODLAND

by

Patricia Ν. Manley

Approved by the Master's Thesis Committee

Barry R. Noon, Chairman

Gerald Allen

C.J.Director Ralph

Natural Resources Graduate Program

88/W/-145/09/30 Natural Resources Graduate Program Number

Approved by the Dean of Graduate Studies

John C. Hennessy ABSTRACT

Bird community structure was described in an oak woodland near

Hopland, Mendocino County, California, between March 1986 and June

1987. The bird community was censused, the vegetative community was

measured, and the foraging behavior of 20 insectivorous and/or

granivorous bird species was observed on 23 5 ha study plots.

Resource partitioning between bird species in the foliage-foraging

guild was investigated.

Seventy-three bird species were detected during censuses.

Thirty-three species were insectivorous and confirmed breeders.

Cavity nesters comprised a large proportion of the breeding bird

community in terms of species (24.5 percent) and individuals (58.1

percent).

Deciduous oak trees dominated most study plots, and blue oak

was the most abundant deciduous oak. Floristics, structure, and

elevation varied among study plots. The 800 m elevation gradient that

existed among study plots was partially responsible for the variance

in vegetative characteristics among plots.

Three guilds (bark-, ground-, and foliage-foragers) were

delineated based on frequency of use of substrates where foraging attempts were directed. The foliage-foraging guild consisted of 10 species. These 10 species were widespread, with an average of 90

percent frequency of occurrence across study plots. Year-round

residents (three species) appeared to be more abundant than summer

residents (seven species).

iii iv Resource partitioning between bird species in the foliage-foraging guild occurred on both spatial and behavioral "axes".

Spatial overlap was measured by habitat association, plant species use, and location on the plant. Behavioral overlap was measured by substrates and maneuvers used to obtain prey, search movements, and prey type. Spatial and behavioral overlap was extensive among species, but the abundance of each species and the frequency of use of resources and behaviors varied between species. All species differed significantly on one or more resource axes. The influence of residency, body size, and nest type on resource use is discussed. ACKNOWLEDGMENTS

Funding for this research was provided by the California

Department of Forestry and Fire Protection (CDF) and a

McIntire-Stennis grant from Humboldt State University. Additional support was provided by the Donald Morris Hegy Memorial Fellowship from The Rotary Club of Eureka and the Alfred Piltz Scholarship from

Humboldt State University.

Ken Mayer, the former Wildlife and Range Ecologist for CDF, provided valuable logistic support. Alfred Murphy, the past director of the Hopland Field Station, facilitated the use of the field station for the study and provided information about the management and ecology of habitats on the field station. Many other members of the staff at the Hopland Field Station provided helpful information and friendly assistance, including Fran Lyle, Art Butler, Bob Keifer, and

Nancy 0'Ferrel. Many thanks to Robert Schmidt (University of

California Cooperative Extension) and Carolyn Shugart for their unending hospitality and good company.

Equipment was provided by Humboldt State University, Jared

Verner (Pacific Southwest Forest and Range Experiment Station, Fresno,

California), and Britt Lumber Company (Arcata, California).

I am very appreciative of the hard work and dedication provided by my field assistants Sara Nook, Randy Sellers, Myrnie

Mayville, Gary Perlmutter, Tracy Tennent, David Solis, and Kent Swick.

ν vi Their efforts and contributions resulted in the successful completion and outcome of this study. Special thanks to Randy Wilson, a fellow graduate student, for his support and cooperation.

The design and implementation of the study was greatly improved by constructive comments from my major professor, Barry Noon, and by David Solis, Sara Mook, and Joe Schell. Pat Collins provided invaluable assistance with the use of various computer systems and

programs. Many thanks to the members of my committee, Barry Noon,

Jerry Allen, and C. J. Ralph, and the Director of the Νatural

Resources Graduate Program, Larry Fox, for their timely and thorough review of my statistical analyses and manuscript. Additional thanks are due to Jack Kahl (Six Rivers National Forest) for his support and encouragement.

Most of all I want to thank my husband, David Solis. His support, encouragement, knowledge, hard work, and love was a shining light throughout all phases of the project. TABLE OF CONTENTS

Page

ABSTRACT

ACKΝOWLEDGMEΝTS V

LIST OF TABLES xi

LIST OF FIGURES xV

INTRODUCTION 1

STUDY AREA 3

MATERIALS AND METHODS 7

Study Plots 7

Estimation and Analysis of the Bird Community 7

Sampling Methods 7

Data Analysis 9

Estimation and Analysis of Vegetation 9

Census Methods 9

Data Analysis 9

Bird/Habitat Relationships 11

Estimation and Analysis of Foraging Behavior 12

Sampling Methods 12

Data Analysis 18

Determination of Independence 18

Variable Categories Grouped for Analysis 21

Cluster Analysis 23

Principal Component Analysis 23

Discriminant Function Analysis 24

Vii viii

TABLE OF COΝTENTS (continued) Page

Contingency Tests 27

RESULTS 29

Bird Community 29

Vegetative Community 36

Definition of Foraging Guilds 40

Results of Analysis of 1986 Data 40

Cluster of Bird Species Into Guilds 40

Major Axes of Variation in Substrate Use 44

Habitat Relationships of Foliage-foraging Species 47

Distribution Across Study Plots 47

Abundance Across Study Plots 47

Linear Relationships Between Bird Species Abundance

and Vegetative Characteristics 48

Analysis of Bird Groups 51

Analysis of Individual Bird Species 51

Foraging Behavior 53

Use of Substrates and Maneuvers by

Foliage-foraging Species 53

Major Axes of Variation in Substrate Use 53

Cluster of Species by Maneuver Use 55

Contingency Table Analyses 61

Comparison of Sampling Methods for Plant Species Use . . 61

Plant Species Use by Bird Species 62

Goodness-of-fit for Use Versus Availability of

Plant Species 62 ix

TABLE OF CONTENTS (continued) Page

Location in the Canopy of Plants Used by Bird Species . . 64

Goodness-of-fit for Use Versus Availability of

Location in the Canopy of Plants 67

Discrimination Between Bird Species 67

Discrimination by Searching Behavior 67

Discrimination by Foraging Site Characteristics 73

Differences in Prey Use by Foliage-foraging Species . . . 79

DISCUSSION 82

Bird Community Patterns 82

Vegetative Community 83

Guild Delineation 84

Resource Partitioning Among Foliage-foraging Species 86

General Patterns 86

Comparisons of Species 89

Resource Use by Year-round Residents 100

C0NCLUSI0NS 105

REFERENCES CITED 107

APPENDIXES 112

A. Proportion of Use of Substrates Upon Which Prey Were Attacked by 20 Selected Bird Species at Hopland, California. Values Represent a Mean Proportion of Use for a Randomly Sampled Substrate for Each Observation. N = Sample Size 112

B. Summary of Plant Group Use by Bird Species in the Foliage-foraging Guild at Hopland, California. Species Are Listed in Taxonomic 0rder 113 x

TABLE OF COΝTENTS (continued) Page

C. Summary of Use of Plant Canopies Proximal and Distal to the Trunk by Bird Species in the Foliage-foraging Guild at Hopland, California. Species Are Listed in Taxonomic Order 116

D. Means and Standard Deviations of Variables Describing Search Movements of Bird Species in the Foliage-foraging Guild at Hopland, California. The Number of Foraging Sites (n) and the Cummulative Number of Seconds (sec) Are Indicated. Species Are Listed in Taxonomic Order . . . . 118

E. Description of Characteristics of Plants and Locations Within Plants Used by Bird Species in the Foliage- foraging Guild at Hopland, California. Means and Standard Deviations are Shown for Continuous Variables and Species Are Listed in Taxonomic Order 119

F. Summary of Use of Height Intervals by Bird Species in the Foliage-foraging Guild at Hopland, California. Species Are Listed in Taxonomic Order 120 LIST OF TABLES

Table Page

1 Variables, and Their Sampling Method, Used to Describe Vegetation of Study Plots at Hopland Field Station, Hopland, California 10

2 Variables, and Their Sampling Method, Used to Describe Each Observation, Site, and Attack Used by Foraging Birds 15

3 Summary and Definition of Variables Used to Describe the Foraging Behavior of Avian Species. The 0riginal Categories used to Describe each Variable Were Lumped into Groups for Analyses 19

4 Variables Used for Discriminant Function Analysis of Foraging Sites and Search Movements by a Guild of Foliage-foraging Bird Species 26

5 List of Bird Species Detected on Study Plots at Hopland Field Station, Hopland, California. The Residency Status (Res. code), Number of Νests Located, Frequency of Occurrence (% Freq), Mean Density, and Standard Error of the Mean (s.e.) Across All Study Plots Are Listed for Each Bird Species. Data Were Collected During the Breeding Seasons (March-June) of 1986 and 1987. Bird Species Are Listed in Taxonomic Order 30

6 Ten Most Abundant Breeding Bird Species across All Study Plots. Abundant Estimates Reflect the Mean Number of Detections per Species Over All Plots (1986-1987) 34

7 Proportion of the Breeding Bird Community Comprised of Cavity Nesting Species. Νumbers Are Derived From Census Data for All 23 Study Plots during Both Breeding Seasons (1986 and 1987) 35

8 Tree and Shrub Species Observed on Study Plots. Mnemonic for Species Group, Common and Scientific Names of Species in Group, Percent Frequency of 0ccurrence (% Freq) by Study Plot, Mean Number of Stems Per Plot, and Range in Number of Stems Are Listed 37

xi xii

LIST 0F TABLES (continued)

Table Page

9 Variables Describing the Floristics and Structural Characteristics of the Vegetation in Study Plots. The Table Includes How Variables Were Derived and Transformed for Regression Analyses. SORT = Square Root 39

10 List of Species, Guild Membership Based on la986 Observations, Frequency of Occurrence Across Study Plots, and Mean Abundance of Birds for Which Foraging Behavior was Characterized. Abundance Estimates Reflect the Mean Number of Detections per Species Over All Plots (1986-1987). Species Whose Foraging beahviors Were Observed in Both 1986 and 1987 Are Indicated by an Asterisk . . . . 41

11 Factor Loadings for Principal Component Factors I-IIl for Frequency of Substrate Use by 20 Selected Bird Species. Factor Loadings for Each Variable Were Derived by a Varimax Rotation. The Eigenvalue, Proportion of the Variance Explained by Each Factor, and the Total Variation Explained by the Three Factors Are Given 45

12 Factor Loadings for Principal Component Factors I-IV for Abundance of Members of the Foliage-foraging Guild on Study Plots. Factor Loadings for Each Bird Species Were Derived by a Varimax Rotation. The Eigenvalue, Proportion of the Variance Explained by Each Factor, and the Total Variation Explained by the Three Factors Are Given 49

la3 Results of All Possible Subsets Regression Analyses of Bird Groups and Individual Species Regressed on Vegetation Variables. The Independent Variables and Their Signs Are Listed for Each Dependent Variable. The "Best" Subset Was Selected on the Basis of Mallow's Cp Algorithm. The T-statistic Is the Coefficient of the Variable Divided by Its Standard Error and Is an Indication of the Contribution Each Variable Makes to the Adjusted R . The Significance of Each T-statistic (Two-tailed Test, df = 11) Is Indicated as Follows: * = p < 0.05, ** = p < 0.01, *** = p < 0.001 52 xiii

LIST OF TABLES (continued)

Table Page

14 Factor Loadings for Principal Component Factors -III for Frequency of Substrate Use by Bird Species in the Foliage-foraging Guild. Factor Loadings for Each Variable Were Derived by a Varimax Rotation. The Eigenvalue, Proportion of the Variance Explained by Each Factor, and the Total Variation Explained by the Three Factors Are Given 56

15 Comparison of two methods of Summarizing Plant Group Use by Bird Species in the Foliage-foraging Guild. Use Is Summarized by the First Plant Used and by All Plants Used in Observations 63

16 Summary of Significant Differences for Use Versus Availability of Plant Species by Bird Species in the Foliage-foraging Guild. Significant Differences Were Based on the Availability of Plant Species that Did Not Occur within 95% Confidence Intervals for Proportions of Use by Birds 66

17 Standardized Discriminant Coefficients Are Listed for Those Functions with Significant (p < 0.05) Chi-square Values for Search Movements of Bird Species in the Foliage-foraging Guild. The Eigenvalue, Percent of the Variance Explained by Each Function, and the Total Variation Explained by the Two Functions Are Given 69

18 Analysis of Variance of Mean Frequency of Search Flights and Hops Used by Bird Species in the Foliage-foraging Guild 70

19 Multiple Comparisons of the Frequency of Search Flights Used by Bird Species in the Foliage-foraging Guild. Mean Number of Search Flights per Minute (R fly) and Sample Size (n) for Each Species Are Shown. Differences Between Means (Below Diagonal) and Minimum Significant Differences (Above Diagonal) Are Indicated for All Pairwise Combinations of Species. Significant (p < 0.05) Differences Are Indicated by an Asterisk 71 xiv

LIST OF TABLES (continued)

Table Page

20 Multiple Comparisons of the Frequency of Hops Used by Bird Species in the Foliage-foraging Guild. Mean Νumber of Hops per Minute (R hop) and Sample Size (n) for Each Species Are Shown. Differences Between Means (Below Diagonal) and Minimum Significant Differences (Above Diagonal) Are Indicated for All Pairwise Combinations of Species. Significant (p < 0.05) Differences Are Indicated by an Asterisk 72

21 Standardized Discriminant Coefficients Are Listed for Those Functions with Significant (p < 0.05) Chi-square Values for the Foraging Site Character- istics of Bird Species in the Foliage-foraging Guild. The Eigenvalue, Percent of the Variance Explained by Each Function, and the Total Variation Explained by the Two Functions Are Given. DBH = Diameter at Breast Height 74

22 Analysis of Variance of Two Foraging Site Characteristics, Mean Diameter at Breast Height of Trees and Mean Foraging Height, of Bird Species in the Foliage-foraging Guild 75

23 Multiple Comparisons of the Mean Diameter (cm) at Breast Height (DBH) of Trees Used by Bird Species in the Foliage-foraging Guild. The Mean DBH (X DBH) and Sample Size (n) for Each Species Are Shown. Differences Between Means (Below Diagonal) and Minimum Significant Differences (Above Diagonal) Are Indicated for All Pairwise Combinations of Species. Significant (p < 0.05) Differences Are Indicated by an Asterisk . . 77

24 Multiple Comparisons of Mean Foraging Height for Bird Species in the Foliage-foraging Guild. Mean Foraging Height (R ht) and Sample Size (n) for Each Species Are Shown. Differences Between Means (Below Diagonal) and Minimum Significant Differences (Above Diagonal) Are Indicated for All Pairwise Combinations of Species. Significant (p < 0.05) Differences Are Indicated by an Asterisk 78

25 Summary of Results Describing Resource Partitioning by Bird Species in the Foliage-foraging Guild. Values for Each Resource Are Underlined when Adjacent Species Have the Same Value 90 LIST OF FIGURES

Figure Page

1 Location of Study Area at the Hopland Field Station, 8 km East of Hopland, Mendocino County, California . . . 4

ο 2 Distribution of Blue Oaks (Quercus d uglassί i) in California (from Griffin and Critchfield 1972). Shaded Areas Indicate Stands of Trees Over two Miles in Diameter. Small x's Indicate Stands Less Than Two Miles Across or of Unknown Size 5

3 Density and Species Composition of Plant Species Groups on Each of the 23 Study Plots 38

4 Dendrogram of 20 Selected Bird Species Based on Frequency of Substrate Use. Substrate Use Associated With the Three Groups Defined at 50% Relative Similarity Is Indicated .. 43

5 Two-dimensional Ordination of 20 Selected Bird Species with Respect to the First Two Principal Components (PC-I and PC-II) Based on Substrate Use. Use was Described by the Standardized Proportion of Use of Substrates for Foraging Attacks 46

6 Two-dimensional Ordination of Bird Species in the Foliage-foraging Guild with Respect to the First Two Principal Components (PC-I and PC-II) Based on Bird Abundance. Locations of Species Are Based on Factor Scores 50

7 Percentage of Use of Substrate for Foraging Attacks by Bird Species in the Foliage-foraging Guild at Hopland, California. Sample Sizes Are Indicated at the Top of Each Bar 54

8 Three-dimensional Ordination of Bird Species in the Foliage-foraging Guild with Respect to the First Three Principal Components (PC-I, PC-II, and PC-III) Based on Substrate Use. Use Was Described by Standardized Proportions of Use of Substrates for Foraging Attacks 57

9 Percent Use of Foraging Maneuver Groups by Bird Species in the Foliage-foraging Guild at Hopland, California. Sample Sizes Are Indicated at the Top of Each Bar . . . 58

xv

xvi LIST OF FIGURES (continued)

Figure Page

10 Dendrogram of Bird Species in the Foliage-foraging Guild Based on Frequency of Use of Foraging Maneuvers. Foraging Strategies Associated With the Two Groups Defined at 25% Relative Similairy Are Indicated 59

11 Percent Use of Plant Species Groups as Foraging Sites by Bird Species in the Foliage-foraging Guild at Hopland. California. Sample sizes Are Indicated at the Top of Each Bar 65

12 Percent Use of Prey Types Observed Being Captured by Bird Species in the Foliage-foraging Guild at Hopland, California. Sample Sizes Are Indicated at the Top of Each Bar 80

13 Bill depth Versus Bill Lenth for the Three Resident Bird Species in the Foliage-foraging Guild. The ● 's Locate Mean Bill Depth and Length for Each Species, Lines Represent Ranges, and Crosshatches on the Range Lines Represent One Standard Deviation on Either Side of the Mean. Data are from Wagner (1981) 103 INTRODUCTION

A variety of environmental factors influence the composition and structure of avian communities (Austin and Smith 1972, Wagner

1981, Landres and MacMahon l980, 1983). Specific factors influencing avian communities vary across habitat types and within habitat types across geographic areas, seasons, and successional stages.

Contrasting results have been reported by studies investigating the influence of changing environmental factors on avian community structure (Austin and Smith 1972, Wagner 1981). Most apparent is the controversy over the importance of interspecific competition in avian community structure (Schoener 1982).

Studies of guilds of insectivorous birds in oak woodlands are good examples of contrasting results regarding factors influencing the structure of avian communities. Wagner (1981) found no significant seasonal changes in the foraging behavior of insectiyorous birds, despite major changes in habitat characteristics and the composition of the avian community. Landres and MacMahon (la983) found no evidence of interspecific competition influencing the foraging behavior of birds during the breeding season and concluded that opportunism was the primary factor influencing the foraging behayior of these bird species. Conversely, Austin and Smith (1972) found that during winter, the foraging behavior of bird species changed in response to the species composition of foraging flocks.

In an eastern deciduous forest, Robinson and Holmes (1984) found that the foraging behavior of bird species during the breeding

1 2 season changed in response to the distribution of foliage within and between plants. Results reported by Austin and Smith (1972) and

Robinson and Holmes (1984) suggest that some avian communities respond to changes in habitat characteristics and that interspecific competition can be an influential factor.

Further clarification of the relationship between changes in habitat characteristics and their affect on avian community structure is needed, particularly in oak woodlands. The objective of my thesis was to describe how a foliage-foraging guild of insectivorous birds partitioned resources in a primarily deciduous oak woodland during the breeding season. Specifically, I investigated three questions: (1) what is the species composition of the guild of insectivorous birds which obtained the majority of their prey from foliage; (2) how does the abundance of guild members change relative to changes in habitat characteristics; and (3) how do guild members partition food resources?

A guild may be considered a subset of a community which uses similar resources in a similar manner (Root 1967). Competition is potentially greater between members of a guild than between members of different guilds (Landres and MacMahon 1980). Consequently, guilds are a useful level of community organization to study changes in resource partitioning in response to changes in habitat structure and composition. Restricting data collection to one localized study area reduced variation in environmental factors (e.g., weather) between study plots. STUDY AREA

The study area was located at the University of California

Hopland Field Station, 8 km east of Hopland, in Mendocino County,

California (Fig. 1). The area was near the northern limit of blue oak

(Quercus dοuglassii) woodlands in the coastal range (Fig. 2). The dominant vegetation of the study area was described as the blue oak phase of the Coast Range foothill woodland (Griffin 1977). Blue oak was the dominant tree species throughout the field station. It was found in association with valley oak (Q. lobata), interior live oak

(0. wislizenii), California black oak (Q. kellοgii), Oregon white oak

(Q. garrvana), California bay (Umbellularia californica), and buckeye

(Aesculus californica). Density of tree cover varied relative to local environmental conditions and management regimes. Annual grasses and forbs dominated the ground cover. Adjacent vegetation types included chapparal, mixed hardwood and conifer forest, and meadows.

Sheep grazing was the primary land use. Portions of the study area, however, were retained as biological reserves (sheep and/or deer grazing are not allowed). Woodcutting was allowed to a limited degree.

The topography of the area was characterized by moderate to steeply sloped hills. The study area had a westerly aspect and had an elevational gradient ranging from 200 to 1000 m. The steep westerly aspect of the field station promoted vegetative growth along east-west strips aligned with intermittent streams and ravines. Ravines, dominated by live oaks, bay, and shrubs, dissected upland

3 4

Figure 1. Location of Study Area at the Hopland Field Station, 8 km East of Hopland, Mendocino County, California. 5

Figure 2. Distribution of Blue Oaks (Quercus douglassii) in California (from Griffin and Critchfield 1972). Shaded Areas Indicate Stands of Trees Over Two Miles in Diameter. Small x's Indicate Stands Less Than Two Miles Across or of Unknown Size. 6 habitats, dominated by deciduous oaks. The climate was Mediterranean

(Griffin 1977). Average annual rainfall was 90 cm, occurring primarily between September and June. Average summer and fall temperatures ranged from 20-25 degrees centigrade (Pitt 1975).

Winters were mild with some frost in the valleys and infrequent, light snow at higher elevations. MATERIALS AND METHODS

Study Plots

Twenty-three 5 ha (100 X 500 m) study plots were established

throughout the continuum of available tree densities within blue oak habitats, defined as areas in which the proportion of the total tree density was greatest for blue oaks. A uniform density and distribution of trees within and adjacent (within 50 m of plot

boundaries) to plots was an important criterion for plot placement to reduce within plot variation and the effect of dissimilar adjacent vegetation. Plot width was based on the ability to detect and accurately nap the locations of birds during censusing. Plots were spaced at least la00 m apart and dispersed throughout the 2,143 ha study area. Wooden stakes (3 cm X 8 cm Χ 2 m) spaced 50 m apart delineated a center transect line through each plot and 50 m reference

points on either side. Stakes were numbered and lettered to indicate their location within the plot. All field work was conducted during

March to June, 1986 and March to August, 1987.

Estimation and Analysis of the Bird Community

Sampling Methods

A modified spot-mapping method was used to describe species composition and abundance of the bird community on each study plot

(International Bird Census Committee 1969, Emlen 1977, Christman

1984). Each plot was censused once per week between late-March and

7 8 mid-June of 1986 and 1987 for a total of ten censuses per plot per year. Censuses were conducted between 0500 and 0900 Pacific Standard

Time. Α census consisted of walking along the center transect line of a plot at a fixed pace while mapping the location and recording the behavior of all birds detected.

The rectangular shape of the plots increased the amount of edge and, therefore, the number of partial territories overlapping plot boundaries. Partial territories generally decrease the accuracy of estimates of territory density in conventional spot-mapping

(Marchant 1981, Verner 1985)). To reduce the possible bias introduced by partial territories, we censused birds out to 100 m on either side of the center transect line and 25 m on either end of each plot. This

"buffer area" around each plot increased the effective size of the censused area to 11 ha.

Each census required one hour to complete. We rigorously adhered to the fixed duration of each census. The pace (50 m every 6 min) was regulated by a stop watch that beeped at two minute intervals. The pace of the census was based on those conditions (high density of vegetation and/or an abundance of birds) requiring the most observer effort. The pace ensured an adequate amount of time to detect birds and accurately map their locations. Therefore, our slow pace reduced differences in detection of birds due to differences in vegetation density. Α total of four people collected census data; two collected data in both years. 9

Data Analysis

Bird species abundances were described by estimating the mean number of detections occurring within 50 m of the transect line per census for each bird species on each study plot, 1986 and 1987 data combined. The openess of the habitat and the slow pace of the census census promoted a high level of detectability. I assumed that detection probabilities out to 50 m from the center transect line were equal for each species across plots. We attempted to keep double counting of birds to a minimum, and I believe double counting occurred infrequently.

Estimation and Analysis of Vegetation

Census Methods

Vegetation on each study plot was described by mapping the location and recording descriptive information on each individual woody plant. Eleven variables were measured and estimated for each plant (Table 1). The variables characterize the species, size, shape, vigor, and foliage volume of plants. Branching pattern, canopy radius, and canopy density were estimated as an index to foliage density. Six people collected vegetation data; all measurements occurred within a four month period. Rigorous training occurred before and during field work to reduce variability within and among observers for measured and estimated parameters.

Data Analysis

I examined the frequency distribution of each variable to identify outliers and deviations from a normal distribution. Square

10

Table 1. Variables, and their Sampling Method, Used to Describe Vegetation of Study Plots at Hopland Field Station, Hopland, California.

Variable Method

Plant species Direct observation; scientific names based on Munz (1975).

Plant height Measured with a clinometer or estimated to nearest one-half meter.

Canopy height above Occular estimate to the nearest decimeter. ground

Plant vigor Direct observation (after Thomas 1979:64).

Diameter at breast Measured with a DBH tape to the nearest height (DBH) decimeter.

Number of stems with Diameters measured with a DBH tape and stems DBH >5 cm counted.

Branching pattern Direct observation. Variable served as an index to shape of the canopy. Canopy shapes were categorized as an inverted triangle (branching pattern = 1-3), ellipsoid (branching pattern = 4), or one-half ellip- soid (branching pattern = 5) (after Pillsbury and Stephens 1987:12).

Canopy radius Occular estimate to nearest decimeter. Variable serves as an index to canopy volume. Canopy radius was the horizontal distance from trunk to edge of canopy. Radius was measured at either the top, middle, or bottom of the canopy for trees with a shape of inverted triangle, ellipsoid, and one-half ellipsoid, respectively.

Canopy density Occular estimate recorded as 0-33%, 34-66%, or > 66% of the canopy occupied by leaves.

Number of cavities Direct observation. The occurrence of natural and excavated cavities was characterized as containing 0, l-3, or > 3 cavities.

Granary tree Direct observation. Trees were characterized as no evidence, < 10%, or > 10% used for storing acorns. 11 root transformations of count data and arcsine transformations of percentage variables were used to improve fit of data to normal distributions (Sokal and Rohlf la981b:421,427). Bivariate scattergrams and correlation coefficients were evaluated to discern relationships between variables.

I estimated the value of a large number of structural and floristic vegetation variables, many of which were highly redundant.

To simplify these data, but retain most of their information, I performed a series of data reduction steps. First, I greatly reduced the dimensionality of the data by grouping variables on the basis of the absolute value of their correlation coefficients (r > 70 percent).

I retained at most two variables from each group for subsequent analysis. Second, I selected for retention those variables which were easiest to discuss with land managers and of most revelence to those making land use decisions. The first and second steps were often considered simultaneously.

Bird/Habitat Relationships

Multiple regression (BMDP program 9R, Dixon et al. 1983) was used to describe macrohabitat associations that existed between the abundance of individual bird species in the foliage-foraging guild and the vegetative characteristics of the study plots. All possible subsets regression (Mallow's Cp criterion) was used to select a "best" subset of vegetative variables (independent variables) to explain variation in the abundance of individual bird species (dependent variables). Mallow's Cp criterion minimizes the total mean squared error of fitted values (Neter et al. 1985:421). 12

Independent variables available for selection in the all possible subsets regression were those which best described the overall structural and floristic characteristics of plots. Care was taken to use variables which best described an entire resource (e.g., total number of blue oak trees) versus portions of a resource (e.g., number of blue oak trees in DBH intervals) to minimize the amount of correlation between independent variables. The tolerance limit for the largest multiple correlation was set at 0.01 to maximize the accuracy of the inverted cross-products matrix. Frequency distributions of each independent and dependent variable were examined to identify outliers and deviations from a normal distribution.

Estimation and Analysis of Foraging Behavior

Sampling Methods

Foraging behaviors of insectivorous bird species were described from March through June, 1986 and la987. Five observers were involved in data collection; two observers collected over 75 percent of the observations.

The intent of sampling efforts in la986 was to describe resource use by insectivorous and/or granivorous bird species. Bird species chosen for observation in 1986 met the following criteria:

(l) they were abundant, locally or throughout the study area; (2) their diet was primarily insectivorous or granivorous; and (3) they were not a flocking species (e.g., quail species). Data collected in

1986 were used to describe resource use by the bird community as a whole and to delineate guilds. Cluster analysis of the 1986 data indicated three broad guilds (bark, ground, and foliage foragers). 13

The intent of sampling efforts in 1987 was to obtain

additional observations on bird species tenatively identified as

members of the foliage-foraging guild. In 1987, the foraging behavior

of 10 bird species, including nine species identified as members of

the foliage foraging guild, was described from March to June.

A focal-animal sampling technique was used to characterize

foraging behavior (Altmann 1974). The focal-animal was located by

meandering through a study plot and focusing on the first individual

sighted of a bird species of interest. Observations began at the

first opportunity for an accurate location of the individual and ended

when either an accurate location was no longer possible or five

minutes had elapsed. An accurate location was defined as knowing the

location of the bird within a one meter radius. I attempted to obtain

only one observation per individual bird per day. However, more than

one observation per day per individual may have occurred since

individuals were not marked.

All observations were performed by watching a bird through

binoculars and verbally describing its actions onto a portable

cassette tape player. Tapes were transcribed daily using a stop watch

to time activities. If the exact movements of a bird were not visible during an observation, the time period(s) was identified and deducted from the total length of the observation. The reduced time period was used to estimate variables describing the movements of the bird.

Duration of observations varied between ten seconds and five minutes. Observations of ten seconds to five minutes were long enough

to obtain an adequate estimate of the location and activity of a foraging individual and short enough to avoid observer fatigue (pers. 14 obs.). Biases introduced by more visible behaviors were minimized by observing individuals for a period of time. All observations occurred within 100 m on either side of the transect line of each study plot.

Each study plot was sampled equally in an attempt to obtain 35 observations per species each year.

Variables measured and estimated for each foraging observation were parameters of importance in studies of similar habitats and bird communities (Root 1967, Landres and MacMahon 1980, 1983, Wagner 1981).

Behavioral data described for each observation included foraging sites, movements, and foraging attacks.

The foraging site was defined as the object (e.g., plant species, ground, fence) the bird was located in or on. The description of a foraging site consisted of two to seven variables, depending upon whether the foraging site was a plant or some other object, respectively (Table 2). Locality, described for birds observed in plants, was characterized by four variables which described the distance of the bird to the trunk (or center of plant) and the edge of the canopy, and the occlusion by the foliage within

0.5 m of the bird. These variables were described where the first observed foraging act occurred or, if a foraging act did not occur, the general location of the bird. If the bird did not perform a foraging act and changed its location more than 2 m in any direction, then the location where the observation first began was described.

Locality was described only once for each observation because of the difficulty in accurately recalling multiple locations during lengthy observations. 15

Table 2. Variables, and their Sampling Method, Used to Describe Each Observation, Site, and Attack Used by Foraging Birds.

Variable Method

Bird species Direct observation.

Sex Direct observation (if determinable).

Age Same as sex.

Foraging site Direct observation of plant or object that birds perched in or on.

Date Month, Day, and Year.

Time of day Pacific Standard Time.

Observer Observer's initials.

Study plot Plot number.

VARIABLES DESCRIBED FOR EACH FORAGING SITE:

Plant:

Bird heighta Occular estimate to nearest meter.

Type of percha Direct observation.

Plant species Same as type of perch.

Plant height Measured with a clinometer or estimated to nearest one-half meter.

Plant vigor Occular estimate based on Thomas (1979).

Number of plant stems Measured with a DBH tape to nearest decimeter with DBH > 5 cm and counted.

DBH of stem Measured with a DBH tape to nearest decimeter. 16

Table 2. Variables, and their Sampling Method, Used to Describe Each Observation, Site, and Attack Used by Foraging Birds. (continued)

Variable Method

VARIABLES DESCRIBED FOR EACH FORAGING SITE: cont.

Locale:

Occulsion by foliage Occular estimate to nearest 5%. within 0.5 m above bird

Occulsion by foliage Same as preceeding. within 0.5 in below bird

Horizontal distance Occular estimate to nearest decimeter. to trunk

Horizontal distance Same as preceeding. to edge of canopy

Movement:

Number and distance Direct observation and occular estimate to of strides nearest decimeter, respectively.

Number and distance Same as strides. of hops

Number and distance Sames as strides. of search flights

Distance flown to the Same as strides. foraging site

Length of time Direct observation of total length of time movements of bird to nearest second that movements were were visible visible.

Length of time Direct observation of total length of time bird was at at foraging site. foraging site 17

Table 2. Variables, and their Sampling Method, Used to Describe Each Observation, Site, and Attack Used by Foraging Birds. (continued)

Variable Method

VARIABLES DESCRIBED F0R EACH FORAGING ATTACK:

Substrate Direct observation.

Maneuver Same as substrate.

Attempts at each prey Same as substrate. item

Success Same as substrate.

Prey type Same as substrate (if successful).

aBird height and type of perch used were also recorded for objects other than plants. 18

Movement variables described search tactics of birds. Six movement variables were summarized for each foraging site occupied during an observation (Table 2). Hops were movements which did not involve use of the wings. Flights were stratified into three types:

(1) site flights used to move from one foraging site (plant, ground, rock, etc) to another; (2) search flights used to search for prey

(e.g., movement within a foraging site); and (3) foraging flights used to obtain prey (e.g., flycatch or hover-glean) (Robinson and Holmes

1982). Strides were walking or running movements used primarily by ground foragers.

Α foraging attack was defined as an attempt to acquire a prey item. Five variables were described for each foraging attack (Table

2). Foraging maneuvers were defined as the behavior used to attack a prey item. Maneuvers were stratified into eight categories (Table 3).

Substrate was defined as the exact part of the plant or environs toward which a foraging maneuver was directed. Substrates were stratified into la7 categories (Table 3). The number of times a maneuver was used to secure the prey item, if the attempt was visibly successful, and what type of prey was acquired were also recorded.

Data Analysis

Determination of Independence. Independence within and between observations is difficult to achieve with behavioral data.

The primary concern is whether sequential behaviors within an observation are dependent in some fashion. Defining a level of stratification at which behaviors become independent varies widely among researchers. Independence has been claimed for observations 19

Table 3. Summary and Definition of Variables Used to Describe the Foraging Behavior of Avian Species. The Original Categories used to Describe each Variable Were Lumped into Groups for Analyses.

Variable group Variable categories Definition

SUBSTRATE:

LEAF Leaf surface Surface of leaf. Leaf base Base of leaf. Leaf bud Bud of leaf. Leaf shoot New shoot of leaf.

TWIG Twig Branch diameter < 1 cm.

BRANCH Small branch Branch diameter l-5 cm. Medium branch Branch diameter > 5-15 cm. Large branch Branch diameter > 15 cm.

TRUNK Trunk Main verticle support of plant.

FLOWER Flower Flower (predominantly catkins on oak trees). Fruit Fruit.

MOSS Moss Moss. Lichen Lichen.

GRASS Grass Grass stalks and seeds.

GROUND Ground Bare ground. Leaf litter Dead leaves on ground.

AIR Air Air.

MANEUVER:

GLEAN Glean Bird removes prey from surface of substrate while perched. Pluck Bird pulls prey from surface of substrate while perched. Hang-glean Bird performs a glean while hanging upside down. 20

Table 3. Summary and Definition of Variable Used to Describe the Foraging Behavior of Avian Species. Original Variable Categories Were Lumped into Groups for Analyses. (continued) 21 taken at 15 sec intervals on the same individual (Landres and MacMahon

1983), all behaviors occurring during the first observation of an individual on a given day (Holmes 1980), and the first behavior occurring during the first observation of an individual on a given day

(Morrison 1984, Hejl et al., in press).

I assumed that observations on the same individual were at least one day apart, and that observations were independent. In reality, the time span between observations on the same individual was

probably closer to five to seven days, the typical time interval between visits to each plot. Mapped locations of observations were generally dispersed and, therefore, supported the assumption that multiple observations on the same individual were uncommon.

Therefore, I believe that whatever level dependency occurred in my data, it did not occur between observations.

I tested the independence of behaviors occurring within an observation using a goodness-of-fit test (Zar 1984:52-53) whenever sample sizes allowed. If no dependency was indicated fcr a particular

behavior, then all occurrences of that behavior for each observation were included in the analysis. If independence could not be tested, I

randomly selected one behavior to represent use for the observation.

Typically, the first observed behavior is used to represent the observation. However, this sampling scheme has the potential of being

biased toward more visible behaviors (Morrison 1984). Random selection of a behavior eliminates this bias.

Variable Categories Grouped for Analysis. Categories of some

variables (substrate, maneuver, and plant species) were lumped for 22 analysis because of infrequent use or small sample sizes (Table 3).

Substrates were lumped into nine groups. Four leaf related categories were combined to form the LEAF group. Three branch size classes were combined to form the BRANCH group. Reproductive parts of plants

(flower and fruit) were combined to form the FLOWER group. Epiphytes

(moss and lichen) were combined to form the MOSS group. And finally, bare ground and leaf litter were combined to form the GROUND group.

Twig, trunk, grass, and air were considered unique and, therefore formed their own independent groups.

Maneuvers were lumped into four groups (Table 3). Stationary maneuvers directed toward surface prey (glean, pluck, and hang-glean) were combined to form the GLEAΝ group. Stationary maneuvers directed toward subsurface prey (peck and probe) were combined to form the PECK group. Two of the dynamic maneuvers (hover-glean and fly-glean) were combined to form the HOVER group. Sallying is a specialized, dynamic maneuver directed toward the ground or air, and therefore it formed the unique group, SALLY.

Plant species were lumped into six groups (Table 3). Oregon white oak and valley oak are deciduous oaks with similar growth forms and often hybridize. They were combined to form the WHITE OAK group.

All evergreen trees were combined to form the EVERGREEN TREES group.

This group was composed primarily of live oaks because madrone and bay, the only other evergreen trees, were generally uncommon on study plots. Oracle oak (Quercus morheus) is a hybrid between live oak and black oak. Its leaves turn yellow in the fall, but they are not completely deciduous. Black oak and oracle oak were combined to form the BLACK 0ΑK group. All shrub and vine species were combined to 23 form the SHRUB group. Blue oak and buckeye trees had unique growth forms, and therefore they formed their own independent groups, BLUE

OAK and BUCKEYE.

Cluster Analysis. Cluster analysis (BMDP program 2M, Dixon et al. 1983) was performed on the relative proportion of substrate use by each bird species to group birds into guilds. Cluster analysis was also used to segregate the foliage-foraging guild into subgroups based on the relative proportion of use of maneuver groups. A dendrogram was formed by a centroid-linkage algorithm. Initially, each species is considered a cluster. Then, the two closest clusters are amalgamated into one cluster by summing the frequencies of the two clusters. Amalgamation of the two closest clusters into one cluster is repeated until there is only one cluster. The distance between clusters was measured by the chi-square statistic.

Principal Component Analysis. Principal component analysis

(PCA) (BMDP program 4M, Dixon et al. 1983) was used to describe major axes of variation in resource utilization by all bird species. The data matrix used in PCA contained the proportion of use of each of nine substrate groups over all observations for each bird species. For example, if in 10 observations the two available substrates, A and B, were used six and four times, respectively, then the associated proportion of use would be 0.60 and 0.40, respectively. The result was 20 vectors (one for each species), each with nine elements, which represented estimates of the probabilities of observing the use of each substrate given a random observation taken on that bird species.

Vectors for all bird species (n = 20) were combined to form a 20 X 9 24 matrix. This matrix was standardized (each substrate category had a mean of zero and variance of one) and the covariance matrix computed from the standardized data (the equivalent of a correlation matrix).

I extracted principal components from the correlation matrix to reduce dimensionality of the foraging space and to investigate relative positions of species in this space. I retained factors with an associated eigenvalue > 1.0.

Subsequent to estimating these principal components, I performed a varimax rotation and then ordinated each of the bird species by its standardized factor score. The rotated factor loadings

(sample correlations between component scores and the original variables) > 0.50 were used to interpret principal components.

PCA was also used to describe major axes of variation in the abundance of members of the foliage-foraging guild among study plots.

Bird abundance was characterized by the mean number of detections per census per plot for each species. These data were arranged in a 20

(species) by 23 (plots) matrix. Principal components were extracted from the correlation matrix.

Discriminant Function Analysis. I used stepwise discriminant function analysis (DFA) (SPSS program DISCRIMINANT, Nie et al. 1975) to determine how variation in search movements and foraging site characteristics differed among members of the foliage foraging guild.

The grouping variable was bird species, and foraging sites were the units of replication. All variables used in DFA were continuous, and were square root transformed to fit a normal distribution. Νominal 25 variables, such as plant or prey species, were examined graphically or tested by G-tests.

Five continuous variables were used to describe foraging site characteristics (Table 4). Characteristics of plants (DBH and height) and the localities within plants (distance to the trunk and occlusion by the foliage around the bird) used for foraging were described. The proximal or distal location of a bird to the trunk (or center of a plant) was determined by the following process. First, the estimated distances from the bird to the trunk and the edge of the canopy were summed. Then, the mid-point distance between the trunk and the canopy edge was calculated. Finally, the bird's location in the canopy was assigned based on whether the distance to the trunk was greater

(distal) or less than (proximal) the mid-point distance.

Search movements were used to describe the rate and style of search of each guild member. Six continuous variables describing search movements were calculated for each foraging site (Table 4).

Search rate was described by the number of hops per minute, search flights per minute, and distance moved per minute. Search style was described by mean distance per hop, mean distance per search flight, and ratio of hops per search flights (H/F ratio). The H/F ratio is a useful index of how species move while searching for prey (Robinson and Holmes 1982).

Discriminant function analysis is an objective method for defining variables which best separate groups (Noon 1981). The importance of each variable in separation of groups is proportional to the absolute value of the standardized discriminant function coefficient. Group sizes were made approximately equal by randomly 26

Table 4. Variables Used for Discriminant Function Analysis of Foraging Sites and Search Movements by a Guild of Foliage-foraging Bird Species.

Variable Derivation

FORAGING SITES:

DBH Stem diameter at breast height.

Plant height Height of the plant.

Relative height Height of bird relative to the height of the plant.

Occlusion above Percent occlusion by foliage 0.5 m above the bird

Occlusion below Percent occlusion by foliage 0.5 m below the bird

SEARCH MOVEMENTS:

Hops per min Νumber of hops/minutes visible.

Search flights per min Number of search flights/minute visible.

Hop-flight ratio Number of hops/number of search flights.

Mean hop distance Distance hopped/number of hops.

Mean search flight Distance flown/number of search flights. distance

Distance moved per min Distance hopped plus distance flown/minutes visible. 27 selecting a proportion of observations from each group, and prior probabilities were set equal. Classification procedures to determine how well the model predicts group membership were performed with observations not selected to create the model.

I used analysis of variance to compare the use of height and movement characteristics within and between bird species. To determine which bird species differed from each other, multiple comparison tests of means by the GT2-method (Sokal and Rolf 1981:248) were performed. The GT2-method adjusts the alpha level to reduce the probability of type II error (Zar 1984:162-163), which is increased by multiple unplanned comparisons.

Continency Tests. Contingency tables were developed to compare the frequency of use of plant species, height intervals, and location in the canopy between species in the foliage-foraging guild.

For the purposes of this investigation, observations were pooled over years, months, observers, time of day, and sex.

Goodness-of-fit tests with Bonferoni 95 percent confidence intervals (Nue et al. 1974) were used to investigate use versus availability of resources by each species in the guild. Bonferoni 95 percent confidence intervals were calculated to determine whether availability occurred within the 95 percent confidence intervals for proportion of use (Neu et al. 1974).

Availability of each plant species group was calculated for each bird species by weighting the plant species composition of each plot by the total number of foraging observations occurring within the plot. Weighted plant species compositions were then summed across 28 plots and the proportion of the total contributed by each plant species was calculated. Weighted plant species compositions served to eliminate any biases introduced by the unequal distribution of foraging observations among the 23 study plots. The plant community was described for only the area within 50 m of the transect line of each plot (a 5 ha area). Only those foraging observations occurring within this 5 ha area οn each plot were used to characterize plant species use.

The availability of foliage volume (the area occupied by the canopy of plants) was calculated for each interval and compared to use. Foliage volume was calculated for each plant with two basic parameters, canopy shape and dimension. The foliage volume was divided equally among each vertical meter occupied by the canopy of the plant. Available foliage volume was calulated by summing the foliage occurring in each height interval across all study plots. The proportion of foliage volume in each height interval was used to calculate the expected frequency of use for each bird species.

I arbitrarily selected an alpha level of 0.05 for all tests o f significance. RESULTS

Bird Community

Territory sizes were too large and detections too few to enable the use of Christman's (1984) method of territory estimation for most bird species. Analyses of abundances of bird species were

based on detections occurring within 50 m of the center transect line, a 5 ha area.

Seventy-three bird species were detected during censusing

(Table 5). Fifty-five bird species were observed in territorial

behaviors and presumed breeding on the study plots; 60 percent (33 of

55) were insectivorous to some extent. Thirty-six species were

confirmed as breeders by the discovery of nests. The ten most

abundant bird species over all study plots are listed in Table 6. Six

of these 10 species are cavity nesters. The most common cavity

nesting species were violet-green swallow (Tachycineta thalassina),

plain titmouse (Porus inornatus), acorn wocdpecker (Melanerpes

formicivorous), and white-breasted nuthatch (Sitta carolinensis).

Cavity nesters comprised 24.5 percent (12 of 49 species) of the

breeding non-raptorial bird species and 58.l percent of the breeding

individuals (Table 7). Secondary cavity nesters alone comprised

almost half (47.5 percent) of breeding individuals.

29 30

Table 5. List of Bird Species Detected on Study Plots at Hopland Field Station, Hopland, California. The Residency Status (Res. code), Number of Nests Located, Frequency of Occurrence (% Freq), Mean Density, and Standard Error of the Mean (s.e.) Across All Study Plots Are Listed for Each Bird Species. Data Were Collected During the Breeding Seasons (March-June) of 1986 and la987. Bird Species Are Listed in Taxonomic Order. 311

Table 5. List of Bird Species Detected on Study Plots at Hopland Field Station, Hopland, California. The Residency Status (Res. code), Number of Νests Located, Frequency of Occurrence (% Freq), Mean Density, and Standard Error of the Mean (s.e.) Across All Study Plots Are Listed for Each Bird Species. Data Were Collected During the Breeding Seasons (March-June) of 1986 and 1987. Bird Species Are in Listed in Taxonomic Order. (continued) 32

Table 5. List of Bird Species Detected on Study Plots at Hopland Field Station, Hopland, California. The Residency Status (Res. code), Number of Nests Located, Frequency of Occurrence (% Freq), Mean Density, and Standard Error of the Mean (s.e.) Across All Study Plots Are Listed for Each Bird Species. Data Were Collected During the Breeding Seasons (March-June) of 1986 and 1987. Bird Species Are in Listed in Taxonomic Order. (continued) 33

Table S. List of Bird Species Detected on Study Plots at Hopland Field Station, Hopland, California. The Residency Status (Res. code), Number of Nests Located, Frequency of Occurrence (% Freq), Mean Density, and Standard Error of the Mean (s.e.) Across All Study Plots Are Listed for Each Bird Species. Data Were Collected During the Breeding Seasons (March-June) of 1986 and 1987. Bird Species Are in Listed in Taxonomic Order. (continued) 34

Table 6. Ten most Abundant Breeding Bird Species across All Study Plots. Abundance Estimates Reflect the Mean Number of Detections per Species Over All Plots (1986-la987).

Species x Birds/40 ha

Violet-green swallow 58.4

Plain titmouse 48.8

Acorn woodpecker 25.4

Lesser goldfinch 20.6

American robin 16.2

White-breasted nuthatch 13.5

Scrub jay 11.0

Mourning dove 10.4

European starling 10.2

Western bluebird 10.0 35

Table 7. Proportion of the Breeding Bird Community Comprised of Cavity Nesting Species. Numbers Are Derived from Census Data for all 23 Study Plots during both Breeding Seasons (1986 and la987). 36

Vegetative Community

Approximately 18,000 woody plants were mapped and described.

Eight oak tree species, four other tree species, and nine shrub species occurred on study plots (Table 8). Study plots showed extensive variation in tree density, ranging from 21 to 392 trees per ha (Fig. 3). Most plots were dominated by blue oak trees, however a variety of other tree and shrub species were also abundant. Elevation

varied among study plots (200 to 950 m) and tree species composition

varied along the elevational gradient (Noon et al. 1988). The primary

trend was a gradient of plots with abundant blue oak and buckeye trees at lower elevations to plots with abundant white oaks and California

bay at higher elevations. Tree species diversity tended to increase

with elevation.

Many of the original vegetation variables were highly

correlated (Noon et al. 1988). Total basal area per plot, the density of snags > 10 cm DBH, and the density of shrubs were all positively

correlated with tree density (r = 0.69, r = 0.63, and r = 0.71,

respectively, p < 0.01). In contrast, mean basal area per tree was negatively correlated with tree density (r = -0.76, p < 0.01).

Because of the high degree of redundancy among the vegetation

variables, 11 variables with minimum intercorrelations were selected

to represent the vegetative characteristics of the study plots in subsequent analyses (Table 9). 37

Table 8. Tree and Shrub Species Observed on Study Plots. Mnemonic for Species Group, Common and Scientific Names of Species in Group, Percent Frequency of Occurrence (% Freq) by Study Plot, Mean Number of Stems Per Plot, and Range in Number of Stems Are Listed. Figure 3. Density and Species Composition of Plant Species Groups on Each of the 23 Study Plots. 39

Table 9. Variables Describing the Floristics and Structural Characteristics of the Vegetation in Study Plots. The Table Includes How Variables Were Derived and Transformed for Regression Analyses. SQRT = square root. 40

Definition of Foraging Guilds

Results of Analysis of 1986 Data

Foraging behaviors were recorded for 20 bird species in la986

(Table la0). Proportion of use of nine substrate groups was used in

cluster analysis to tenatively identify guild membership for these 20

bird species. Only substrates were used to delineate guilds because I

wanted to first identify all species which directed the majority of

their prey attacks toward foliage dwelling prey items.

Three tentative groups of species were defined by cluster

analysis: bark-foragers (four species); ground-foragers (six

species); and foliage-foragers (10 species) (Table 10). Resource

associations of each group were assigned by reviewing proportions of

substrate use.

Cluster of Bird Species Into Guilds

Cluster analysis was used to define the final species

composition of each guild. In this final analysis, the proportion of

use of nine substrate groups for all 20 original species was

recalculated using foraging observations from 1986 and 1987 (Appendix

A). In 1987, foraging observations were limited to the 10 tentative

members of the foliage-foraging guild. Sample sizes were small for

some species, in particular some members of the bark- and

ground-foraging guild. However, this analysis was intended only to

describe general patterns.

The resulting dendrogram is a heirarchial grouping of species

that may reflect biological significance, and it displayed several

levels of association. At 0 percent similarity, all species are 41

Table 10. List of Species, Guild Membership Based on 1986 Observations, Frequency of Occurrence Across Study Plots, and Mean Abundance of Birds for Which Foraging Behavior Was Characterized. Abundance Estimates Reflect the Mean Number of Detections per Species Over All Plots (1986-1987). Species Whose Forgaging Behaviors Were Observed in Both 1986 and 1987 Are Indicated by an Asterisk. 42 insectivorous and/or granivorous (my criterion for observation of species). A 50 percent level of relative similarity on the resulting dendrogram was subjectively used to define three guilds which represented broad adaptive strategies (Fig. 4).

The bark-foraging guild consisted of four species (acorn woodpecker, brown creeper (Certhia americans), Nuttall's woodpecker

(Picoides pubescens), and white-breasted nuthatch). The ground-foraging guild consisted of six species (western bluebird,

Sialic mexicana, lesser goldfinch, Carduelis psaltria, lark sparrow,

Chondestes grammacus, american robin, Turdus migratorius. dark-eyed junco, Junco hyemalis, and chipping sparrow (Spizella passerine)).

Finally, the foliage-foraging guild consisted of 10 species

(ash-throated flycatcher (Myiarchus cinerascens), plain titmouse, bushtit (Psaltriparus minimus), blue-gray gnatcatcher (Polioptila caerulea), solitary vireo (Vireo solitarius), Hutton's vireo (Vireo hutttonii), orange-crowned warbler (Vermivora celata), western tanager

(Piranga ludoviciana), black-headed grosbeak (Pheucticus

melanocephalus), and northern oriole (Icterus galbula bullockii)).

Cluster analysis of the combined 1986 and la987 data set yeilded the same results as those produced by the 1986 data set, with two exceptions. Analysis of the 1986 data placed ash-throated flycatcher in the ground-foraging guild and chipping sparrow in the foliage-foraging guild. Analysis of the combined 1986 and 1987 data set placed chipping sparrow in the ground-foraging guild and ash-throated flycatcher in the foliage-foraging guild. Consequently, ash-throated flycatcher was determined to be a member of the 43

Figure 4. Dendrogram of 20 Selected Bird Species Based on Frequency of Substrate Use. Substrate Use Associated With the Three Groups Defined at 50% Relative Similarity Is Indicated. 44 foliage-foraging guild based on the final analysis. Howeyer, it is the only member for which I have only one year's data.

Major Axes of Variation in Substrate Use

Principal component analysis was used to describe the major axes of variation in substrate use. PCA was performed on standardized proportions of use for eight of the nine substrate categories. The substrate group LEAF was removed from the analysis because its variation was highly correlated (i.e., squared multiple correlation =

1.0) with many other variables, namely BRANCH, TRUNK, MOSS, GRASS, and

GROUND (r = -0.531, r = -0.414, r = -0.521, r = -0.385, r = -0.447, respectively). Principal component axes were interpreted in light of these relationships.

PCA yielded three principal components which described 52.6 percent of the variation among bird species (Table 11). Principal component I (PC-I; 19.6 percent) was characterized by a contrast between the frequent use of trunks and moss and the frequent use of aerial foraging. PC-II (18.6 percent) was characterized by variation in the frequency of use of the ground and grasses. PC-III (14.4 percent) was characterized by variation in the frequency of use of flowers, fruits and twigs.

Ordination of bird species along PC-I and PC-II showed the relationship of the constituents of each guild to principal component axes and relationships between guilds (Fig. 5). The bark-foraging guild was positiyely associated with PC-I (bark) and negatively associated with PC-II (ground). Species in this guild were confirmed as bark-foragers. The ground-foraging guild varied in its association 45

Table 11. Factor Loadings for Principal Component Factors I-III for Frequency of Substrate Use by 20 Selected Bird Species. Factor Loadings for Each Variable Were Derived by a Varimax Rotation. The Eigenvalue, Proportion of the Variance Explained by Each Factor, and the Total Variation Explained by the Three Factors Are Given. Figure Two-dimensional Ordination of 20 Selected Bird Species with Respect to the First Two Principal Components (PC-Ι and PC-II) Based on Substrate Use. Use Was Described By the Standardized Proportion of Use of Substrates for Foraging Attacks. 47 with PC-I (bark), but was positively associated with PC-II (ground)

(Fig. 5). Species in this guild were confirmed as ground-foragers.

The foliage-foraging guild was negatively associated with PC-I (bark) and PC-II (ground) (Fig. 5). The species in this guild were confirmed as foliage-foragers. Resource partitioning within the foliage-foraging guild was the focus of all subsequent analyses.

Habitat Relationships of foliage-foraging Species

Distribution Across Study Plots

The majority of species were present on most of the study plots. Plain titmouse, bushtit, and orange-crowned warbler occurred on every plot. Northern oriole, Hutton's vireo, and ash-throated flycatcher occurred on all but one or two plots (91-95 percent occupancy). Blue-gray gnatcatcher, black-headed grosbeak, solitary vireo, and western tanager were absent on four to six plots (74-82 percent occupancy).

Abundance Across Study Plots

The abundance of individual species influence the impact each will have on food resources, and, therefore, on the potential for interspecific competition. If species had greatly different detection probabilities, then direct comparisons of abundance estimates between species would not be valid. However, it is unlikely that detection probabilities differed significantly among bird species in the foliage-foraging guild. All of the foliage-foragers defended the same type of territory (mating, nesting, and feeding occur within it) by 48

relatively frequent and audible vocalizations. For example, abundance estimates for plain titmouse were three to four times greater than all other species in the guild across all plots. I belieye a difference of this magnitude reflected real differences in the abundance of this species.

Principal component analysis was performed on the abundance of foliage-foraging bird species on each study plot to determine which species covaried in their abundance. PCA yeilded fcur principal

components which described 73.8 percent of the variation in the abundance of bird species among study plots (Table 12). Principal

component I (PC-I; 32.6 percent) represented covariation in the abundance of solitary vireo, black-headed grosbeak, western tanager, and blue-gray gnatcatcher. PC-II (18.l percent) represented

covariation in the abundance of bushtit, plain titmouse, and

orange-crowned warbler. PC-III (1la.8 percent) represented a negative

covariation between the abundance of Hutton's vireo and the abundance

of ash-throated flycatcher. PC-ΙV (11.3 percent) represented

variation in the abundance of northern oriole. 0rdination of birds in

plot space, based on factor scores, showed the grouping of species as

described by the principal components (Fig. 6).

Linear Relationships Between Bird Species Abundance and Vegetative Characteristics

All possible subsets regression was performed to describe

vegetative characteristics associated with the abundance of each of

the bird species groups indicated by PC-I and PC-II of the PCA on bird abundance (Fig. 6). Species not a part of a group in this space were 49

Table 12. Factor Loadings for Principal Component Factors I-IV for Abundance of Members of the Foliage-foraging Guild on Study Plots. Factor Loadings for Each Bird Species Were Derived by a Varimax Rotation. The Eigenvalue, Proportion of the Variance Explained by Each Factor, and the Total Variation Explained by the Three Factors Are Given. Figure 6. Two-dimensional Ordinations of Bird Species in the Foliage-foraging Guild with Respect to the First Two Principal Components (PC-I and PC-II) Based on Bird Abundance. Locations of Species Are Based on Factor Scores. 51

analyzed separately. Independent variables used to describe the

vegetative community are listed in Table 9.

Analysis of Bird Groups. Approximately 46 percent of the

variation in the combined abundance of the bird species represented by

PC-I (solitary vireo, western tanager, blue-gray gnatcatcher, and

black-headed grosbeak) was explained by three independent variables

(Table 13). The abundance of these birds varied negatively with mean

basal area per tree (MEAN BASAL AREA) and BUCKEYE, and positively with

WHITE 0AK. These bird species were generally more common on study

plots with dense trees and with white oak trees as a component.

Forty-four percent of the variation in the combined abundance

of bushtit, plain titmouse, and orange-crowned warbler was explained

by one independent variable (Table 13). The abundance of this group

yaried positively with the number cf cavities (CAVITIES). It also

varied positively with EVERGREEN TREES. These species were generally

more common on study plots with abundant evergreen trees, which

inherently had more natural cavities (Noon et al. 1988).

Analysis of Individual Bird Species. Approximately 70 percent

of the variation in the abundance of northern oriole was explained by

one independent variable (Table 13). Northern oriole abundance varied

positively with MEAN BASAL AREA. In addition, the abundance of

northern oriole had a positive relationship with WHITE 0AK and

negative relationships with BLUE OAK, BUCKEYE, EVERGREEN TREES,

SHRUBS, and percent canopy closure (CAΝOPY CLOSURE). Νorthern orioles

were generally more common in study plots with a few large trees,

primarily white oaks, and more open understories. 52

Table 13. Results of All Possible Subsets Regression Analyses of Bird Groups and Individual Species Regressed on Vegetation Variables. The Independent Variables and Their Relationship (Sign) to the Dependent Variable Are Listed. The "Best" Subset Was Selected on the Basis of Mallow's Cp Algorithm. The T-statistic Is the Coefficient of the Variable Divided by Its Standard Error and Is an Indication of the Contribution Each Variable Makes to the Adjusted R . The Significance of Each T-statistic (Two-tailed Test, df=lll) Is Indicated as Follows: * = p < 0.05, ** = p < 0.01, *** = p < 0.001. 53

Approximately 35 percent of the variation in the abundance of ash-throated flycatcher was explained by four independent variables

(Table 13). Ash-throated flycatcher abundance varied positively with

with SHRUBS and CAVITIES, and yaried negatiyely with BLUE OAK and

EVERGREEN TREES. Ash-throated flycatcher abundance also varied

positively with WHITE OAK. Therefore, ash-throated flycatcher was generally more common on study plots with numerous white oaks and shrubs.

0nly 29 percent of the variation in the abundance of Hutton's vireo was explained by one independent variable (Table 13). Hutton's vireo varied positively with SHRUBS. It was also positively associated with CANOPY CLOSURE, SNAGS, BLUE OAK, and EVERGREEN TREES.

Hutton's vireo was generally more common on study plots with small, dense trees, abundant blue oak and evergreen trees, and a dense understory.

Foraging Behavior

Use of Substrates and Maneuvers bv Foliage-foraging Species

Major Axes of Variation in Substrate Use. Principal component analysis was applied to the standardized proportion of substrate use by the 10 members of the foliage-foraging guild to better estimate axes of variation within the guild (Fig. 7). The substrate catagory

LEAF was again removed from the analysis because of its high negative correlation (multiple R2 = 1.0) with MOSS, BRANCH, TWIG, and GROUND (r

-0.721, r = -0.660, r = -0.615, and r = -0.547, respectively). Figure 7. Percentage of Use of Substrate for Foraging Attacks by Bird Species in the Foliage-foraging Guild at Hopland, California. Sample sizes Are Indicated at the Top of Each Bar. 55

Three principal components had eigenvalues > 1.0, and they described

63.5 percent of the variation among bird species (Table 14).

Principal component I (PC-I; 27.5 percent) was characterized

by the contrast between the frequent use of leaves to the use of

branches (often covered with moss) and the ground. PC-II (22.1

percent) was characterized by variation in the frequency of use of

trunks and grass. Trunk and grass were represented by the same axis

because species that used one of these substrates typically also used

the other. PC-III (13.9 percent) was characterized by the contrast between the use of flowers and/or fruits, and the air.

Ordination of bird species on PC-I, PC-Il, and PC-III indicated that the majority (six) of species varied from a relatively high proportion of use of woody substrates (solitary vireo) to almost exclusive use of leaves (ash-throated flycatcher) (Fig. 8). Four species (black-headed grosbeak, plain titmouse, orange-crowned warbler, and western tanager) specialized in their use of a specific resource in addition to leaves; the resources they exploited were trunks, a variety of woody substrates, catkins, and aerial prey, respectively.

Cluster of Species Maneuver Use. Proportion of use of four maneuver groups was used in a cluster analysis to define subgroups for the foliage-foraging guild (Fig. 9). The dendrogram for maneuver use clarifies the functional relationships among species (Fig. 10). Two foraging strategies, the primary use of dynamic versus stationary maneuvers, were revealed at 25 percent relative similarity. The next level of similarity (50 percent) began to reflect the breadth and 56

Table 14. Factor Loadings for Principal Component Factors I-III for Frequency of Substrate Use by Bird Species in the Foliage-foraging Guild. Factor Loadings for Each Variable Were Derived by a Varimax Rotation. The Eigenvalue, Proportion of the Variance Explained by Each Factor, and the Total Variation Explained by the Three Factors Are Given. 57

Figure 8. Three-Dimensional Ordination of Bird Species in the Foliage-foraging Guild with Respect to the First Three Principal Components (PC-I, PC-ΙΙ, and PC-III) Based on Substrate Use. Use Was Described by Standardized Proportions of Use of Substrates for Foraging Attacks. Figure 9. Percent Use of Foraging Maneuver Groups by Bird Species in the Foliage-foraging Guild at Hopland, California. Sample Sizes Are Indicated at the Top of Each Bar. Figure 10. Dendrogram of Bird Species in the Foliage-foraging Guild Rased on Frequency of Use of Foraging Maneuvers. Foraging Strategies Associated With the Two Groups Defined at 25% Relative Similarity Are Indicated. 60 combinations of maneuvers used by species in each of the two maneuver categories (dynamic and stationary). At 75 percent similarity, the breadth of maneuvers used by species became even more well defined.

Species that are in the same cluster at this level of similarity have the potential to compete for food should it become limiting.

The 75 percent level of relative similarity on the resulting dendrogram was subjectively used to define four subgroups (Fig. 10).

Species using primarily dynamic maneuvers split into three groups:

"hovering" (Ash-throated flycatcher); "hovering and gleaning"

(Hutton's vireo, solitary vireo, and blue-gray gnatcatcher); and

"hovering, gleaning, and sallying" (western tanager). The ash-throated flycatcher was unique in its use of maneuvers.

Ash- throated flycatcher used HOVER and SALLY maneuvers over 90 percent of the time. However, the sample size was small (n = 15) and the apparently unique combination of behaviors could be a spurious result.

Western tanager was also unique in its approximately equal use of

GLEAN, HOVER, and SALLY maneuvers. Hutton's vireo, blue-gray gnatcatcher, and solitary vireo formed a group that primarily used

GLEAN and HOVER, and occasionally (5-13 percent of the time) used

SALLY maneuvers (Fig. 10).

Species using primarily stationary maneuvers formed the fourth subgroup, the "gleaning" group. Bushtit, orange-crowned warbler, northern oriole, black-headed grosbeak, and plain titmouse were the members of this group. This group used GLEAN maneuvers over 70 percent of the time, employing HOVER maneuvers the remainder of the time. 61

Contingency Table Analyses

Three contingency table analyses were performed: (1) bird species by plant species group; (2) bird species by location in the canopy; and (3) bird species by foraging height interval. In conjunction with each analysis, a goodness-of-fit test was also performed to compare use to availability for each bird species.

My data set was too small (matrices had expected frequencies less than 1.0 and/or > 20 percent were less than fiye) to partition into three (or more) way contingency tables (Zar 1984:70). However, I realize that three-way interactions may have existed between these variables. Similarly, I was also unable to investigate interactions of bird species by substrate use or maneuver use by contingency table analysis because of small sample sizes.

Comparison of Sampling Methods for Plant Species Use. Plant species use by each bird species was summarized by two methods: all foraging sites used; and only the first foraging site used during each observation. Hejl et al. (in press) found that dependent samples had less precise estimates of standard errors and differed in their estimates of means and standard errors for uncommon behaviors. Hejl et al. (in press) suggest that Pearson's contingency coefficient be used to test for dependency between samples. However, the test statistic is based on the chi-square distribution, and my sample sizes were too small to use this test. Proportions derived from all foraging sites used and the first site used during each observation were very similar (Table 15). 62

A comparison of the two summaries was accomplished by a

goodness-of-fit test. .Four plant species groups (BLUE OAK, WHITE OAK,

EVERGREEN TREES, AND BLACK OAK) had adequate sample sizes for use in

the comparison. There were no significant differences (p > 0.90, G <

0.37, df = 3) between the sampling methods for any bird species.

Therefore, all foraging sites were used for subsequent analyses of

plant species use.

Plant Species Use by Bird Species. The same four plant groups

of the preceeding analyses were used to create a 4 X 10 contingency

table which described the frequency of plant species use by bird

species. The plant group was considered the response variable. Plant

species use differed significantly (p < 0.001, G = 204.01, df = 27)

between bird species.

Non-significant subsets of the contingency table were

investigated (BIOME program RXC, Sokal and Rolf 1981a). Two groups of

bird species were not significantly different in their use of all the

plant species groups: (l) plain titmouse, ash-throated flycatcher,

blue-gray gnatcatcher, western tanager, and solitary vireo; and (2)

the same list of species except with Hutton's vireo substituted for solitary vireo.

Goodness-of-fit for Use Versus Availability of Plant Species.

A goodness-of-fit test was used to test for differences between observed and expected use of plant species by each bird species. Four

(BLUE OAK, BLACK 0AK, WHITE OAK, and EVERGREEN TREES) of the six plant species groups were used to characterize plant species use and availability. The BUCKEYE and SHRUB groups were combined to form a 63

Table 15. Comparison of Two Methods of Summarizing Proportion of Use of Plant Groups by Bird Species in the Foliage-foraging Guild. Use Is Summarized by the First Plant Used and by All Plants Used in Observations. Ν = Sample Size. 64 fifth group representing understory vegetation (UNDERSTORY). The proportional use of plant species groups by each bird species is shown in Figure 11.

Eight of the 10 members of the foliage-foraging guild used the plant species groups in significantly different proportions (p < 0.05,

G > 11.46, df = 4) than expected based on availability. Ash-throated flycatcher and western tanager were the only bird species to use plant species groups in proportion to their availability (p > 0.05, G <

6.66, df = 4).

Significant test results based on simultaneous Bonferoni confidence intervals (Appendix B) are summarized in Table 16. Plain

titmouse used the WHITE OAK, EVERGREEN TREES, and BLACK OAK groups significantly greater than expected based on availability. In contrast, plain titmouse used UNDERST0RY less than expected. Bushtit used EVERGREEN TREES greater than expected and UNDERSTORY less than expected based on availability. Blue-gray gnatcatcher did not use any one plant species group significantly different than expected.

Hutton's vireo used EVERGREEN TREES greater than expected and

UNDΕRSΤΟRY less than expected based on availability (Table 16).

Solitary yireo used BLACK OAK greater than expected and UNDERST0RY less than expected. Orange-crowned warbler used BLUE 0AK less than expected based on availability. Νorthern orioles used WΗITΕ OAK greater than expected and black-headed grosbeak used UNDERSTORY less

than expected based on availability.

Location in the Canopy of Plants Used Bird Species.

Location in the canopy was described by the proximal or distal portion Figure 11. Percent Use of Plant Species Groups as Foraging Sites by Bird Species in the Foliage-foraging Guild at Hopland, California. Sample Sizes Are Indicated at the Top of Each Bar. 66

Table 16. Summary of Significant Differences for Use Versus Availability of Plant Species by Bird Species in the Foliage-foraging Guild. Significant Differences Were Based on the Availability of Plant Species that Did Not Occur within 95% Confident Intervals for Proportions of Use by Birds. 67 of a canopy relative to the trunk or center of a plant. A 2 X 10 contigency table described the frequency of use of proximal and distal locations in the canopy by each bird species. Location in the canopy

was considered the response variable. The distal portion of the

canopy was used significantly more frequently (p < 0.01, G = 24.8, df

= 9) compared to the proximal portion of the canopy by all members of

the foliage-foraging guild.

Goodness-of-fit for Use Versus Availability of Location in the

Canopy of Plants. A goodness-of-fit test was used to test for

differences between observed and expected use of location in the

canopy of a plant. Availability of the two locations in the canopy

(proximal and distal to the trunk) were set equal to each other. All

bird species used the two locations in the canopy in significantly

different (p < 0.05, G < 6.1, df = 1) proportions than expected based

on availability.

Bonferoni 95 percent confidence intervals were calculated for

the proportion of use for two locations in the canopy (proximal and

distal to the trunk) by each bird species (Appendix C). The

availability of each location was compared to values encompassed by

the 95 percent confidence interyals based on use. All bird species

used the portion of the canopy proximal to the trunk significantly

greater than expected based on availability.

Discrimination Between Foliage-foraging Species

Discrimination by Searching Behavior. Five yariables were

derived to characterize the search movements of bird species (Table 68

4). The values for each variable used in this analysis are listed in

Appendix D. Box's M test (Klecka 1975) indicated that group

variance-covariance matrices were significantly different (p < 0.001).

Therefore, the pooled variance-covariance matrix was used to compute

probabilities of group membership. Ash-throated flycatcher was

excluded from this analysis because of sample sizes (n = 9) for some

variables.

Two discriminant functions had significant (p < 0.05)

chi-square values and explained over 87 percent of the variation in

search movements used by foliage-foraging bird species (Table 17).

Discriminant function I (DF-I; 47.3 percent) was described by the

number of search flights per minute. DF-II (40.3 percent) was

described by the number of hops per minute. Since each function was

dominated by a single variable, these variables were further analyzed

with univariate statistics (ANOVA, GT2-method, and goodness-of-fit

tests).

Separate ANOVA tests were perfomed for hops and flights and

both tests detected significant differences between species (Table

18). The comparison of the frequency of search flights between

species showed several significant differences (Table 19). Species

were grouped into three flight classes: frequent fliers (bushtit and

Hutton's vireo); intermediate fliers (plain titmouse, solitary vireo,

blue-gray gnatcatcher); and infrequent fliers (ash-throated flycatcher, northern oriole, orange-crowned warbler, black-headed grosbeak, and western tanager).

The comparison of the frequency of hops between species also showed several significant differences (Table 20). Species were 69

Table 17. Standardized Discriminant Coefficients Are Listed for Those Functions with Significant (p < 0.05) Chi-square Values for Search Movements of Bird Species in the Foliage-foraging Guild. The Eigenvalue, Percent of the Variance Explained by Each Function, and the Total Variation Explained by the Two Functions Are Giyen. 70

Table 18. Analysis of Variance of Mean Frequency of Search Flights and Hops for Bird Species in the Foliage-foraging Guild. Table 19. Multiple Comparisons of the Frequency of Search Flights Used by Bird Species in the Foliage-foraging Guild. Mean Ν umber of Search Flights per Minute (x fly) and Sample Size (n) for Each Species Are Shown. Differences Between Means (Below Diagonal) and Minimum Significant (p < 0.05) Differences (Above Diagonal) Are Indicated for All Pairwise Combinations of Species. Significant Differences Are Indicated by an Asterisk. Table 20. Multiple Comparisons of the Frequency of Hops Used by Bird Species in the Foliage-foraging Guild. The Mean Number of Hops per Minute (x hop) and Sample Size (n) for Each Species Are Shown. Differences Between Means (Below Diagonal) and Minimum Significant Differences (Above Diagonal) Are Indicated for All Pairwise Combinations of Species. Significant (p < 0.05) Differences Are Indlcated by an Asterisk. 73 grouped into three hop classes: frequent hoppers (blue-gray gnatcatcher, orange-crowned warbler, plain titmouse, and bushtit); intermediate hoppers (solitary vireo, black-headed grosbeak, and northern oriole); and infrequent hoppers (ash-throated flycatcher, western tanager, and hutton's vireo).

Discrimination Foraging Site Characteristics. Five variables were used to characterize foraging sites used by bird species (Table 5). Foraging height was highly correlated (r = 0.642, p < 0.001) with plant height, and therefore it was not included in the analysis. The values for each variable used in this analysis are given in Appendix E. The three variables represented by percentages were arcsine transformed. The other two variables approached normality. Box's M test (Klecka 1975) indicated that group variance-covariance matrices were significantly different (p < 0.001).

Therefore, the pooled variance-covariance matrix was used to compute probabilities of group membership.

Two discriminant functions had significant (p < 0.05) chi-square values, and they explained 76.4 percent of the variation in foraging site characteristics among bird species (Table 21).

Discriminant function I (DF-I; 52.7 percent) was described primarily by the DBH of the tree. DF-II (23.7 percent) was described primarily by plant height. Since each function was dominated by a single variable, these variables were further analyzed with univariate analyses.

Bird species used significantly (p < 0.001, F = 9.01, df = 9) different diameter trees (Table 22). Multiple comparisons between 74

Table 2la. Standardized Discrminant Coefficients Are Listed for Those Functions with Significant (p < 0.05) Chi-square Values for the Foraging Site Characteristics of Bird Species in the Foliage-foraging Guild. The Eigenvalue, Percent of the Variance Explained by Each Function, and the Total Variation Explained by the Two Functions Are Given. DBH = Diameter at Breast Height. 75

Table 22. Analysis of Variance of Two Foraging Site Characteristics, Mean Diameter at Breast Height (DBH) of Trees and Mean Foraging Height, of Bird Species in the Foliage-foraging Guild. 76

species showed that northern oriole used significantly larger trees

than all members of the foliage-foraging guild (Table 23). Hutton's

vireo use the smallest diameter trees and used significantly smaller

diameter trees than black-headed grosbeak, as well as northern oriole.

All other species did not differ significantly in the mean diameter of

trees used for foraging.

Foraging height of birds was substituted for plant height in

the following analyses because foraging height is a more direct

measure of resource use. Foraging height is generally considered an

important mechanism for partitioning resources in avian communities

(MacArthur 1972). Mean foraging height of each species was

significantly different (p < 0.001, F = 9.48, df = 9, Table 22).

Multiple comparisons of mean foraging heights used by each species

showed several differences (Table 24). Hutton's vireo had the lowest

foraging heights and foraged significantly lower than all other

species except bushtit. Northern oriole, western tanager, and

black-headed grosbeak had the highest foraging height and foraged

significantly higher than species (Hutton's vireo, bushtit, and plain

titmouse) that foraged at the lowest heights.

Foraging heights were stratified into intervals to further

investigate how foraging heights were partitioned between species.

Six height intervals were established: 0-2 m, >2-5 m, >5-10 m, >10-15

m, >15-20 m, >20-30 m. Frequency of use of each height interval was

compared by a contingency test. Frequency of use of height intervals

was highly significantly (p < 0.001, G = 140.6, df = 45) different

between species. Avian foraging height is often correlated with the

height distribution of foliage (Baldy 1969). Table 23. Multiple Comparisons of the Mean Diameter (cm) at Breast Height ,(DBH) of Trees Used by Bird Species in the Foliage-foraging Guild. Mean DBH (X DBH) and Sample Size (n) for Each Species Are Shown. Differences Between Means (Below Diagonal.) and Minimum Significant Differences (Above Diagonal) Are Indicated for All Pairwise Combinations of Species. Significant (p < 0.05) Differences Are Indicated by an Asterisk. Table 24. Multiple Comparisons of Mean Foraging Height (m) of Bird Species in the Foliage-foraging Guild. Mean Foraging Height (x ht) and Sample Size (n) for Each Species Are Shown. Differences Between Means (Below Diagonal) and Minimum Significant Differences (Above Diagonal) Are Indicated for All Pairwise Combinations of Species. Significant (p C 0.05) Differences Are Indicated by an Asterisk. 79

comparison of use versus avallability showed several significant differences (Appendix F). Each species used height intervals significantly (p < 0.005, G > 17.9, df = 5) different than expected based on the availability of foliage volume (Appendix F).

Bonferoni 95 percent confidence interyals showed that discrepencies between use and availability varied between species. All species, except plain titmouse, use the first three height intervals (0-la0 m) equal to or less than expected. Plain titmouse used the first interyal (0-2 m) greater than expected. This is probably because plain titmouse foraged on grass seeds and the ground more often than any other species. Plain titmouse was often observed exploiting heads of grasses and the ground from low perches.

The fourth and fifth height intervals (> 10-20 m) were used equal to or greater than expected by all species (Appendix F). And, the highest interval (> 20-30 m) was used equal to its availability by all species except northern oriole, which used this interval less than expected. Northern oriole was most abundant on lower elevation study plots where the trees rarely exceeded 20 m in height. Therefore, foliage in the upper most height interval may not have been as abundant in areas occupied by northern oriole as the foliage volume proportions indicated.

Differences in Prey Use Foliage-foraging Species

Bird species in the foliage-foraging guild were graphically compared according to the percent of attacks directed toward four prey type categories (Fig. 12). Sample sizes were small, therefore the data only indicate trends. Observed prey use was biased by the Figure 12. Percent Use of Prey Types Observed Being Captured by Βird Species in the Foliage-foraging Guild at Hopland, California. Sample Sizes Are Indicated at the Top of Each Bar. 81 visibility of prey captures. However, a relative comparison between bird species is legitimate, assuming that visibility of prey capture between bird species is constant.

Seven of the ten bird species were observed foraging on insects of some type 100 percent of the time (Fig. 12). Howeyer, a gradient from the exclusive capture of larvae (sedentary prey) to the exclusive capture of other insects (including aerial prey) was observed. The three remaining species foraged to some extent on other

prey types. Blue-gray gnatcatcher and black-headed grosbeak foraged

on grass seeds occasionally (11 and 18 percent, respectively). Plain

titmouse also foraged on grass seeds (10.5 percent), as well as being

the only species in the foliage-foraging guild to utilize acorns (7.4

percent) as a food source. Plain titmouse was observed foraging on acorns stored in trees (granaries) by acorn woodpeckers, as well as acorns which had fallen to the ground. DISCUSSION

Bird Community Patterns

Bird species richness observed in my study area was similar to other oak woodland habitats in California (Verner 1980). At least 49 species of birds bred at the Hopland Field Station in the years 1986

to 1987. Comparing these results with those based on Breeding Bird

Censuses published in American Birds, my study area is in the upper 10

percent of habitat types in the continental United States in terms of

species richness.

Density estimates for birds on my study plots are less

reliable than the majority of those published in American Birds

because of my relatively small plot size, and I was not able to use a

territory mapping procedure. Verner (1985) has shown that small plot

sizes can lead to large positive biases in the estimation of breeding

bird density. However, if I assume that my census methodology

provided perfect detection out to 50 m on either side of the transect

line and that all detections represented territorial birds, I can

compare my density estimates with other habitat types in the

continental United States. The comparison shows that the breeding

density in my study area was in the upper 25 percent of habitat types.

The high species richness and breeding density of the bird

community in my study area is characteristic of most oak woodland

habitats in California (Verner 1980, Block and Morrison 1987) and

indicates the relative importance of California oak woodlands to the

82 83

United States avifauna. The high number of insectivorous bird species breeding in oak woodland habitats suggests that resources are sufficiently abundant to meet these species' needs for food and nest sites. The abundance of insectivorous bird species makes this habitat a good forum for studying resource partitioning.

Cavity nesters (12 species) comprised a substantial proportion

(25 percent) of the breeding bird community and the total breeding density (58 percent) on the study plots. The number of cavity-nesting species is comparable to conifer dominated habitats across the western

United States (Scott et al. 1980, Raphael la981). However, my estimates of the relative density of cavity-nesters is considerably higher than most habitats (Scott et al. 1980). The dominance of cavity-nesters on my study plots was largely attributable to the abundance of violet-green swallows.

Vegetative Community

My study plots were not randomly located throughout the available habitats at the Hopland Field Station. Rather, they were placed systematically to represent a smooth, but steep, gradient in oak tree density. As a consequence, I cannot make inferences to relative proportions nor general characteristics of all the vegetative communities at the field station. However, I believe that the study plots sampled a range of vegetative conditions and were representative of much of the variation at the Station.

Woodlands at the Hopland Field Station were heterogeneous both in species composition and the density of the vegetation. There were no descrete boundaries separating woodland vegetative communities. 84

Study plots reflected yariation in floristics as well as density of

plants.

Variation in plant species composition was influenced by the

800 in elevation gradient that existed among study plots. Colder,

wetter, harsher climates typically accompany an increase in eleyation.

Vegetation structure and floristics are influenced by climate, even at

a local scale (Whittaker and Woodell 1972). The plant species

composition of study plots at higher elevations consisted of species

whose distributional centers lie to the north of Hopland.

Specifically, Oregon white oak, California black oak, Pacific madrone,

and California bay were most abundant on mid tc high elevation plots.

These species were common in coniferous forests north of Hopland,

unlike blue oak and buckeye, species more common on lower eleyation

plots, which are restricted to the more southern (Hopland and south)

portion of the coast range. The structure and species composition of

vegetation may influence bird communities by differing in the quantity

and quality of nest, roost, and foraging sites, and prey (Holmes and

Robinson la981, Robinson and Holmes 1982, Martin 1988).

Guild Delineation

Several researchers have partitioned groups of species by

cluster analysis techniques and used the guild concept to describe how

species partition resources (Landres and MacMahon 1980). Still others

haye selected bird species with similar life history strategies (based

on qualitatiye information), labeled the group a guild, and described

how resources are partitioned within a guild (Robinson and Holmes

1982, Airola and Barrett 1985, Morrison et al. 1985). The subjective 85 selection of species for guild membership can render conclusions about how resources are partitioned invalid if the species composition of the guild is incomplete or incorrect. Few researchers identify guilds using quantitatiye data, and then study one or more guilds in detail.

Based on the cluster analysis of substrate use, the insectivorous and/or granivorous members of the bird community I studied segregated into three groups. Ordination of these species on principal component axes showed similar groupings as the cluster analysis, but the division between groups was less distinct. This

suggests that resource exploitation is accomplished by a range of strategies that form a continuum along one or more environmental axes.

For example, acorn woodpecker, a well studied species, is often classified as a bark forager. Howeyer, acorn woodpecker also performs air sallies, and in some areas it sallies more often than it forages on bark (Landres and MacMahon 1983). In the community I studied, both acorn woodpecker and many members of the foliage-foraging guild air sallied to some extent. As a result, the ordination (based on substrate use) of acorn woodpecker oyerlapped with some members of the foliage-foraging guild on the air to bark axis. A similar lack of distinction occurred between the foliage-foraging and ground-foraging guilds. The potential for interspecific competition existed to some extent between members of different guilds, as well as within guilds. 86

Resource Partitioning Among Foliage-foraging Species

General Patterns

The foliage-foraging guild I defined included 10 species which foraged at least 50 percent of the time on foliage. Species in this group varied in the type and frequency of use of other substrates.

Those species that used substrates other than leaves used prodominantly one additional substrate, either a variety of bark surfaces or the air.

The partitioning of resources between community members theoretically has two components: a coevolutionary component with a genetic basis; and an adaptive component that is plastic and able to respond quickly to changes in resources availability (Schoener 1982).

The coevolutionary effect on resource partitioning would be effective primarily on morphological characteristics (affecting the ability to effectively exploit various resources) and macrohabitat associations

(affecting sympatry and frequency of interaction) of species. The product of coevolution is a set of morphological and behavioral characteristics which enable species to use an exclusive sub-set of resources, minimizing overlap in resource use between species during times of lean food resources (Baker and Baker 1973, Boag and Grant

1981, Schoener la981).

Actual resource use is the combination of the availability of resource types and the morphological characteristics of the animal.

Morphological characteristics are genetically fixed and dictate the energetic cost of resource exploitation, defining a "general resource niche" (Alatalo 1980). Actual resource use is generally a subset of 87 the "general resource niche", limited by resource availability.

The behavioral adaptability of species enables them to exploit seasonally abundant resources, as well as cope with negative impacts on resource availability. For example, during times of resource abundance, similar species overlap more in resource use, compared to lean times, because of the ability to exploit an abundant resource by changing the kinds of food taken and where searching occurs (Schoener la982). This adaptive ability enables a greater overlap or

"convergence" of resource use between species. Foraging theory predicts that this convergence would be most likely to occur if: (l) resources were patchy, the benefits of a localized abundance of food could offset other energetic costs resulting from less than efficient foraging methods; and (2) the resource was very energetically profitable and, therefore, profitable for many species simultaneously.

I am not able to determine whether the patterns of resource use between species in the foliage-foraging guild were the result of

coevolutionary processes and/or behavioral adaptability. However, many researchers believe that basic habitat associations and some resource use patterns, such as movement characteristics, substrate, and maneuver use are significantly affected by the morphological

(i.e., genetically fixed) characteristics of the bird (Alatalo 1980,

Robinson and Holmes 1982). The limits of the "general resource niche" are defined by these and other resource use characteristics observed over the range of the species. The other resource use characteristics, such as plant species use, area of the plant occupied, and to some extent prey type, may be adaptive because they change between areas or over relatively short periods of time in 88 response to resource abundance. Resource availability can be limited by a variety of factors, including interspecific competition and predation. Actual resource use in a given area is a product of all these factors and may reflect a reduction in the overlap of resource use between species to minimize interspecific competition.

Habitat selection studies show that the habitat associations of bird species are correlated with features of the vegetation structure (James la971, Anderson and Shugart 1974). Habitat is most often the resource that differs between the niches of species which forage on similar resources (Schoener 1974). Bird species composition

does not typically show sharp zones of demarcation, but varies on a

continuum with vegetation structure and physical features of the

habitat (Bond 1957, Beals 1960). In concordance, most species in the foliage-foraging guild had widespread occurrence across study plots, averaging presence on greater than 90 percent of the plots (range =

74-100 percent). This indicates that variations in vegetation structure and floristics between study plots did not exceed the

"general resource niche" of most species and that competitive

exclusion was not a major factor in resource partitioning within the

foliage-foraging guild.

The species that covaried in abundance across plots had similar habitat associations. Resource depletion may have occurred on

those plots where they were most abundant, thereby invoking the

potential for interspecific competition. The impact of any one

species on resource availability was probably low. However, the

cumulative impact of a number of relatively abundant species on

resource availability may be significant. 89

Seven of 10 species in the foliage-foraging guild formed two groups which showed covariations in the abundance of their individual members across study plots. The abundance of the groups varied as a function of tree species composition; specifically one group was associated with deciduous trees and the other with evergreen trees.

Tree species preferences by bird species are common in a variety of habitats (Hartley 1953, Balda 1969, Franzreb 1978, Holmes and Robinson

1981, and Morrison et al. 1985). The three remaining guild members had unique variation in abundance across study plots. The density of shrubs was an important habitat component to two of these three species. Resource availability associated with evergreen versus deciduous trees is discussed below.

Comparisons of Species

The results of analyses pertaining to resource partitioning among species in the foliage-foraging guild are summarized in Table

25. Resources are listed in order of importance in reducing overlap in resource use. Habitat association, plant species use, and tree diameter use are described first because they can separate species horizontally. Foraging height is described next because it can separate species vertically. The remaining resources and behaviors can serve to partition resources where species overlap spatially.

Species which covary in abundance have an increased

probability of interaction in habitats where they are most abundant.

These species must differ in other aspects of their behavior if interspecific competition is to be avoided during periods of resource shortage. 0ne group of four species (western tanager, blue-gray Table 25. Summary of Results Describing Resource Partitioning by Bird Species in the Foliage-foraging Guild. Values for Each Resource Are Underlined when Adjacent Species Have the Same Value. Table 25. Summary of Results Describing Resource Partitioning by Bird Species in the Foliage-foraging Guild. Values for each variable are underlined when adjacent species have the same value. (continued) 92 gnatcatcher, solitary vireo, and black-headed grosbeak) was associated with abundant white oaks, which occurred primarily on higher elevation plots. Higher elevation plots also had taller trees and unique tree species composition, including species (Oregon white oak, Pacific madrone, and California bay) common in montane forests to the north of my study area.

These white oak associated species were summer residents.

They are also typically not associated with oak woodlands (with the exception of blue-gray gnatcatcher) and do not breed in oak woodlands south or east of my study area (Verner 1980, Wagner 1981, Landres and

MacMahon 1983, Block and Morrison 1987). They are typically associated with montane forests and riparian woodlands (Miller 1951).

The proximity of my study area to montane forests to the north may be responsible for their presence on study plots. All of these species had low abundances, suggesting that oak woodlands are suboptimal habitats for these species. These results support the hypothesis of

Holmes and Robinson (1981) and the findings of Sabo and Holmes (1983) that summer residents breeding in suboptimal habitats key into areas that most resemble the "niche gestalt" of their optimal breeding habitat.

All four species in the group associated with abundant white oaks differed on one or more resource "axes" (Table 25). Black-headed grosbeak differed from the other three species in its almost exclusive use of stationary maneuvers, and a less than expected frequency of use of understory vegetation. Western tanager differed from the other three species in its frequent use of air sallying. Substrate use, maneuver use, and foraging height of these two species were similar to 93 observations in a mixed-conifer forest in California, even though the plant species composition in the two communities was very different

(Airola and Barrett 1985).

Solitary vireo and blue-gray gnatcatcher were similar in many apsects of their resource use. They foraged at similar heights, indicating that they did not partition resources vertically. Their use of maneuvers, flight frequency, and frequency of use of plant groups were also very similar. Interspecific competition can be reduced by staggering the breeding cycles of species, especially summer migrants (Schoener 1982). However, these species arrived at the study area and started exhibiting breeding behavior within days of each other.

Subtle differences in foraging behavior did exist between blue-gray gnatcatcher and solitary vireo (Table 25). Blue-gray gnatcatcher sallied more often, moved through the vegetation faster, captured a lower proportion of larvae, foraged less frequently on bark and occasionally on grass seeds, and used plant groups in proportion to their availability. Subtle differences in maneuvers and movement characteristics may have resulted from morphological differences.

Blue-gray gnatcatcher has a smaller body size (11.5-la2.5 cm long) than solitary vireo (la2.5-15.0 cm long) (Farrand 1983). Body size influences the energetic cost of various maneuvers, substrate uses, and movement strategies (Holmes et al. 1979). The primary factor limiting overlap in resource use between these two species was the low abundance and patchy distribution of blue-gray gnatcatcher. The low frequency of co-occurrence (65 percent) on study plots and the low overall abundance of blue-gray gnatcatcher may have reduced the 94 frequency of interactions between these two species such that resource depletion by blue-gray gnatcatcher was insignificant.

Solitary vireo is a widespread species in the western and eastern United States (Farrand 1983). The foraging behavior of solitary vireo varies between areas (Robinson and Holmes 1982, Airola and Barrett 1985). The types of substrates and maneuvers used do not differ, however the proportional use of each substrate and maneuver does vary between areas. Solitary vireo feeds primarily on foliage and bark surfaces by using glean and hover maneuvers. The solitary vireo's use of leaves was less common and use of bark more common in other habitats compared to my observations (Robinson and Holmes 1982,

Airola and Barrett la985). The frequency of use of hops and flights by this species in my study area differed from that in a northern hardwood forest in New Hampshire (Robinson and Holmes 1982), suggesting that movement characteristics are influenced by habitat structure.

The proportional use of evergreen and deciduous trees by blue-gray gnatcatcher was very similar to use observed by Wagner

(1981). Substrate and maneuver use reported by Landres and MacMahon

(1983) was similar in proportion to my observations with one exception. The birds I observed air sallied to a limited (10 percent) extent, whereas Landres and MacMahon (1983) did not observe any air sallying.

Ash-throated flycatcher had unique habitat associations. The reported substrate use by ash-throated flycatcher varied between study areas and habitat types (Landres and MacMahon 1980, 1983). Some combination of air, foliage, and ground was used in all studies. 95

Ash-throated flycatcher used substrates and maneuvers in proportions similar to western tanager, as well as having similar movement characteristics (Table 25). In addition, they used plant groups in proportion to their availability and foraged in similar diameter trees. Foraging height differed between the two species, which serves to separate them yertically. Differences in resource use between the two species were primarily limited slower overall movement and more frequent gleaning and sallying by western tanager compared to ash-throated flycatcher. These behaviors may function to shorten the search radius for prey and affect the types of prey available to western tanager. These two species co-occurred on 70 percent of the plots, and the relatively low abundance of western tanager makes it unlikely that resource use by western tanager was a significant impact on resource availability for ash-throated flycatcher.

Ash-throated flycatcher and western tanager used dynamic maneuvers a greater proportion of the time and had the lowest number of perch changes, as compared to other guild members. The consistant use of dynamic maneuvers by flycatchers is well documented (Cody

1974). Both the ash-throated flycatcher and western tanager are

"sit-and-wait predators" (sensu Pianka 1971) because they move infrequently and wait for suitable prey to enter their field of view.

They typically pursue active (e.g., aerial) prey with dynamic maneuvers. The other eight species in the foliage-foraging guild are

"gleaners" (sensu Eckhardt 1979) which are more active searchers, moving quickly through the foliage, and pursuing stationary prey.

Dynamic foraging maneuvers are energetically more expensive

than stationary maneuvers (Holmes et al. 1979), however these 96

energetic costs maybe at least partially offset by infrequent perch

changes. The behavior of ash-throated flycatcher was a good example

of this strategy. The opposite of this pattern also holds true, as

demonstrated by the foraging behavior of bushtit. Bushtit had the

greatest frequency of perch changes, but used stationary maneuvers

(i.e., gleaning) almost exclusively. The other guild members had

intermediate and varied combinations of maneuver and movement

characteristics. My observations lend support to the hypothesis that

there is an upper limit for energy expediture for passerine birds

beyond which the bird must find a balance by reduced reproductive

expenses or metabolic responses (Holmes et al. 1979).

Northern oriole had a unique habitat association; it was most

abundant on plots with sparse, large white oaks. Black-headed

grosbeak was also associated with white oaks, but was most abundant on

plots with greater tree density. Habitat association was one of the

few factors separating the foraging niches of these two species (Table

25). The similarities in resource use included substrate use, use of

stationary maneuvers, movement characteristics, and foraging height.

Subtle differences in their use of plant species, tree sizes, and prey

types may have served to separate their resource use in areas where

they both had moderate abundances.

The unique association of northern oriole with large, sparse

white oaks may be attributed to nest site requirements. Νorthern

oriole built pendulous nests, and large white oaks have a unique

growth form of overhanging branches which offer an abundance of

suitable sites for pendulous nests. Compared to observations in an

oak woodland south of my study area (Landres and MacMahon 1983), 97 substrate and maneuver use by northern oriole were both similar in type, but substrate use varied in frequency. Northern orioles in my study area used leaves a greater proportion of the time and bark lower proportion of the time compared to use reported by Landres and

MacMahon (1983).

Plain titmouse, bushtit, and orange-crowned warbler were more abundant on study plots with abundant evergreen trees (Table 25).

These species foraged a large proportion of the time in blue oak and used stationary maneuvers to exploit their prey. Orange-crowned warbler differed from the other two species in a number of ways: the frequency of use of plant groups relative to their availability, frequent use of leaves and catkins as substrates, exclusive capture of larvae as a prey type. and frequent perch changes but slow movement through the vegetation. In addition, orange-crowned warbler foraged on shrubs more than any other species in the guild. Wagner (1981) observed orange-crowned warbler foraging on shrubs in similar proportions (10-15 percent).

Plain titmouse and bushtit were both year-round residents, were more abundant than all other guild members, and had similar habitat associations and foraging behaviors (Table 25). They used different types of nests (cavity versus pendulus), potentially causing predation pressures to differ between species. Nevertheless, it appeared that these two species had a high potential for frequent interactions and interspecific competition.

Similarities between members of the family Paridae are well documented (Hertz et al. 1976, Landres and MacMahon 1980, Wagner 1981,

Robinson and Holmes 1982). Morphological similarities between these 98 species should lead to overlap in resource use. Plain titmouse and bushtit overlapped in the use of most resources and behaviors, but they differed in the frequency of use of some resources. Plain titmouse used the greatest number of substrates, and foraged on bark surfaces and grass more frequently than any other species. As a result, plain titmouse had the greatest overlap in substrate use with the other guilds in the bird community. Plain titmouse was three to four times as abundant as all other guild members. The superabundance of plain titmouse may be partially explained by its being a year-round resident, and that it is one of the few species in the foliage-foraging guild that reaches peak abundances in blue oak woodlands.

The types of substrates and maneuvers used by plain titmouse were similar to observations by other researchers, including the use of grasses and acorns as food sources (Hertz et al. 1983, Landres and

MacMahon 1983). The proportion of use of leaves was higher and the use of bark lower on my study area compared to Hertz et al. (1983). The proportion of use of evergreen and deciduous trees was also similar to observations by Wagner (1981).

Landres and MacMahon (1980, 1983) and Hertz et al. (1983) observed bushtit foraging on foliage a greater proportion of the time and bark surfaces a lower proportion compared to my observations.

However, both studies observed that foliage was the dominant substrate and bark surfaces the secondary substrate, as I did. The frequent use of evergreen trees was also observed by Wagner (1981). The other two year-round residents (plain titmouse and Hutton's vireo) in my study area used shrubs and/or occupied the first few meters of vegetation 99

more frequently than bushtit. This suggests that food resources were

partitioned vertically by the year-round residents, which was not

observed between any of the other members of the guild. Hertz et al.

(1983) also observed a low frequency of use of shrubs by bushtit.

Wagner (1981) reported more frequent use of shrubs compared to my

observations.

Hutton's vireo was unique in its association with dense shrubs

and blue oaks. However, it was also associated with eyergreen trees,

as were plain titmouse and bushtit (Table 25). Plant species use by

Hutton's vireo was very similar to plain titmouse and bushtit. All

three species (plain titmouse, bushtit. and Hutton's vireo) foraged on

blue oak and evergreen trees over 70 percent of the time. Therefore.

the potential for overlap between the three resident species on a

horizontal scale was high. Wagner (1981) observed Hutton's vireo

using a similar proportion of use of deciduous and evergreen trees and

the understory in a blue oak woodland located in the coast range

approximately 200 miles south of my study area. The association with

shruby habitats was also observed by Airola and Barrrett (1985) in

mixed conifer forest in California.

Behavioral differences existed between Hutton's vireo and the

other two year-round residents (Table 25). Hutton's vireo used a

greater proportion of dynamic maneuvers than both plain titmouse and

bushtit, and moved most quickly through the vegetation of all the

members of the foliage-foraging guild. 100

Resource Use Year-round Residents

The frequent use of evergreen trees by year-round residents supports the observations of Karr (1975,1976) that resident species

tend to use permanent, less seasonal resources more frequently than summer residents. Evergreen and deciduous trees differ in structure and associated insect populations (Jackson 1979). Evergreen trees

provide a more constant structural environment for both birds and

insects by retaining their leaves year round. However, the tough

cuticles which protect leaves from dessication may also limit the

number or types of insects (and birds) associated with them (Jackson

1979). Consequently, evergreen trees offer a trade-off between

predictable food resources and less abundant food resources. Perhaps

evergreen trees are a year round "staple" for resident species,

supplimented by seasonaly abundant resources, such as lepidoptera

larvae.

The structure of evergreens trees also differ from deciduous

trees in the density of foliage. Live oaks (the most abundant

evergreen tree on the study plots) had a greater density of foliage

compared to deciduous oaks. The observed similarities in foraging

behavior among year-round residents may be attributed to convergent

adaptations to foraging in evergreen yegetation, since behavior is

shown to vary with foliage structure (Robinson and Holmes 1984). The

year-round residents were smaller-bodied birds (R = 11.83 cm, s.d. =

2.57 cm), based on body length, compared to the summer residents (R =

16.46 cm, s.d. = 3.48 cm) (Farrand 1983). These results support the

contention that as foliage density increases, the size cf the

associated birds decreases (Pearson 1975). 101

Alternatively, these smaller-bodied birds may be competitively

inferior to the larger-bodied birds, and therefore may be forced to

exploit poorer foraging sites (Alatalo and Moreno 1987), given that

evergreen trees have a potentially lower abundance of insects relative

to deciduous trees (Jackson 1979). In communities where body size

influences the dominance heirarchy, the smaller-bodied birds were

relagated to foraging in the portion of canopies distal to the trunk

(Alalto and Moreno 1987), where predation rates are potentially higher

(Ekman 1986 in Alatalo and Moreno 1987). All of the species I

observed used the proximal portion of the canopy greater than

expected. If a dominance heirarchy existed in the community I

studied, it did not influence the portion of the canopy occupied by

birds.

Year-round residents are typically more diverse

morphologically and behaviorally compared to summer residents (Morse

1971, Herrera 1978). During periods of high resource abundance,

increased similarity in foraging behavior and prey type use has been

observed (Herrera 1978). The timing of the reproductive efforts of

insectivorous birds have been shown to coincide with the abundance of

lepidoptera larvae, a common defoliator in oak woodlands (Jackson

1979). Varlet' (1970) found the nesting cycle of titmice coincided

closely with the timing of winter moth larvae (a lepidopteran).

Similarly, plain titmouse, bushtit, and Hutton's vireo began nesting

activities early (March 20-25) in the breeding season relative to the

summer residents, feeding nestlings while lepidoptera larvae were

abundant (Tieje and Foott 1987). These data suggest that the observed

behavioral similarities between the resident species may be a product 102 of resource abundance and similar enough morphologies to exploit the same resources. In addition, breeding earlier than summer residents lowers the frequency of interactions and competition for resources

(Morse 1971).

The year-round residents had the greatest overall abundance of all the foliage-foraging species. During the non-breeding season, habitat associations may shift because of the expansion or dissolution of territories and changes in the availability of various food resources. Changing resources may, in turn, affect the maneuvers used to acquire food items. However, the morphology of the birds will not change. Many researchers have shown that food type and size were important factors segregating food resources between bird species, especially during times of resource scarcity (Gibb 1954, Newton 1967,

Baker and Baker 1973). Bill measurements are considered a reliable means of sorting foliage-foragers by the type and size of food they capture (Cody 1974, Eckhardt 1979). Wagner (la981) compared bill depth and bill length for nine bird species, including plain titmouse,

bushtit and Hutton's Vireo (Fig. 13). The specimens were taken from an area approximately la00 miles from my study area.

The bills of all three species were longer than they were deep

(Fig. 13), characteristic of insect gleaning birds (Welty 1962).

Values for bill depth and bill length were non-overlapping (to one standard deviation away from the mean) for all pairwise combinations of species. Bill size was positively related to body size. Bushtit

had the smallest bill in length and depth. Hutton's vireo was

intermediate and plain titmouse had the largest bill. These

morphological differences may serve to reduce the overlap in resource 103

Figure 13. Bill Depth Versus Bill Length for the Three Resident Bird Species in the Foliage-foraging Guild. The ●'s Locate Mean Bill Depth and Length for Each Species, Lines Represent Ranges, and Crosshatches on the Range Lines Represent One Standard Deviation on Either Side of the Mean. Data Are from Wagner (1981). 104 use between species during the breeding season, as well as the non-breeding season. Morphological differences may be a manifestation of food limitations that occurred in the past, because food limitations cause species to specialize to reduce the amount of overlap in food resource use (Morse 1971). CONCLUSIOΝS

Resource use by bird species in the foliage-foraging guild overlapped extensively in my study area as evidenced by the frequency of occurrence across study plots and extensive use of existing resources. Species differed in their relative frequency of use of the existing resources, as found by other researchers (e.g., Alatalo

1980). Species apparently separate themselves on one or more resource axes to reduce the potential for interspecific competition.

Comparisons of my results with research in other areas and habitats showed a consistent pattern of use of substrates and maneuvers for a number of bird species. This supports the contention that these axes of resource exploitation are more limited by the morphological characteristics of the birds compared to the other resource axes I measured. The other axes of resource exploitation were more variable between areas and habitats. Many of the species I observed foraged on foliage more often and bark less often than reported in other areas. The northern location of my study area may be a more productive environment for vegetation and folivorous insects compared to oak habitats in drier climates. All species differed significantly on one or more resource axes.

Habitat associations and plant species use differed most

between species in the foliage-foraging guild, and served to reduce

the frequency of interactions by separating species on a horizontal scale. 0verlap in foraging height was high between most species.

105 106

Vertical partitioning of food resources was only observed to a limited extent between year-round residents. Thus, foraging height did not appear to be an important aspect of resource partitioning within the foliage-foraging guild. Landres and MacMahon (1983) found a similar lack of partitioning on a vertical scale. The high amount of overlap in resource use among species suggests that the breeding season is a time of food resource abundance in my study area. Abundant resources allow for overlap in resource use without depleting resources and invoking interspecific competition. Resource use was most similar among resident species as one would expect, given that they are adapted to the same habitat type and their adaptive behaviors and morphologies were shaped by the same environmental pressures. REFERENCES CITED

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Zar, J. H. 1984. Biostatistical analysis. Prentice-Hall, Inc. Englewood Cliffs, New Jersey. 718 pp. Appendix A. Proportion of Use of Substrates Upon Which Prey Were Attacked by 20 Selected Bird Species at Hopland, California. Values Represent a Mean Proportion of Use for a Randomly Sampled Substrate for Each Observation. N = Sample Size. Appendix B. Summary of Plant Group Use by Bird Species in the Foliage-foraging Guild at Hopland, California. Species Are Listed in Taxonomic Order. Appendix B. Summary of Plant Group Use by Foliage-foraging Bird Species at Hopland, California. Species Are Listed in Taxonomic Order. (continued) Appendix B. Summary of Plant Group Use by Foliage-foraging Bird Species at Hopland, California. Species Are Listed in Taxonomic Order. (continued) Appendix C. Summary of Use of Plant Canopies Proximal and Distal to the Trunk by Bird Species in the Foliage-foraging Guild at Hopland, California. Species Are Listed in Taxonomic Order. Appendix C. Summary of Use of Plant Canopies Proximal and Distal to the Trunk by Bird Species in the Foliage-foraging Guild at Ηορland, California. Species Are Listed in Taxonomic Order. (continued) Appendix D. Means and Standard Deviations of Variables Describing Search Movements of Bird Species in the Foliage-foraging Guild at Hopland, California. The Number of Foraging Sites (n) and the Cummulative Number of Seconds (sec) Are Indicated. Species Are Listed in Taxonomic Order. Appendix E. Description of Characteristics of Plants and Locations within Plants Used by Bird Species in the Foliage-foraging Guild at Hopland, California. Means and Standard Deviations Are Shown for Continuous Variables and Species Are Listed in Taxonomic Order. Appendix F. Summary of Use of Height Intervals by Bird Species in the Foliage-foraging Guild at Hopland, California. Species are listed in Taxonomic Order. Appendix F. Summary of Use of Height Intervals by Bird Species in the Foliage-foraging Guild at Hopland, California. Species are listed in Taxonomic Order. (continued)