Conservation Assessment and Management Plan for Breeding Western and Clark’s Grebes in California Gary L. Ivey, Wildlife Consultant PO Box 2213 Corvallis, OR 97339-2213 [email protected] June 2004 TABLE OF CONTENTS LIST OF TABLES AND FIGURES.............................................................................................. iii ACKNOWLEDGMENTS ............................................................................................................. vi EXECUTIVE SUMMARY...........................................................................................................vii INTRODUCTION ...........................................................................................................................1 BACKGROUND .............................................................................................................................2 Taxonomy ...............................................................................................................................2 Legal status .............................................................................................................................2 Description..............................................................................................................................2 Geographic distribution ..........................................................................................................3 Life history .............................................................................................................................5 Habitat requirements......................................................................................................5 Reproductive ecology. ...................................................................................................6 Demography...................................................................................................................6 Feeding ecology.............................................................................................................7 Movements and migration. ............................................................................................7 Mortality ........................................................................................................................8 Population status and trends....................................................................................................9 Threats to populations...........................................................................................................13 Natural factors..............................................................................................................13 Habitat loss...................................................................................................................13 Water management ......................................................................................................14 Contaminants ...............................................................................................................14 Disturbance ..................................................................................................................15 Disease. ........................................................................................................................15 Summary of recent conservation actions ..............................................................................15 STUDY AREA ..............................................................................................................................16 METHODS ....................................................................................................................................22 RESULTS ......................................................................................................................................23 Colony size ...........................................................................................................................23 Relative importance of sites surveyed ..................................................................................24 Eagle Lake ...................................................................................................................24 Tule Lake NWR...........................................................................................................25 Clear Lake....................................................................................................................25 Lake Almanor. .............................................................................................................25 Thermalito Afterbay and Bridgeport Reservoir...........................................................25 Goose Lake. .................................................................................................................25 Other sites. ...................................................................................................................26 Colony and nest habitat.........................................................................................................26 Eagle Lake ...................................................................................................................26 Tule Lake and Lower Klamath NWRs. .......................................................................28 i TABLE OF CONTENTS (con't) Clear Lake....................................................................................................................28 Lake Almanor ..............................................................................................................30 Thermalito Afterbay.....................................................................................................31 Bridgeport and East Park reservoirs ............................................................................32 Goose Lake. .................................................................................................................33 Nest success ..........................................................................................................................35 Eagle Lake. ..................................................................................................................35 Clear Lake....................................................................................................................35 Lake Almanor. .............................................................................................................35 Other areas. ..................................................................................................................36 Brood surveys .......................................................................................................................36 Lake Almanor. .............................................................................................................36 Clear Lake....................................................................................................................36 Eagle Lake and Bridgeport Reservoir..........................................................................36 Thermalito Afterbay.....................................................................................................36 Other areas. ..................................................................................................................36 Nesting and fledging chronology..........................................................................................38 Species composition..............................................................................................................42 Mortality ...............................................................................................................................42 DISCUSSION................................................................................................................................42 Conservation issues...............................................................................................................43 Reduce mortality..........................................................................................................43 Increase nesting success...............................................................................................44 Enhance and maintain breeding habitat. ......................................................................44 Monitor efficacy of implemented conservation measures...........................................45 Recommendations for selected sites .....................................................................................45 Eagle Lake. ..................................................................................................................45 Tule Lake and Lower Klamath NWRs ........................................................................48 Clear Lake....................................................................................................................50 Lake Almanor. .............................................................................................................53 Thermalito Afterbay.....................................................................................................55 Bridgeport Reservoir....................................................................................................58 Goose Lake. .................................................................................................................59 East Park Reservoir......................................................................................................60