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