Food Abundance and Habitat Use of Varied Thrushes
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FOOD ABUNDANCE AND HABITAT USE OF VARIED THRUSHES NEAR REDWOOD FOREST EDGES By Gregory G. Brown A Thesis Presented to The Faculty of Humboldt State University In Partial Fulfillment Of the Requirements for the Degree Masters of Science In Natural Resources: Wildlife October, 2007 ABSTRACT Food abundance and habitat use of Varied Thrushes near redwood forest edges Gregory G. Brown Previous research suggests that Varied Thrushes (Ixoreus naevius) in redwood forests (Sequoia sempervirens) are sensitive to forest edges and are unlikely to breed in small forest fragments. Food abundance is likely to vary at the forest edge, and has been linked to habitat use and edge avoidance in some other forest-interior passerines, particularly other ground foraging insectivores. To determine the degree to which Varied Thrushes avoid forest edges, and how this might be influenced by food abundance, I monitored third-order habitat selection of individual birds using radio telemetry, and sampled relative abundance of ground-dwelling invertebrates with pitfall traps across edge gradients in redwood forests of north coastal California. In addition, I measured vegetation structure and other habitat variables that may vary near forest edges and influence Varied Thrush habitat use either directly or through effects on invertebrate distribution. I predicted that Varied Thrush habitat use would be associated with invertebrate abundance, with both decreasing in proximity to forest edges, and increasing in proximity to streams. Of the 10 Varied Thrushes monitored, only two used locations significantly farther from the edge than expected, and overall there was no clear pattern of edge avoidance or preference for streams in the birds studied. Invertebrate abundance was greater near forest edges, contrary to predictions, and was not affected by distance to streams or any of the habitat variables measured. Previous research indicating edge avoidance by Varied Thrushes did not distinguish between second-order and third-order iii habitat selection. Since neither third-order edge avoidance nor edge effects on food abundance were apparent in this study, future research should focus on other potential edge effects at larger scales. The lack of evidence for edge effects also suggests that other mechanisms, such as conspecific attraction, may account for the absence of Varied Thrushes from small redwood forest fragments. iv ACKNOWLEDGEMENTS I would like to thank my advisor, Dr. Luke George, for inspiration, advice, and a steady guiding hand throughout all phases of this project. Thanks also to the other members of my graduate committee, Dr. Rick Golightly and Dr. Matt Johnson, for their valuable input as well as for teaching engaging and useful courses which provided a good foundation for this research. Financial support and project funding was generously provided by the Humboldt Area Foundation. For additional material support, I thank Dr. Matt Johnson for the use of telemetry equipment, Dr. Mark Colwell and fellow graduate students Chris Tonra and Jesse Conklin for transmitters, and Dr. Mike Camann for the generous loan of his pitfall traps. I am also grateful to Anthony Desch in the Wildlife Stockroom for cheerfully and patiently helping with innumerable other equipment needs. Field work was possible only with the help of outstanding field assistants: Jordan Muir, Tom Pacholik, Katie Cowan, and John Short all deserve special thanks for working long hours for an occasionally difficult supervisor. I also extend thanks to Mark Andre with the City of Arcata and Tom Coyle with the City of Eureka for facilitating permits and access to the Arcata Community Forest and Sequoia Park, respectively. Finally, I would like to thank my family, especially my wife Jen and son Benjamin for all of their love, support, tolerance, and encouragement. v TABLE OF CONTENTS Page ABSTRACT.................................................................................................................. iii ACKNOWLEDGEMENTS.............................................................................................v LIST OF TABLES ........................................................................................................vii LIST OF FIGURES......................................................................................................viii LIST OF APPENDICES ................................................................................................ix INTRODUCTION...........................................................................................................1 METHODS .....................................................................................................................6 Study Area.......................................................................................................6 Varied Thrush Habitat Use...............................................................................8 Invertebrate Abundance and Habitat Structure ...............................................10 Analyses ........................................................................................................15 RESULTS.....................................................................................................................20 DISCUSSION...............................................................................................................33 LITERATURE CITED..................................................................................................38 APPENDICES ..............................................................................................................43 vi LIST OF TABLES Table Page 1 Summary of radio telemetry monitoring and estimates of minimum convex polygon and 95% fixed kernel home range size for individual Varied Thrushes. Arcata Community Forest and Sequoia Park, Humboldt County, California in 2005 and 2006. ....................................................................................................................21 2 Edge distance (m) of observed Varied Thrush locations compared to random locations in 95% fixed kernel or minimum convex polygon home ranges. Arcata Community Forest and Sequoia Park, Humboldt County, California in 2005 and 2006. ....................................................................................................................23 3 Comparison of observed number of bird locations in edge zone with expected number (based on proportion of each minimum convex polygon home range within edge zone). Arcata Community Forest and Sequoia Park, Humboldt County, California in 2005 and 2006..................................................................................24 4 Distance (m) to streams of observed Varied Thrush locations compared to random locations in 95% fixed kernel or minimum convex polygon home ranges. Arcata Community Forest and Sequoia Park, Humboldt County, California in 2005 and 2006 .....................................................................................................................25 5 Summary of invertebrates captured in pitfall traps in Varied Thrush home ranges, Arcata Community Forest and Sequoia Park, Humboldt County, California in 2005 and 2006, compared to proportion of invertebrates in stomach samples of 58 Varied Thrushes (location unknown) from October to April (Beal 1915)..........................26 6 Vegetation and habitat characteristics measured in Varied Thrush home ranges less than and greater than 100 m from forest edge. Arcata Community Forest and Sequoia Park, Humboldt County, California in 2005 and 2006. Bold P values (Wilcoxon rank sum test) indicate variables with significant differences...............31 7 Model covariates, Akaike Information Criterion (AICc), model likelihood, and AICc weight of a priori linear regression models of habitat covariates affecting invertebrate abundance in Arcata Community Forest and Sequoia Park, Humboldt County, California in 2005 and 2006 ....................................................................32 vii LIST OF FIGURES Figure Page 1 Study areas in the Arcata Community Forest (circled areas) and Sequoia Park, Humboldt County, California in 2005 and 2006......................................................7 2 Illustration of grid used for invertebrate and habitat sampling in the Arcata Community Forest, Humboldt County, California in 2005. Four parallel transects were established 50 m apart extending from the forest edge 200 m into the forest, and 16 sample points were randomly distributed among eight distance categories on the four transects so that there were four points per transect and two points per distance category ..................................................................................................11 3 Example of stratified random sample points for invertebrate and habitat sampling in Varied Thrush home ranges, Arcata Community Forest and Sequoia Park, Humboldt County, California in 2005 and 2006. A minimum convex polygon home range was generated for each bird, the home range was divided into three edge- distance zones of equal width, and two sample points randomly placed in each zone13 4 Mean number (± SE) of invertebrates (> 5 mm) captured in pitfall traps over four 2- week sample periods in the Arcata Community Forest, Humboldt County, California in 2005................................................................................................. 27 5 Number of invertebrates (> 5 mm) captured in pitfall traps vs. distance from forest edge in Arcata Community