HABITAT USE, BEHAVIOR, AND MORPHOLOGY OF HERONS AND EGRETS IN ELKHORN SLOUGH, CALIFORNIA I \ A thesis submitted to the faculty of San Francisco State University in partial fulfillment of the requirements for the degree Master of Science in Marine Science / by Pamela Elaine Byrnes ~' San Francisco, California December, 1997 Habitat Use, Behavior, and Morphology of Herons and Egrets in Elkhorn Slough, California. Pamela Elaine Byrnes San Francisco State University 1997 Great Blue Heron (Ardea herodias), Great Egret (Ardea alba), and Snowy Egret (Egretta thula) were surveyed from January to August 1996 in Elkhorn Slough and surrounding wetlands. Ninety-one wetland sites were surveyed 68 to 72 times during daylight hours throughout the tidal range. Hierarchical loglinear analyses indicated that ardeid occurrence at wetland sites was dependent upon time of day, tide level, and area. Logistic regression analyses identified three environmental cHaracteristics associated with the presence of Great Blue Herons (channels/ditches, pickleweed flats, ponded water) and three characteristics associated with the presence of Great E~rets and Snowy Egrets (pickleweed, roosting structures, water depth). Three additional characteristic were associated with Snowy Egrets (mud, mid-tides,and late daylight hours). I certify that the Abstract is a correct representation of the content of this thesis. ------- ,.-----. .1 I. \Jmni' I, Nlt!l!i/'f'i, 11 0/. /71;- (Chair, Thesis Committee) ) (Date) ACKNOWLEDGMENTS l thank my advisory committee; Dr. James T. Harvey, Dr. Shannon M. Bros, and Dr. Luis F. Baptista for their encouragement, guidance, and skill at sharing their knowledge. Thanks to the Moss Landing Marine Laboratories and San Francisco State University staff, especially Sheila Baldridge, Gail Johnston, Lynn McMasters, and Kathleen Baker. Sincere thanks to the San Francisco State University for the Graduate Equity Fellowship awards and to the University Women's Association for the scholarship that helped to finance the completion of this study. Special thanks to my mother and father whose generosity, hard work, and computer expertise were invaluable. This study could not have been done without the talents and assistance of many people who helped me along the way. Thanks to Oliver Boisseau, Chris Wright, and Christy Walker who each spent many days and months observing herons and egrets with interest and enthusiasm despite the rigors of data collection. Thanks to Mark Silb~rstein for his encouragement and assistance. Thanks to Daniela Maldini Feinholtz for suggesting this project. I also thank Tony Orr, Tomo Eguchi, Joe Caesar, and Doreen Moser. I am deeply grateful to John Oliver for hi?. energetic direction that kept me outside and moving. Thank you to the many people who assisted me on the boat during surveys. Their help and company were much appreciated. Special thanks v - to Carol Keiper, Peter Slattery, Ky Springer, Jonathan Finger, Terry Byrnes, Teresa Friend, Linda Hannigan Kanihan, Jim Curland, Peggy Byrnes, Iris Finger, Terry Darcy, Sue Shaw and her team of wetland restoration workers, and the many MLML students, interns and other people who also helped. Thank you to Terrence Byrnes and Jim Curland for recording the morphological measurements and providing such enjoyable company. I also thank John "JD" Douglas for help with data collection and for his thoughtfulness and skill in keeping the boats ready to go. Special thanks to Carol Keiper and Kip Keiper who provided me a place to live near Elkhorn Slough. Their generosity made it possible for me to be on site sunrise to sunset as required by the sampling design. I am deeply grateful for the friendship and kindness of my family and friends. Thanks to Holly Sargent-Green and Ann Woodmansee who were with me throughout, despite the geographic separation; Jim Curland whose love, humor, and company were always welcome; Ted Byrnes, Peggy Byrnes, and Terry Byrnes for their continued support and assistance. I also thank John Finger for his example of diligence and for the wonderful illustration of an ardeid leg; Jim and Linda K9nihan for their help and \ assistance and friendship; Jan Sargent for his' empathetic ear and generosity; Rob Suryan and Kim Raum for help and encouragement; and Genevieve Hoffman for inspiring me to pursue a higher education. Thanks to Juan Ramirez and Ignacio Ramirez for keeping my car running safely for the 150,000 miles of driving while in graduate school. vi TABLE OF CONTENTS List of Tables ......... viii List of Figures . xi Introduction . 1 Method . 9 Study Area . 9 Surveys . 11 Analysis . 18 Results . .. ' ....... ' . ' ... ' . ' ...... ' ....... 28 Discussion . ' ... '.'. ''.'.' .. ' .. '' ... '.'. 47 literature Cited . '. ' ..... '' ' ........ '. 69 Tables ................. ' ................... 77 Figures . ..................... 89 vii ::-:::,<:-" _/; _,-:- LIST OF TABLES Tables Page 1. Mean percentages and standard error (SE) of birds per survey (n = 68) in Elkhorn Slough (study area A) observed in four macro-habitats (channel, mudflat, pickleweed flat, upland) and five microhabitats (mud, pickleweed, flooded/submerged mud, flooded/submerged vegetation, and 'other' -which included dikes, structures, branches, and air). NP =never present . 77 2. Two-way associations between high levels of birds in four macro­ habitat categories (channel, mudflat, pickleweed flat, upland) and tide level or time of day were tested using hierarchical loglinear analysis (n =68). A survey was classified as having high levels of birds when the percentage of birds in a habitat was greater than mean percentage birds in the same habitat. Surveys were conducted in Elkhorn Slough (study area A), near Moss Landing, California, January to April and June to August, 1996. 'S' denotes significant association between the frequency of high or low levels of birds in a macro-habitat and factor teSted. NS denotes non-significance (p > 0.025). NP indicates that birds were never present . 78 3. Two-way associations between high levels of birds in five microhabitats (mud, pickleweed, flooded mud, .flooded vegetation, and other (e.g., dikes, structures, and branches) and tide level or time of day were tested using hierarchicalloglinear analysis (n = 68). A survey was classified as having high levels of birds when the percentage of birds in a habitat was greater than mean percentage birds in the same habitat. 'S' denotes significant association (p < 0.025) between high and low levels of birds in a microhabitat and factor tested. NS denotes non­ significance . 79 viii 4. Results of Mann-Whitney U tests for comparing mean bird density between Elkhorn Slough (area A) and surrounding wetlands (area B) at two different scales. Values are presented as the mean number of birds per hectare. NS indicates no significant (p > 0.025) difference . ........... ' ... 80 5. Associations among the presence or absence of birds and three factors (survey area, tide level, or time of day) tested using hierarchicalloglinear analysis. Sample size (n) is equal to the total number of times large or small scale sites were surveyed for a species. Sites located in Elkhorn Slough (area A) and in surrounding wetlands (area B), near Moss Landing, California. p = probability of a type I error for all significant associations relating to bird presence and absence; NS = No significant association (P>0.025). Asterisk= association between factors .. 81 6. Percentage of large and small scale wetland sites that birds occurred at one or more times between January and August 1996 during surveys conducted at Elkhorn Slough (area A) and surrounding wetlands (area B), near Moss Landing, California... 82 7. Mean density when one or more birds were present and standard error (SE) of ardeids at small sites (mean size= 2.6 hectares}-------- and large sites (mean size= 14.2 hectares). A site was classified as having a high abundance of birds when the density was greater than mean density of sightings at all sites of the same size class. Sites were located in Elkhorn Slough (area A) and surrounding wetlands (area B), near Moss Landing, California .. 83 8. Percentage of correct predictions of logistic regression models for high and low abundances of ardeids at small scale sites (mean= 2.6 + 1 .6 hectares) and large scale sites (mean = 14.2 + 0.5 hectares), January to August1996. Sites located in Elkhorn Slough (area A) and surrounding wetlands (area B), near Moss Landing, California. 84 ix 9. Coefficients (Bj) for logistic regression models predicting high abundances of herons and egrets in and around Elkhorn Slough, California. An asterisk denotes non-significant variables {p > 0.025); balded coefficients denote significant variables (p < 0.025); ' -' indicates variables excluded from final models, 'c' =variables correlated (> 0.60) with another variable (var), therefore not included in the analysis. Correlations were indicated as follows: pf = pickleweed flats, pw = ponded water, vu = vegetated flats or upland, m = mud, sw = shallow water, mw =mid~~r ............................ 85 10. Results of analysis of variance of the mean percentage time (with arcsine transformation) that Great Blue Herons (GBHe), Great Egrets (GrEg) and Snowy Egrets (SnEg) spent foraging, roosting/preening, or traveling/interacting during daylight hours. Midday hours were within three hours of the sun at its apex, early/late hours were defined as the first and last three hours of daylight. Fifteen minute focal animal observations made of birds near Elkhorn Slough in surrounding