Changes in Breeding Population Size of Brandt’s and Double-crested Cormorants in , 1975-2003

Phillip J. Capitolo1, Harry R. Carter1,4, Richard J. Young1, Gerard J. McChesney2, William R. McIver1,5, Richard T. Golightly1, and Franklin Gress3

1Department of Wildlife, Humboldt State University, Arcata, California 95521 USA

2San Francisco Bay Complex, U.S. Fish and Wildlife Service, P.O. Box 524, Newark, California 94560 USA

3California Institute of Environmental Studies, 3408 Whaler Avenue, Davis, California 95616 USA

Final Report for Contract # 10154-2-G106 between Humboldt State University Foundation and U.S. Fish and Wildlife Service

July 2004

4Current Address: 5700 Arcadia Road, Apt. #219, Richmond, British Columbia V6X2G9 Canada

5Current Address: U.S. Fish and Wildlife Service, Ecological Services, 2493 Portola Road, Suite B, Ventura, California 93003 USA

Suggested Citation: Capitolo, P. J., H. R. Carter, R. J. Young, G. J. McChesney, W. R. McIver, R. T. Golightly, and F. Gress. 2004. Changes in breeding population size of Brandt’s and Double- crested Cormorants in California, 1975-2003. Unpublished report, Department of Wildlife, Humboldt State University, Arcata, California.

TABLE OF CONTENTS

ABSTRACT...... 1 INTRODUCTION ...... 2 METHODS ...... 4 AERIAL PHOTOGRAPHIC SURVEYS...... 4 Surveys ...... 4 Photograph Counting ...... 5 Data Archiving...... 7 BOAT AND GROUND SURVEYS...... 7 Boat Surveys ...... 7 Boat and Ground Surveys...... 8 Ground Surveys of Colonies...... 8 RESULTS ...... 9 BRANDT’S CORMORANT...... 9 California Summary...... 9 Northern California (2003) ...... 9 Central California (2003)...... 9 (2001)...... 10 DOUBLE-CRESTED CORMORANT...... 11 California Summary...... 11 Northern California (2003) ...... 11 Central California (2003)...... 11 Southern California (2001)...... 12 J CORRECTION FACTOR ...... 13 DISCUSSION...... 13 BRANDT’S CORMORANT...... 13 California Summary...... 13 Patterns of population size in California...... 14 El Niño in 2003 ...... 15 1989-91 Comparisons...... 16 1975-80 Comparisons...... 17 New Colonies ...... 18 South ...... 18 DOUBLE-CRESTED CORMORANT...... 19 California Summary...... 19 Patterns of population size in California...... 20 El Niño in 2003 ...... 21 New Colonies ...... 21 South Farallon Islands...... 22 Colonies on bridges in San Francisco Bay...... 22 ACKNOWLEDGEMENTS...... 24 LITERATURE CITED ...... 25 TABLES...... 31 FIGURES...... 43 APPENDICES ...... 48

i ABSTRACT

In 2003, Humboldt State University (Department of Wildlife) and the U.S. Fish and Wildlife Service (San Francisco Bay National Wildlife Refuge) cooperated to conduct a survey of all coastal colonies of Brandt’s and Double-crested Cormorants (Phalacrocorax penicillatus, P. auritus) in California. Our goals were: a) to continue annual aerial photographic surveys of these colonies, which have been conducted since 1985-91; and b) to rapidly assess possible changes in populations of these species through comparisons to previous state-wide summaries in 1975-80 and 1989-91. In northern and central California, numbers of nests at each colony in 2003 were counted from aerial photographs, except at two large colonies of Double-crested Cormorants on bridges in San Francisco Bay, where nests were counted from boat surveys. Due to insufficient funds to count all 2003 photographs, 2001 aerial photographic survey data were substituted for southern California colonies. For Anacapa Island colonies, 2001 boat and ground survey data collected by the California Institute for Environmental Studies were used. Numbers of Common Murres (Uria aalge) and Caspian Terns (Sterna caspia) also were counted at specific colonies in California in 2003.

In 2001-03, 26,718 Brandt’s Cormorant nests were counted at 97 active colonies and 6,477 Double-crested Cormorant nests were counted at 42 active colonies in coastal California. The nest total for Brandt’s Cormorants was 29.1% lower than in 1989-91 surveys and less than 1% lower than in 1975-80 surveys. The nest total for Double- crested Cormorants was 48.2% higher than in 1989-91 surveys and 592.0% higher than in 1975-80 surveys. Numbers of Brandt’s Cormorants in northern California declined from 1975-80 to 2003. Numbers of Brandt’s Cormorants in central and southern California increased from 1975-80 to 1989-91 before declining between 1989-91 and 2003 and 2001, respectively. In central California, numbers of Brandt’s Cormorants declined at the South Farallon Islands from 1975-80 to 2003, but increased from 1975-80 to 1989-91 at all nearshore colonies combined. Some Brandt’s Cormorants apparently moved from this large offshore colony to nearshore colonies in central California by 1989-91. While numbers of Double-crested Cormorants increased from 1975-80 to 2003 in northern and central California, numbers in southern California increased from 1975-80 to 1989-91 but declined from 1989-91 to 2001. Survey techniques and seasonal timing for counting nests at colonies were similar in 1975-80, 1989-91, and 2001-03, and surveys did not occur immediately following cyclic population dips of Brandt’s Cormorants related to severe El Niño events in 1983, 1992, and 1998. Numbers of cormorant nests were generally comparable between these three surveys. However, survey error may amount to + 10%, which must be taken into account when interpreting comparisons. Mild to moderate El Niño conditions existed in the California Current prior to and during 2003 surveys. Mild effects on breeding Brandt’s Cormorants were noted in central California (but not in northern California) and no effects were observed on Double-crested Cormorants. Upwelling conditions in southern California in 2001 were stronger than normal but had no major observed effect on either species.

In 2003 at Castle Rock National Wildlife Refuge, 62,504 Common Murres were counted. Using a k correction factor of 1.67, a breeding population of 104,400 birds was

1 estimated. At Arcata Bay Sand Islands (a colony lost in 1969 with recolonization after 1989), 262 Caspian Terns were counted. At Knight Island in (a colony established in the 1970s), 321 terns were counted.

INTRODUCTION

In 2003, the U.S. Fish and Wildlife Service (Region 1, Office of Migratory Birds and Programs, Portland, Oregon; MBHP) coordinated and partly sponsored a survey of all coastal colonies of Brandt’s and Double-crested Cormorants (Phalacrocorax penicillatus, P. auritus) in California, Oregon, and Washington. Major goals of the survey were: a) to assess the current status of these species along the west coast by gathering updated information on the distribution of breeding colonies and estimates of breeding population size for all colonies; b) to determine whether the world population of the Brandt’s Cormorant (most of which occurs in the study area) had declined since the last complete surveys of all colonies in 1989-91; and c) to measure to what degree the Pacific coast population of the Double-crested Cormorant had increased since the last complete surveys of all colonies in 1989-91 (Carter et al. 1992, 1995a,b). As part of this west coast survey, Humboldt State University (Department of Wildlife, Arcata, California; HSU) and the U.S. Fish and Wildlife Service (San Francisco Bay National Wildlife Refuge Complex, Newark, California; SFBNWRC) cooperated to conduct a state-wide aerial photographic survey of Brandt’s and Double-crested Cormorant colonies in 2003. While state-wide surveys had been occurring annually for several years in California, insufficient funding had been available to count photographs from all colonies since 1989-91 and southern California surveys were no longer funded after 2002. With additional funding from MBHP, a new state-wide survey and summary could be provided. After Carter et al. (1992), we defined: a) “northern California” from the Oregon border south to but not including , including Arcata and Humboldt Bays; b) “central California” from Point Reyes south to and including Point Conception, including the Farallon Islands and San Francisco Bay; and c) “southern California” from Point Conception to the Mexico border, including the Channel Islands and San Diego Bay. Substantial in-kind support was provided by the California Department of Fish and Game (Office of Spill Prevention and Response) who coordinated flight support with the California Department of Fish and Game (Air Services) for surveys in northern, central and southern California, and the U.S. Navy (Naval Air Warfare Center) who funded additional flight support for surveys in southern California by Aspen Helicopters.

In northern and central California, aerial photographic surveys in 2003 were conducted by SFBNWRC and HSU through the Common Murre Restoration Project (funded by the Apex Houston Trustee Council). In southern California, surveys in 2003 were conducted solely by HSU (with MBHP funding). Similar surveys have been conducted by SFBNWRC and HSU annually since 1985 in northern and central California and since 1991 in southern California. In northern and central California, these surveys include Common Murres (Uria aalge), in addition to Brandt’s and Double- crested Cormorants. Due to insufficient funding, an incomplete assessment of population size at all cormorant and murre colonies has occurred in northern California in all years

2 except 1989 (Carter et al. 1992, 2001). In southern California, most cormorant colonies were assessed from 1991-2001. Photographs were taken in southern California in 2002 but have not yet been assessed. Thus, a complete assessment of all Brandt’s and Double- crested Cormorant colonies in coastal California had not taken place since 1989-91 (Carter et al. 1992, 1995a,b).

In 2003, aerial photographs for several colonies in central California (including the South and North Farallon Islands) were counted by the Common Murre Restoration Project. However, most colonies in central California and all colonies in northern California were counted by HSU (with MBHP funding). In addition, HSU conducted boat surveys of two large Double-crested Cormorant colonies on bridges in San Francisco Bay in 2003, which cannot be surveyed with aerial photographs. Insufficient funds were available to count 2003 aerial photographs in southern California, but the most recent and available data from photographs counted by HSU in 2001 (through a separate project by HSU and the U.S. Geological Survey [USGS]) were substituted. In addition, we included 2001 data collected from boat and ground surveys by the California Institute of Environmental Studies (CIES) at Anacapa Island, which was only partly surveyed with aerial photographs. These surveys were conducted with CIES funding and data handling was assisted with MBHP funding.

In this report, we provide summaries of: a) 2003 surveys; b) factors affecting 2003 surveys at certain colonies; c) numbers of nests counted at each colony in 2003 in northern and central California and in 2001 in southern California; d) total numbers of nests for all colonies combined in northern and central California in 2003 and in 2001 in southern California; e) total numbers of nests from 1975-80 and 1989-91, as derived from previous sources (Hunt et al. 1979, Sowls et al. 1980, Carter et al. 1992, 1995a,b); and f) changes in population size between state-wide surveys in 1975-80, 1989-91 and 2001-03. Further analyses of these data and assessment of population trends will be performed at a later time, when data from Oregon, Washington, and British Columbia become available and all 2003 west coast data can be summarized and analyzed together. In addition, annual monitoring data in coastal California will be analyzed in the future, once all past aerial photographs have been counted.

In Appendix A, we provide information on the numbers of Common Murres (Uria aalge) at Castle Rock National Wildlife Refuge in 2003. Recent concerns about this colony have been expressed (Carter et al. 2001), so its current status was assessed, as requested and funded by MBHP.

In Appendix B, we provide information on the numbers of Caspian Terns (Sterna caspia) at one colony in northern California (Arcata Bay Sand Islands) and one colony in central California (Knight Island). This information will assist other researchers who are assessing the status and trends of this species on the west coast, as requested by MBHP.

3 METHODS

AERIAL PHOTOGRAPHIC SURVEYS

Surveys

In 2003, we conducted aerial photographic surveys of Brandt’s and Double- crested Cormorant colonies on the coasts of northern, central and southern California with standardized techniques used since 1985 (Takekawa et al. 1990, Carter et al. 1992, 1995a,b, 1996, 1998, 2000, 2001, 2003, Sydeman et al. 1997, McChesney et al. 1998a,b, 1999, 2001, Capitolo et al. 2002, Knechtel et al. 2003). All previously known colonies were inspected and photographed if cormorants were present. Previously known colonies that were empty in 2003 were noted on flight log datasheets, but not necessarily photographed. In addition, we inspected almost all stretches of coastline between known colonies to identify and photograph any newly-formed colonies.

Each colony in northern and central California was photographed by GJM and HRC, or GJM and PJC, once during the census period from late May to early June. Standard seasonal survey timing was originally determined to correspond with the mid- to-late incubation period for Common Murres, whose colonies are photographed during the same surveys as those for cormorants. In addition, surveys were timed to cover murre colonies between 10:00-14:00 hours (PDT) (Takekawa et al. 1990, Carter et al. 2001). However, cormorant-only colonies were surveyed as early as about 09:00 h and as late as about 18:00 h on certain days in 2003. We have found that this seasonal timing of aerial surveys often captures peak or near-peak numbers of nests at cormorant colonies during the mid-incubation to early-chick period in northern and central California (Carter et al. 1992). Thus, in most cases, we believe that numbers of nests counted from aerial photographs act as a reasonable index of population size for that year, although numbers counted at many colonies would be expected to be slightly lower than peak numbers, if each colony was monitored weekly through the breeding season to determine highest peak numbers. Exceptions to capturing near-peak numbers can occur when colonies are partly or completely abandoned due to severe El Niño conditions or human disturbance (Carter et al. 1998, 2003).

Persistent fog along the coast in Del Norte and Humboldt counties (northern California) and along the Big Sur coastline in Monterey and San Luis Obispo counties (central California) delayed completion of surveys of all colonies until 11 and 6 June for northern and central California, respectively (slightly later than in most other years). However, we do not believe that slightly later surveys affected counts to a great degree. Northern California colonies from Point Reyes to Point Arena were photographed during central California surveys for the most efficient use of flight time, as often done in previous years.

In southern California in 2003, most colonies were surveyed in mid May, except those at Santa Barbara Island which were not surveyed due to persistent fog. Additional southern California surveys were conducted in late April and mid June at certain colonies

4 where the peak number of nests has been detected in prior years substantially earlier or later than the mid May period (Carter et al. 1992, McChesney et al. 1998a, 2001). Colonies at Santa Barbara, San Nicolas and San Clemente Islands were surveyed in April. A complete survey of Santa Barbara Island was again conducted in June, along with partial surveys of San Miguel, Santa Rosa, Santa Cruz, Anacapa, and San Nicolas Islands. All surveys were conducted by PJC and HRC. Southern California surveys in 2001 also were conducted in April, May, and June by GJM, WRM, and John Mason as part of a separate study during the breeding seasons of 1999-2001 by HSU and USGS (McChesney et al. 2001), and as a continuation of long-term monitoring since 1991 by HSU (Carter et al. 1992, 1996, McChesney et al. 1998a). At West Anacapa Island, aerial surveys of the Double-crested Cormorant colony have not been conducted by HSU since 1991 to avoid possible disturbance to breeding Brown Pelicans (Pelecanus occidentalis) in the same area, but boat and ground surveys have been conducted by CIES.

All 2003 surveys were flown in twin-engine, fixed-wing Partenavia aircraft. All northern and central California surveys and April southern California surveys were flown by the California Department of Fish and Game. May and June surveys in southern California were flown by Aspen Helicopters. See Table 1 for a summary of survey dates, personnel, and pilots. Photographs were taken with handheld 35 mm cameras pointed vertically through the belly of the plane in northern and central California, where most colonies of murres and cormorants occur on the peaks and slopes of offshore rocks. In southern California, photographs were taken obliquely through side windows since most colonies occur on steep cliffs. Two observers simultaneously photographed each colony using a 300 mm lens for close-up photographs and one observer also used a 50 mm lens for overview photographs. Zoom lenses (35-80, 70-200, and 70-300 mm) were also used at times for overviews and close-ups to allow photographers greater flexibility in completely documenting colonies. Color slide film (ASA 200) was used for all photography. Each roll of film was individually numbered. After development, all slides were labeled with roll number, date, photographer, and location, through the aid of flight log datasheets.

Photograph Counting

With the exception of certain central California colonies where cormorants nest among Common Murres, photographs of colonies in northern and central California in 2003 were counted by PJC and RJY. Colonies with murres and cormorants in central California (i.e., North and South Farallon Islands, Point Reyes, Point Resistance, Millers Point Rocks, Double Point Rocks, , Devil’s Slide Rock, Benchmark 227X, Castle Rocks and Mainland, and Hurricane Point Rocks) were counted by the Common Murre Restoration Project (Knechtel et al. 2003). In northern California, we counted only cormorants at colonies where murres and cormorants nested together, except for Castle Rock where murres were also counted (see Appendix A). If Common Murres are counted at these colonies in the future, archived slides used for counting cormorants will need to be re-examined along with remaining unused slides to select the best photographs for counting murres and to ensure complete coverage of murre breeding areas. Many of the photographs used for counting cormorants may also be the best for

5 counting murres. Southern California photographs in 2001 were counted by WRM. For southern California colonies that were surveyed monthly in 2001, we present data from the survey with the highest nest total.

We projected slide images onto 2’ x 3’ white paper and marked (or “dotted”) each bird, nest and site with a felt pen. Figures 1 and 2 provide examples of slide images used for counting Brandt’s and Double-crested Cormorants, respectively. Slide images are much larger than in Figures 1 and 2 when projected during counting. For each colony, we determined the total number of Brandt’s and/or Double-crested Cormorants present, and the total numbers of nests and territorial sites. Nests and sites were categorized by their stage of development following an HSU protocol described in McChesney et al. (1998a,b, 1999). For the 2003 west coast survey, the definition of a nest was standardized between researchers as any location with a well-built or poorly-built nest, whether or not it was attended by an adult cormorant. Using the HSU protocol, this corresponded to including all nest categories (i.e., “X [well-built nest]”, “C [nest with chicks]”, “B [brood]”, “E [empty nest]”, “A [abandoned nest]”, and “P [poorly-built nest]”. However, we excluded all sites attended by adults (i.e., “Z [bird attending a site with little or no nesting material], and “S [bird at an undetermined site or nest due to poor photo quality]”). In almost all cases, sites were identified as locations attended and apparently defended by adults (based on spacing). Sites can serve to indicate that more nests may be built after surveys are completed, though nests often are not built at sites later in the breeding season.

Raw nest totals can be adjusted for nests that were not present during surveys at this time of year, by using rough correction factors determined at sample colonies monitored throughout the season or by examining total numbers of nests and sites combined (Carter et al. 1992, McChesney et al. 1998a,b). While neither adjustment is exact, these approaches serve to provide better estimates of true breeding population size, especially for use in comparing regional population sizes of species. However, it is unclear if it is appropriate to apply correction factors, determined at one or few colonies, on a regional or state-wide basis. For purposes of monitoring and assessing population trends, a better approach is to examine changes in raw nest totals using a standard definition of what constitutes a nest. For the 2003 west-coast survey, MBHP decided to simplify and standardize the process of determining colony and regional population estimates by only summarizing the total number of nests observed. Thus, the reader should be aware that the methods used to determine cormorant population estimates in this report differ from some past sources (Carter et al. 1992, 1995a,b, McChesney et al. 1998a,b, 1999, 2001). In addition, all population estimates in this report should be considered slight underestimates of true population size. With these caveats, we have only included summaries and comparisons of raw nest totals for all years examined in this report.

In addition to cormorants, we also counted roosting Brown Pelicans that were photographed at past or present cormorant colonies. Pelicans photographed at other areas (e.g., roost rocks not adjacent to a cormorant colony) were not counted. Pelicans were

6 aged as “adult” or “immature” based on plumage (Capitolo et al. 2003). Pelican counts are not included in this report, but data will be archived for future use.

Data Archiving

All close-up slides used for counting, as well as one or more overview slides of most colonies, were additionally labeled with specific locations, and were placed in archival slide sheets and stored in 3-ring binders. Raw data from counting of slide images were transcribed from original ‘dotting sheets’ (generated during photograph counting) onto tally sheets and proofed against the dotting sheets. Data were entered into a database (Microsoft Access), proofed against tally sheets, and corrections from proofing were made in the database. All data compilation and data entry work was conducted by PJC and RJY. All archived and non-archived slides, dotting sheets, and tally sheets have been retained by SFBNWRC.

BOAT AND GROUND SURVEYS

San Francisco Bay Boat Surveys

To obtain population estimates of Double-crested Cormorant colonies at the San Francisco-Oakland Bay Bridge East (SFOBB; Figures 3, 4) and Richmond-San Rafael Bridge (RSRB; Figure 5) in San Francisco Bay, HSU conducted boat surveys on 20 June 2003. These bridge colonies and monitoring methods have been described in detail elsewhere (Carter et al. 1989, Stenzel et al. 1995, Rauzon et al. 1999, 2000). Aerial photographic surveys of these colonies are not possible since a high proportion of the colonies occur on the undersides of the bridges, and low-level flying is difficult and not permitted. We launched an inflatable boat from Richmond Marina and conducted counts at RSRB from 08:05-10:00 hours and at the SFOBB from 11:30-14:35 hours. PJC made all counts of nests and adults through 8 x 32 Leica binoculars and HRC directed and recorded counts by mapped area in a field notebook. At the SFOBB, nests were counted by working the boat back and forth under and beside the bridge from piers E2 to E14. Eventually, we approached loud pile driving activities and active barges beginning at pier E6 and counted cormorants between piers E12 and E14 from a greater distance of 150- 200 m to avoid loud noises. At the RSRB, we maneuvered the boat for the best vantage points of the colonies at distances of 25-50 m from the north side of the bridge (for counting the north cord and undersides of the bridge) and 100-200 m from the south side of the bridge (for counting the south cord). These distances reflected our counting along the edge of a temporary boat exclusion zone, marked with buoys around to prevent disturbance to Harbor Seals (Phoca vitulina), which extended farther south of the bridge and only slightly north of the bridge (Figure 5). Counts of birds and nests were recorded for each section of each “bay” (i.e., between bridge piers) and each type of bridge structure utilized (i.e., “cords”, “beams”, “travelers”, etc.), and an estimate of breeding population size was determined by summing all nests counted.

7 Anacapa Island Boat and Ground Surveys

To obtain population estimates and productivity (i.e., chicks fledged per nesting attempt), CIES has conducted boat and ground surveys of Double-crested and Brandt’s Cormorants (as well as Pelagic Cormorants P. pelagicus) at Anacapa Island in each year since 1970 (except 1995), using standard methodology (Lewis et al. 1988). Each month from March to August 2001, the number of nests and number of chicks per nest in a sample were counted from an inflatable boat for each sub-section of West, Middle and East Anacapa Island. FG conducted all counts with 10 x 42 Leica binoculars. In addition, FG conducted ground counts of certain Double-crested Cormorant nesting areas on West Anacapa Island where nests could not be seen from the boat. To obtain breeding population estimates, average high counts or peak counts were summed for each sub- section of Anacapa Island. Since counts are conducted throughout the breeding season, few nests are missed although some abandoned nests may not be detected.

Ground Surveys of Northern California Colonies

To assist possible future adjustments of raw nest totals with correction factors, three Double-crested Cormorant colonies in northern California were monitored twice per month by PJC from April to July 2003, using ground counts from the adjacent mainland. We calculated a j correction factor for each colony as the number of active nests over the entire breeding season divided by the number of nests counted on the single aerial photographic survey in early June (see Carter et al. 1992). A nest was considered to be “active” (i.e., eggs laid) in ground monitoring if at least one incubating adult was present on two consecutive visits. At Radar Station Rocks and White Rock (Figures 6, 7), the entire colony was not visible from mainland vantage points. The total number of nests over the season was determined for distinct subcolonies or study plots. However, we were able to monitor the entire Sea colony. Brandt’s Cormorants also nest at White Rock and were monitored in a study plot. Ground monitoring also aided our distinguishing Double-crested from Brandt’s Cormorant nests in aerial photographs at White Rock.

All areas were monitored from mainland vantage points using a Kowa spotting scope with a 32x wide angle or a 20-60x zoom eyepiece. Nests were numbered on printouts of digital photographs which were taken through the 32x eyepiece of the spotting scope. New photographs were printed for each subsequent visit to the colony and compared to prior photographs. Any new nests were added and numbered sequentially. The numbers of adults in incubation posture and standing at each nest were recorded in a field notebook. We could not determine clutch size at almost all nests, but did note the number of small, medium-sized, and large chicks for each nest. Colony monitoring required about 20-120 minutes, depending on the number of nests and chicks present. In addition to j correction factors, we were able to determine brood sizes and rough estimates of productivity (i.e., chicks fledged per pair).

8 RESULTS

BRANDT’S CORMORANT

California Summary

In 2001-03, a total of 26,718 Brandt’s Cormorant nests were counted at 97 active colonies in California (Tables 2, 4-6). In addition, 61 previously active colonies were documented as empty. Total numbers of nests in 2001-03 were 29.1% lower than in 1989-91 (i.e., 37,690 nests at 102 active colonies) and 0.4% lower than in 1975-80 (i.e., 26,835 nests at 93 active colonies).

In 2001-03, the largest colonies in California occurred at the South Farallon Islands within the Farallon National Wildlife Refuge (6,801 nests or 25%), Bird Island at State Reserve (1,345 nests or 5%), and Castle Rock National Wildlife Refuge (1,034 nests or 4%). Most Brandt’s Cormorants (15,566 nests or 58%) bred in central California (which includes the South Farallon Islands and Bird Island), with the remainder split between northern California (5,397 nests or 20%, which includes Castle Rock) and southern California (5,755 nests or 22%).

Northern California (2003)

A total of 5,397 Brandt’s Cormorant nests were counted at 30 colonies (Table 4). The largest colony was Castle Rock with 1,034 nests. High Bluff South, Mistake Point- Big White Rock, and Soldier Frank Point were new colonies since 1989, but had been previously described as breeding locations for other seabird species (Carter et al. 1992). High Bluff South was first noted as a new colony in 1997, but these photographs have yet to be counted (USFWS, unpubl. data). The Point St. George Lighthouse colony was not inspected, but Brandt’s Cormorants only rarely nest in small numbers at the SW Seal Rock subcolony (H. Carter, pers. obs.).

Central California (2003)

A total of 15,566 nests were counted at 39 colonies (Table 5). All known colonies were surveyed except the small colony at in San Francisco Bay (which had not been noted as active in 1991-94 [J. E. Takekawa, pers. comm.] and low flying is difficult or not permitted adjacent to the Oakland-San Francisco Bay Bridge) and two cliff colonies at Seal Rock Cliffs and Martins Beach in San Mateo County (where small numbers have nested with Pelagic Cormorants in the past and none have been noted in recent years). We presumed that all three colonies had no nests in 2003 but small numbers may have been present. At the South Farallon Islands, the largest Brandt’s Cormorant colony throughout its breeding range, we counted 6,801 nests. The Lone Cypress was a newly described seabird colony in 2003 (Table 7), on a mainland point below the tourist landmark in Pebble Beach in Monterey County. Terrace Point-Point Santa Cruz, Monterey Harbor, Cannery Row, Ragged Point Lodge Colony, Soberanes Point South, Gamboa Point, Point Buchon, and Unnamed Rocks were new

9 colonies, but had been previously described as breeding locations for other seabird species (Carter et al. 1992). Most of these colonies have also been active in recent years prior to 2003, but only Point Buchon has been counted, though not described in available reports (HSU and USGS, unpubl. data; McChesney et al. 2001). At other colonies, cormorants nested at new locations within the colony that had not been previously described, requiring the numbering of new subcolonies following Carter et al. (1992; see Table 7). At Pfeiffer Point, we were unable to determine the exact location of the nesting cormorants and entered this area as an unknown subcolony in the database. Cormorants on “sites” were also noted on a rock 200 m west of Lover’s Point in Monterey Bay and at Bench Mark 223. Nesting Brandt’s Cormorants have not been previously documented at these areas. Since no nests were present during the aerial survey these areas are not included as “colonies” in Table 5. The Cypress Point colony in Pebble Beach in Monterey County was empty in 2003 and was first noted during surveys in 1998, but photographs have yet to be counted (Table 7; USFWS, unpubl. data).

Flight restrictions in 2003 near the Diablo Canyon Nuclear Power Plant prevented photographic surveys of the Pup Rock and Adjacent Mainland, Lion Rock, Diablo Rock and Adjacent Mainland, and Diablo Nuclear Power Plant South colonies in San Luis Obispo County. However, while briefly flying offshore, these colonies could be partially assessed and nests were only detected on Lion Rock. A rough estimate of the number of nests on Lion Rock was recorded on the flight log datasheet. Only the ocean side of Pup Rock and Diablo Rock could be seen, and views of the mainland cliff areas were poor. However, few nests, if any, were apparently missed.

Southern California (2001)

A total of 5,755 nests were counted at 28 colonies (Table 6). Several small cliff colonies found with boat surveys in 1991 were not examined with aerial surveys in 2001 because there was a strong likelihood that nests could be missed or mistaken for Pelagic Cormorants (i.e., Northwest , Hoffman Point Area, Profile Point Area, Cueva Valdaze Area, Coche Point Northeast, Cavern Point Area, Middle Anacapa Island, East Anacapa Island, and Shag Rock). In 1991, these colonies collectively accounted for 78 nests (Carter et al. 1992). If nests were present in 2001 but not seen, this may have reduced the total estimate in 2001 by a small degree. No nests were photographed or seen at these colonies in 2003. The largest colony was at Vizcaino Point South on with 726 nests.

At Anacapa Island, small numbers nesting on cliffs on East Anacapa (7 nests) and Middle Anacapa (8 nests) were detected with CIES boat and ground surveys but were not examined with HSU aerial surveys. However, aerial surveys and boat/ground surveys obtained the same number of nests at one location on Middle Anacapa (9 nests). At West Anacapa, aerial surveys determined a slightly higher total (306 nests) than boat/ground surveys (278 nests), but reasons for this difference were not determined. Brandt’s Cormorants nest about 10-15 m above high water within a “pock-mark” cliff (i.e., an area of small grottos with as many as 3-5 nests per grotto) along the north side of West Anacapa Island. Conditions were excellent during the boat survey and nests were well

10 viewed. While a few nests may have been missed during the boat survey that were seen in aerial photographs, it seemed unlikely that as many as 28 nests were missed for this reason alone. Overall nest totals of Brandt’s Cormorants at Anacapa Island were similar for aerial surveys (315 nests) and boat/ground surveys (302 nests) in 2001. In 2003, CIES recorded 340 nests at Anacapa Island (10 on East; 32 on Middle; and 298 on West).

DOUBLE-CRESTED CORMORANT

California Summary

In 2001-03, a total of 6,477 Double-crested Cormorant nests were counted at 42 active colonies in coastal California (Tables 3-6). In addition, 19 previously active colonies were documented empty. Total numbers of nests in 2001-03 were 48.2% higher than in 1989-91 (i.e., 4,371 nests at 39 active colonies) and 592.0% higher than in 1975- 80 (i.e., 936 nests at 17 active colonies).

In 2001-03, the largest colonies in California occurred at Arcata Bay Sand Islands (809 nests or 13%), the San Francisco Oakland Bay Bridge (745 nests or 12%), and the Richmond San Rafael Bridge (632 nests or 10%). Most Double-crested Cormorants (3,287 nests or 51%) bred in central California (which included the bridge colonies) or northern California (2,500 nests or 39%, including the Arcata Bay Sand Islands colony), with much lower numbers in southern California (690 nests or 11%).

Northern California (2003)

A total of 2,500 nests were counted at 19 colonies (Table 4). The largest colony of 809 nests was found at Arcata Bay Sand Islands. Nesting cormorants had not been previously reported at this location (Carter et al. 1992, 1996; USFWS, unpubl. data). The second largest colony was found at Teal Island (365 nests), a colony that formed in 1993 (Carter et al. 1996). The third largest colony has been long established at Prince Island (323 nests). Hog Island, a newly described seabird colony (Table 7) where cormorants nest in eucalyptus and cypress trees, first formed in 2001 (J. Kelly and S. Allen, pers. comm.) but was not photographed in 2001-02. Arcata Bay Sand Islands and Shell-Wright’s Beach are new colonies, but were previously described as breeding locations for other seabird species (Carter et al. 1992). The Humboldt Bay Duck Blinds colony has not been active since 1996 and the blinds used for nesting have deteriorated and no longer exist (Capitolo et al. in prep.). Double-crested Cormorants did not nest at High Bluff South in 2003 and the colony was first noted during surveys in 1998, but photographs have yet to be counted (USFWS, unpubl. data).

Central California (2003)

A total of 3,287 nests were counted at 17 colonies (Table 5). The two largest colonies of 745 and 632 nests were found at the SFOBB and RSRB, respectively. The SFOBB colony extended from bay section E3-9 into bay E13 (Table 8; see Carter et al. 1989 for a detailed description of bridge structures). At RSRB, cord nests were present

11 from section 51-4 through 56-6, with sections 52-1 and 53-1 empty. South cord nests were not present continuously until section 52-3. Nests under the bridge extended from section 51-5 through 56-6, with one nest also present in section 51-3. Sections 52-1, 53- 1, and 55-1 were empty (Table 9). Major construction and maintenance projects occurred beside these colonies during the breeding season in 2003. Numbers of nests and their distribution on bridges were similar to recent surveys in 1999-2000 (Rauzon et al. 2000; Table 8). The third largest colony occurred at the South Farallon Islands (439 nests), a long established colony.

One historical cliff colony at Pillar Point was not surveyed, but was assumed to be empty because no nesting has been noted in recent years. Power Towers is a newly described seabird colony (Table 7), but has been active since at least 1994 when the San Francisco Bay Bird Observatory noted 5 nests (C. Strong, pers. comm.). This colony was not present during surveys in 1990 (Carter et al. 1992). Aerial photographs of this colony from recent years have not yet been counted. Two colonies within the mouth of the (i.e., estuarine waters) were not surveyed with aerial photographs in 2003. At Donlon Island, no nests were present on the power towers when the colony was last inspected in 2001 and the colony may have been abandoned due to human disturbance (H. Carter, pers. obs.). At Wheeler Island, researchers from noted 126 nests on 18 July 2003 from a ground count on the island. Most nests had fully grown chicks (J. Kelly, pers. comm.). Also, three active colonies in the were not surveyed with aerial photographs in 2003. Double- crested Cormorants have been nesting in eucalyptus trees since 1997 at Lake Merced in San Francisco and since 1998 at Lake Merritt in Oakland (see Table 7). Lake Merced is a freshwater lake, but at least some foraging by cormorants likely occurs in San Francisco Bay. In 2003, 125 nests were counted from the ground at Lake Merced (D. Murphy, pers. comm.). Lake Merritt receives limited tidal flow, and cormorants have been observed foraging in the lake as well as flying toward adjacent waters of San Francisco Bay (P. Capitolo, pers. obs.). We did not attempt aerial surveys of Lake Merritt due to concerns about low flights over residential areas. In 2003, approximately 80 nests were counted from the ground (M. Rauzon, pers. comm.). In south San Francisco Bay, Double-crested Cormorants have been nesting at the Alviso Plant, Pond Nos. A9 and A10 colony since 1999, and 39 nests were counted in 2003 (C. Strong, pers. comm.). The Dumbarton Bridge Power Towers colony, empty in 2003, was noted in 2001, but photographs have yet to be counted (Table 7; USFWS, unpubl. data).

Southern California (2001)

A total of 690 nests were counted at six colonies (Table 6). The two largest colonies were found at West Anacapa Island (330 nests) and Prince Island (140 nests), both long established colonies. In 2003, CIES determined 286 nests at West Anacapa Island.

12 J CORRECTION FACTOR

In 2003, an overall j correction factor of 1.24 was determined for five Double- crested Cormorant areas in northern California (Table 10), similar to the 1.2 value calculated by Carter et al. (1992) from data collected at the South Farallon Islands in central California in 1989. Correction factors for individual areas ranged from 1.00 to 1.64. A j correction factor of 1.00 was determined for the one Brandt’s Cormorant area monitored at White Rock, as the aerial survey nest total was the same as the seasonal nest total (n = 60). Carter et al. (1992) calculated a value of 1.1 for Brandt’s Cormorants from data collected at the South Farallon Islands in 1989. At these study locations in northern California, nest initiations peaked in April and May for Double-crested Cormorants and Brandt’s Cormorants, respectively.

DISCUSSION

BRANDT’S CORMORANT

California Summary

In 2001-03, the total number of Brandt’s Cormorant nests was 29% lower than in 1989-91 and less than 1% lower than in 1975-80. The timing of these three state-wide surveys was comparable with respect to cyclic patterns of Brandt’s Cormorant population sizes. The relatively large degree of decline between 1989-91 and 2001-03 exceeds survey error within one survey year, and little effect of El Niño conditions in 2003 was noted. Thus, we believe that substantial population decline on the order of about 25-35% has occurred in California since 1989-91. Since 1989-91, decline has occurred primarily in northern California (23%) and southern California (56%). Decline in central California (11%) may not exceed potential survey error combined with annual variation in numbers of cormorants attempting to breed. Since 1975-80, decline occurred in northern California (38%), but not in central or southern California. Central California was 3% higher in 2003 and southern California was 91% higher in 2001 (see further discussion below).

The overall distribution of Brandt’s Cormorant colonies has remained similar between 1975-80 and 2001-03. Of 158 colony locations recorded in California, 59%, 65%, and 61% were active in 1975-80, 1989-91 and 2001-03, respectively. However, the relative distribution of Brandt’s Cormorants between northern, central and southern California has changed between 1975-80, 1989-91, and 2001-03: a) northern California has accounted for 32%, 19%, and 20% of California populations, respectively; b) central California has accounted for 56%, 46%, and 58% of California populations, respectively; and c) southern California has accounted for 11%, 35%, and 22% of California populations, respectively (Table 2).

Local movements between adjacent colonies in different years account for many unused colonies in a given survey, but human disturbance has likely caused some of these

13 movements and may prevent Brandt’s Cormorants from nesting at certain locations, especially in south-central California (Carter et al. 1998). At San Nicolas Island in southern California, higher numbers since 1975-80 and much changing of adjacent colony locations between 1991 and 2001 has occurred due to new protected areas on the west end of the island, predation by Island Foxes (Urocyon littoralis) and possibly feral cats, and human disturbance (McChesney 1997; T. Keeney, pers. comm.; G. Smith, pers. comm.). A large colony present in 1991 at Bay Point (San Miguel Island) has not been present since 1991, possibly due to human disturbance and access to the colony by Island Foxes. Most large colonies have occurred at the same or adjacent colony locations in all three survey periods, despite declines at some colonies. Small declines of 11% and 15% occurred at the single largest colony at the South Farallon Islands between 1989 (7,610 nests) and 2003 (6,801 nests), and between 1979-80 (8,025 nests) and 1989, respectively. Notable increases at existing colonies or new colonies that are not adjacent to other colonies occurred at Fish Rocks, Bird Rock (Marin County), Devil’s Slide Rock and Mainland, Año Nuevo Island, , Monterey Harbor, and Morro and Pillar Rock. Some Brandt’s Cormorants may have moved from the South Farallon Islands to nearby mainland areas, resulting in most of these increases and new colonies at Alcatraz and Año Nuevo Islands (see below).

Patterns of population size in California

At the South Farallon Islands, California, the size of breeding populations of Brandt’s Cormorants exhibit much annual variability due to annual differences in the proportion of adults that breed, strength of year classes of potential recruits, degree of recruitment of first-time breeders, and adult and subadult mortality (Ainley et al. 1988, Boekelheide and Ainley 1989, Ainley and Boekelheide 1990). Low population dips can occur when several factors are correlated, such as during severe El Niño years. In these years, a lower proportion of adults breed, lower recruitment of first-time breeders occurs, and high adult mortality occurs, all contributing to low breeding population size during El Niño years regardless of strength of year classes of recruits. Often such dips are followed by several years of colony growth before another dip occurs, usually associated with the next severe El Niño event (Carter et al. 1996, McChesney et al. 1998b, 1999). In contrast, large peaks in population size have not been well shown, likely because factors which can lead to peaks are rarely correlated. Instead, smaller peaks tend to occur when excellent conditions follow a period of colony growth. While the great degree of annual variability observed at the Farallon Islands may reflect the use of more variable offshore prey resources, limited data from other nearshore colonies in California also indicates that variability exists widely in size of Brandt’s Cormorant populations in California. Given this variation, comparisons of population sizes several years apart must consider whether survey years reflect high, medium or low population sizes. Population sizes within similar parts of this cycle can be reasonably compared for roughly assessing trends. However, if data from a severe El Niño year or strong La Niña year is compared with any other type of year, comparisons have much less validity.

14 El Niño in 2003

In winter 2002-03, moderate El Niño conditions developed in the California Current System, but appeared short-lived with strong upwelling conditions returning early in the cormorant breeding season by May 2003. El Niño conditions in the northern part of the California Current System appeared to be overshadowed by an anomalous intrusion of cold, nutrient-rich water from the subarctic (Venrick et al. 2003). In northern California, we did not detect any large-scale impacts of spring 2003 El Niño conditions on population sizes of Brandt’s Cormorant colonies. Possible small-scale impacts from spring 2003 El Niño conditions cannot be detected without data from additional years. The last complete count of all colonies in northern California was in 1989 (Carter et al. 1992) and few colonies have been counted since 1995 (Carter et al. 1996, unpubl. data).

Small-scale impacts were detected at certain colonies in central California. In years of mild El Niño conditions such as 2003, Brandt’s Cormorant have been noted to occupy areas where they do not regularly nest and populations tend to be spread out at more, small colonies rather than fewer, large colonies (H. Carter, G. McChesney, pers. obs.). Little effect was noted in the Gulf of the Farallones region, including the South Farallon Islands. However, the nest total at the relatively large Bird Island colony in northern Monterey County was 41% lower than in 1995, (i.e., when photographs were last counted; Carter et al. 2000), possibly due to cormorants moving to other nearby areas. Just south of Bird Island, The Lone Cypress and Yankee Point were not occupied by Brandt’s Cormorants before 2003, but together accounted for more than 1,000 nests in 2003. Overall, numbers of nests in northern Monterey County were similar in 2003 and 1995.

On the Big Sur coast, the situation was very different in 2003. Two groups of adjacent colonies were empty (i.e., group #1 [McWay Rocks, Partington Ridge South, Anderson Canyon Rocks, and Burns Creek Rocks]; group #2 [Unnamed Rock and Redwood Gulch Rock]) and numbers were reduced at almost all other colonies. However, some birds may have changed colony locations (e.g., some birds from group #2 may have moved to Seastack south of Redwood Gulch or Ragged Point Lodge which had higher numbers of nests than in 1989). Overall, the total number of nests at colonies along the Big Sur coast from Pfeiffer Point through Piedras Blancas Island were 43% lower in 2003 than in 1995. Similarly, from Point Buchon to Point Arguello (southern San Luis Obispo and northwest Santa Barbara Counties) a total of 299 nests was counted in 2003 versus more than 1,000 nests in May 2001 (HSU and USGS, unpubl. data). However, at the Morro Rock and Pillar Rock colony in central San Luis Obispo County, the number of Brandt’s Cormorant nests was more than 300 nests higher in 2003 than in 2001 (HSU and USGS, unpubl. data). Thus, the overall effect of mild El Niño conditions on the number of Brandt’s Cormorant nests in central California in 2003 was small, although relative effects at certain colonies were large.

Nest totals for 2001 in southern California were not affected by El Niño since upwelling conditions were stronger-than-normal in spring and summer 2001 (Venrick et

15 al. 2003). Southern California photographs from 2003 need to be counted to detect possible effects of El Niño conditions in 2003 at these colonies.

1989-91 Comparisons

Prior to 2003, the most recent strong El Niño event occurred during the 1998 breeding season. In northern California, much nest abandonment was noted at Castle Rock in 1998 (Jaques and Strong 2001). Reduced breeding also occurred at the South Farallon Islands and Drake’s Bay Colony Complex in central California (USFWS and HSU, unpubl. data; Carter et al. 2003) and at several colonies in southern California (Carter et al., unpubl. data). By 2003, Brandt’s Cormorant populations had been recovering for five years, and 2003 likely represented a moderately large breeding population within the overall pattern of oscillating population size. In southern California, population size in 1997 had substantially recovered in five years after the severe 1992 El Niño event (McChesney et al. 1998a). Data from surveys in 1989 also likely reflected a relatively large breeding population size in northern and central California since six years had passed since the last strong El Niño in 1983 (Ainley and Boekelheide 1990). Thus, we considered 2003 and 1989 similar types of years for comparisons in northern and central California that were not strongly influenced by dips in population size during severe El Niño events.

Nest totals in 2003 were 11% lower in central California and 23% lower in northern California than in 1989. Given that nest totals are an index of and underestimate population size, and that some survey error may be involved, we considered that the 11% difference in central California might not reflect true change (i.e., decline may not have occurred), while the 23% difference in northern California probably exceeded possible survey error and more strongly suggested a true decline. The absence of Brandt’s Cormorants at Sister Rocks and lower numbers at Redding Rock may reflect human disturbance at these locations, as well as disturbance from California Sea Lions (Zalophus californianus) at Redding Rock (Carter et al. 2001). Lower numbers at Flatiron Rock and False Cape Rocks by 2003 may reflect growth of Common Murre colonies with partial exclusion of Brandt’s Cormorants over time (Carter et al. 2001). However, 2003 is the only year since 1989 when Brandt’s Cormorants have nested at Sugarloaf Island (Capitolo et al. in prep). Much lower numbers in central California in the general vicinity of the Diablo Canyon Nuclear Power Plant (i.e., 60 nests in 2003 versus 812 nests in 1989) may partly reflect disturbance related to greater military overflights since 11 September 2001 related to national security concerns.

In 2001, only three years had passed since the severe 1998 El Niño and Brandt’s Cormorant populations in southern California probably had not yet fully recovered from the population dip. In 1991, eight years of recovery since the 1983 El Niño had occurred and the largest recorded populations were documented prior to a dip during the severe 1992-93 El Niño event (Carter et al. 1992, 1996). Thus, comparisons of 2001 and 1991 data for southern California are complicated by comparing a medium year (2001) with a high year (1991). However, 56% lower numbers in 2001 than in 1991 also likely reflects

16 some degree of decline, based on an overall declining trend shown with annual surveys since 1991 (Carter et al. 1992, 1996, McChesney et al. 1998a, 2001).

1975-80 Comparisons

Brandt’s Cormorant populations in 1979-80 in northern and central California also apparently reflected high years within the pattern of oscillating population size (Boekelheide et al. 1989, Ainley and Boekelheide 1990). Thus, we considered these years to be generally comparable to 1989 and 2003. However, 1979-80 aerial photographic surveys were conducted in less standardized time periods, including late June and July, outside the late May-early June period that has been standard since 1985. In addition, a combination of boat and aerial surveys were used at several colonies and earlier protocols for counting aerial photographs were not well described and likely included rough counts of groups of birds, rather than specific counts of individual birds and nests which have been standard since 1985 (see Carter et al. 2001). Thus, 1979-80 surveys at many colonies either may have over-estimated or under-estimated the raw nest total, compared to how data have been handled in 1989 and 2003. However, the large magnitude of difference in nest totals (i.e., 38% lower total nests in northern California in 2003 than in 1979-80, likely exceeding potential survey error) further suggests that true decline occurred in northern California from 1989 to 2003. For central California, the 2003 nest total was 3% higher than in 1979-80. However, 53% of the central California population in 1979-80 occurred at the South Farallon Islands where boat and ground counts likely underestimated the population size (see below). Thus, true increase in central California from 1979-80 to 2003 likely did not occur and some decline may have occurred.

In southern California, boat and ground surveys (including landing on colonies and flushing breeding birds) were primarily used to determine nest counts in 1975-77, and low effort was expended to conduct counts at some of the Channel Islands which may have resulted in missing small colonies. Some aerial surveys of Channel Islands colonies were also conducted (Hunt et al. 1979; S. Speich, pers. comm.; M. Naughton, pers. comm.). We consider these surveys not directly comparable with 1991 boat and aerial surveys and 2001 aerial surveys, but comparisons still provide rough information on population changes. Much lower numbers at Prince Island and Gull Island in 1975-77 probably reflected researcher disturbance at these locations. At Santa Barbara Island, Brandt’s Cormorants nested mainly on Webster Point which was not visited but studies of Western Gulls on the west side of the island since the early 1970s on slopes adjacent to Webster likely caused flushing with egg or chick losses to gulls at least on occasion. At Vizcaino Point East, lower numbers in 1975-77 likely reflected greater and more widespread military activities which were reduced on the west end of San Nicolas Island by 1991. Nevertheless, much larger numbers in 1991 on the north sides of Santa Rosa and Santa Cruz Islands, where disturbance was much lower, suggests population increase, although low survey coverage occurred in these areas 1975-77. Population increase also has occurred at Anacapa Island, although relatively small numbers occur there (Carter et al. 1992; F. Gress, unpubl. data).

17 New Colonies

Many new Brandt’s Cormorant colonies have formed since 1989-91 surveys, particularly in central California. Given the pattern of oscillating population sizes and similar central California nest totals in 1989 and 2003, formation of these colonies is likely due to several factors. In the Gulf of the Farallones, changing locations or types of prey resources during the warm phase of the Pacific Decadal Oscillation (PDO) may have led to a partial re-distribution of Brandt’s Cormorants from the offshore South Farallon Islands to nearshore colonies in central California (McChesney et al. in prep.; see below). Colonies at Año Nuevo Island and Alcatraz Island have grown steadily since their formations in 1989 and 1995, respectively (Carter et al. 1992, 1996, 2000; USFWS and HSU, unpubl. data).

Several new colonies in Monterey County may reflect local changes in breeding success, prey resources, or disturbance events which lead to shifting of colonies to nearby areas. Human disturbance from aircraft and boats is thought to have affected the distribution and population sizes of colonies in the southern portion of central California (e.g., Carter et al. 1998). The availability of artificial habitat also facilitated colony formation on a jetty in Monterey Harbor in recent years.

South Farallon Islands

The Brandt’s Cormorant colony at the South Farallon Islands is the largest in the world. A peak population size of 28,000 breeding birds (or 14,000 nests) has been reported (Sowls et al. 1980). However, Ainley and Boekelheide (1990) clarified that the actual peak estimate of 23,800 breeding birds (or 11,900 nests) occurred in 1974 and was based on an unverified extrapolation of ground-based bird counts from small plots with k correction factors. For Table 5, we lacked an exact number to use for the South Farallon Islands in 1979-80, because: a) unlike other colonies in northern and central California, it was not surveyed with aerial surveys (Sowls et al. 1980); and b) only a histogram without exact estimates was provided for island-based surveys (Ainley and Boekelheide 1990: 170). In 1979 and 1980 (respectively), about 9,000 and 8,000 nests could be roughly judged from the histogram. However, the 1980 value was based on a direct count of occupied sites around the islands using boat and ground surveys (i.e., more comparable to later aerial surveys), whereas the 1979 value was based on an unverified extrapolation of bird counts from small plots (i.e., less comparable to aerial surveys). For 1979-80, we chose to use the reported mean population estimate (i.e., 8,025 nests) from 1971-86 boat and ground surveys for Table 5 because it apparently was close to the 1980 histogram value.

By the mid 1980s, a large drop in breeding population size occurred at the South Farallon Islands, after the strong El Niño event of 1982-83 and the continued warm phase of the Pacific Decadal Oscillation (PDO) that began in the mid 1970s (Ainley et al. 1988, Ainley and Boekelheide 1990). Little human disturbance occurred at the South Farallon Islands during this time and did not appear to be a factor in this drop. By 1989, bird counts from boat and ground surveys (5,095 birds adjusted with k correction factors)

18 resulted in an estimate of 5,420 nests (Carter et al. 1992), 32% lower than the 1980 value although estimation techniques may not be comparable. However, 1989 aerial surveys demonstrated: a) a much higher total of 7,610 nests counted; b) a much higher total of 9,016 birds counted; and c) that boat and ground surveys had severely underestimated population estimates by 35-48% compared to aerial surveys (Carter et al. 1992). We believe that 1989 and subsequent aerial surveys have provided the first accurate estimates of population size and possibly trends for this colony. However, in the absence of a comparable number for comparison to later aerial surveys, we have simply used the 1980 value of 8,025 nests as a rough estimate. We caution the reader that this value is likely to be a large underestimate of actual breeding population size in 1980 and not directly comparable to aerial survey estimates. Detailed comparisons of breeding population estimates derived from aerial surveys and from boat and ground surveys were made for Common Murres at the South Farallon Islands (Takekawa et al. 1990, Sydeman et al. 1997) and are similarly needed for Brandt’s Cormorants.

The mid-1980s drop in population size at the South Farallon Islands had been associated with the loss of banded individuals, presumably through mortality (Ainley et al. 1988, Boekelheide and Ainley 1989). However, we propose an alternate explanation that some missing birds may have moved from the offshore South Farallon Islands to nearshore colonies in central California. Combined nest totals for all nearshore colonies in central California were 38% higher in 1989 than in 1979-80, demonstrating population growth at the same time as the drop at the South Farallon Islands. Most growth was evident south of Monterey Bay (Table 5). Birds moving from the South Farallon Islands in the mid 1980s likely appended to various colonies in central California, but without detectable increases at colonies north of Monterey Bay (Table 5; McChesney et al. 1998b, Carter et al. 2003). However, by the mid 1990s, new colonies formed at Año Nuevo Island and Alcatraz Island possibly reflecting continued movements of Farallon birds to adjacent mainland locations (Carter et al. 1996).

DOUBLE-CRESTED CORMORANT

California Summary

In 2001-03, the total number of Double-crested Cormorant nests in coastal California was 48% higher than in 1989-91 and 592% higher than in 1975-80. Given the relatively large degree of increase which far exceeds survey error within one survey year and no apparent effect of El Niño in 2003, these data indicate that substantial population increase on the order of about 35-45% has occurred in coastal California since 1989-91 (see further discussion below). Since 1989-91, large increases occurred in northern (80%) and central California (76%), but 38% decline occurred in southern California. Since 1975-80, large increases have occurred in northern (310%), central (2,686%), and southern (232%) California.

The overall distribution of Double-crested Cormorant colonies has remained similar between 1975-80 and 2001-03, except for formation of new colonies on artificial in Humboldt, San Francisco, and San Diego Bays. Of 61 colony locations

19 recorded in California, 28%, 64%, and 69% were active in 1975-80, 1989-91 and 2001- 03, respectively. The relative distribution of Double-crested Cormorants between northern, central and southern California has changed between 1975-80, 1989-91, and 2001-03: a) northern California has accounted for 65%, 32%, and 39% of California populations, respectively; b) central California has accounted for 13%, 43%, and 51% of California populations, respectively; and c) southern California has accounted for 22%, 25%, and 11% of California populations, respectively (Table 3).

In addition to the formation of new colonies, population growth has occurred at most existing colonies in northern and central California, except for some small colonies that were no longer used. Human disturbance does not appear to have been a great problem for Double-crested Cormorants, with some new colonies being established on artificial habitats with some human disturbance. These results refute statements that Double-crested Cormorant populations are declining throughout the west coast region from Baja California to British Columbia, except for increases at East Sand Island, Oregon (Anderson et al. 2004). Population increase associated with new colony formations is occurring over a substantial portion of the central range of the west coast population in northern California, central California, and Oregon.

Patterns of population size in California

Population sizes of Double-crested Cormorant colonies on the California coast have increased in recent decades following historical declines and extirpations at many colonies from the mid 1800s to the 1970s, apparently due to human disturbance, loss of inland nesting and feeding habitats, persecution, and pollutants (Gress et al. 1973, Sowls et al. 1980, Carter et al. 1992, 1995a,b). Historical colony decline is best documented at the South Farallon Islands and Anacapa Island. Recent population growth on the California coast has been facilitated by increased protection, availability of artificial nesting habitats (e.g., bridges, electrical power towers, and man-made levees), and growth of inland populations which are likely connected to some degree with coastal populations. In contrast to Brandt’s Cormorants, coastal Double-crested Cormorants forage primarily in estuaries in California, where cormorant prey resources may be less affected by El Niño conditions. At certain colonies, Double-crested Cormorants can show reduced breeding during some severe El Niño events (but not others) and birds may move to different colony locations in response (Ainley and Boekelheide 1990; Carter et al. 1995; Stenzel et al. 1995; Capitolo et al. in prep.). Overall, Double-crested Cormorants show less annual variation in breeding population size than Brandt’s Cormorants, and Double-crested Cormorant populations appear to be increasing steadily. Thus, less concern exists about comparing single years of data that are a decade apart for a rapid assessment of population trends, unless one or both years compared are severe El Niño events. Although nesting by Double-crested Cormorants in northern and central California is earlier than in southern California, Double-crested Cormorants also appear to vary less than Brandt’s Cormorants with respect to timing of breeding between years. However, as mentioned above for Brandt’s Cormorants, nest totals from aerial photographic surveys are an index of population size and similarly underestimate Double-crested Cormorant population size.

20 El Niño in 2003

In 2003, Double-crested Cormorant colonies in northern and central California appeared unaffected by the moderate El Niño conditions in spring. Most previously known colonies were attended and breeding phenology appeared “normal”, as many Double-crested Cormorant nests had chicks at the time of aerial surveys.

Nest totals for 2001 in southern California were not affected by El Niño since upwelling conditions were stronger-than-normal in spring and summer 2001 (Venrick et al. 2003). However, strong El Niño conditions in 1998 resulted in a greatly reduced breeding population (Carter et al., unpubl. data) and colonies in southern California may not have fully recovered by 2001 (Table 6). Southern California photographs from 2003 need to be counted to examine the possible effects of 2003 El Niño conditions.

New Colonies

The large increase in northern California since 1989 is accounted for primarily by the formation of new colonies in Humboldt, Arcata, and Tomales Bays (Table 4). Colonies in the northernmost part of the region near the Smith and Klamath Rivers increased only slightly. The Teal Island colony in Humboldt Bay formed in 1993 on a man-made levee (Carter et al. 1996) and grew to over 1,000 nests in 1997 (Capitolo et al. in prep). The Arcata Bay Sand Islands colony, where Caspian Terns have recently nested since 2002 (see Appendix B), was newly formed in 2003. Small numbers of nests may have been present in 2002, as ‘velvety black’ birds too small to have fledged from the nearby Old Arcata Wharf colony were noted in late summer (M. Colwell, pers. comm.). Efforts by USFWS to minimize disturbance to the terns from nearby active oyster cultivation (R. Brown, pers. comm) may also have facilitated use of the island by cormorants in 2003. However, no cormorants nested there in 2004 when two scarecrows were observed on Sand Island and human disturbance apparently continued. Cormorants have nested on artificial habitat at the nearby Old Arcata Wharf colony for decades, but disturbance likely prevented them from nesting on Sand Island. Breeding phenology at the Arcata Bay Sand Islands colony in 2003 differed from other colonies, as no nests with chicks were seen in aerial photographs. This colony may have formed later in the season than other colonies, perhaps by birds moving from the Teal Island colony.

Most of the increase in the Russian River area is accounted for by the Hog Island colony in , which first formed in 2001. No nests were present during several visits to the island in 2000 and 12 nests were noted from a ground survey on 6 August 2001. A 30 June ground survey detected a minimum of 56 nests active in 2002, and 96 nests were counted from the ground on 18 July 2003 (J. Kelly and S. Allen, pers. comm.). A slightly higher total (112 nests) was counted in aerial photographs in late May 2003 (Table 4). In addition, Double-crested Cormorants were first noted at Gull Rock in 1991, where they were absent in 1989 (Carter et al. 1992, 1995a).

In central California, five new Double-crested Cormorant colonies have formed since 1989-91 in the San Francisco Bay area. Formation of new colonies at Lake

21 Merced, Lake Merritt, Dumbarton Bridge Power Towers, Bair Island Power Towers, and Alviso Plant, Pond Nos. A9 and A10 may reflect increases at and movements of birds from the RSRB and SFOBB colonies (see below). Increase in the Morro Bay area in San Luis Obispo County is accounted for by a new Double-crested Cormorant colony at Shell Beach Rocks that was first noted in 1999 (McChesney et al. 2001), along with increases at the Fairbank Point colony. Cormorants nest in trees at both of these colonies. In southern California, only two small colonies have formed since 1975-77, South San Diego Bay Saltworks (Carter et al. 1996) and Seal Cove Area on (McChesney et al. 2001).

Formation of new colonies may involve local population increase and inter- colony movements or possibly some immigration from other coastal or inland colonies. However, the relatively small degree of increases observed in northern and central California is consistent with local population increase alone. Similarly, the lack of new colonies and possible declining trend in southern California suggests little or no immigration.

South Farallon Islands

In the mid 19th century, the South Farallon Islands colony in central California numbered in the thousands, but human occupation of the islands, construction of a lighthouse, and disturbance associated with commercial egging of Common Murre colonies caused a dramatic decline in the Double-crested Cormorant population by the end of the 19th century, and the colony became confined to the slopes and peak of Maintop on West End Island (Ainley and Lewis 1974, Ainley and Boekelheide 1990, Carter et al. 1995a, 2001). Less than 50 pairs nested on Maintop until the early 1970s, when the number of nests began to increase (Ainley and Boekelheide 1990). By 1989, the colony had grown to 475 nests and the nesting area on Maintop was near saturation (Carter et al. 1992, 1995a). Biologists from Point Reyes Bird Observatory conduct monitoring and research on Southeast Farallon Island year-round, and human activities near habitations, paths, and within accessed study areas, as permitted by and conducted cooperatively with the U.S. Fish and Wildlife Service (San Francisco National Wildlife Refuge), likely prevent Double-crested Cormorants from recolonizing former nesting areas (Carter et al. 1992). Cormorants banded as chicks on the Farallones have been noted nesting on the RSRB in San Francisco Bay (Stenzel et al. 1995; Rauzon et al. 2000) and movements of birds to San Francisco Bay during the 1982-83 El Niño may have led to the formation of the bridge colonies (Carter et al.1995a; Stenzel et al. 1995). Constraints to growth of the South Farallon Islands colony may additionally cause birds to move to San Francisco Bay or other colonies (Carter et al. 1992).

Colonies on bridges in San Francisco Bay

The SFOBB and RSRB colonies in San Francisco Bay were first noted in 1984 and have since become the largest Double-crested Cormorant colonies in central California (Carter et al. 1992, 1995a). Construction activities for the new east span of the SFOBB began in 2002 and were ongoing during the 2003 breeding season (Figures 3, 4).

22 To minimize disturbance to nesting cormorants during construction, the California Department of Transportation directed the contractor to avoid passage of crew boats beneath the existing bridge between piers E7 through E10 during the breeding season (R. Slabaugh, pers. comm.). Construction of the new span is scheduled for completion in 2009, with dismantling of the existing span after completion of the new span. Potential nesting platforms will be installed inside the new span upon its completion (M. Rauzon, pers. comm.). While it is unclear to us whether or not cormorants will use these platforms, others believe that cormorants will readily use the new platforms on the new span because these cormorants are acclimated to noise and human activity on the existing span and are accustomed to nesting in this general location (M. Rauzon, pers. comm.; M. Melandry, pers. comm.). Cormorants nesting on the existing span will be exposed to high levels of human activity and construction noise for several years, and then may not find suitable nesting habitat for some time after removal of the existing span. During the 2003 breeding season, construction activities including loud pile-driving for new pier bases took place along the north side of the colony. Construction activities and noise levels near the colony during the 2004 breeding season will likely be similar to 2003 levels, but reduced in following years (M. Rauzon, pers. comm.; R. Slabaugh, pers. comm.). At the RSRB, seismic retrofitting of pier bases also was underway in 2003 and will likely require several years for completion. Large barges were anchored near the colony, although little noise occurred during our brief survey in 2003. A boat exclusion zone had been established around Castro Rocks at the east end of the bridge (adjacent to the cormorant colony) to reduce potential disturbances to Harbor Seals (Phoca vitulina). Overall potential sources of disturbance at the RSRB were much less than at the SFOBB.

A comparison of the total numbers of SFOBB nests in 2003 compared to 1999 indicated the large-scale abandonment had not occurred due to construction activities. At SFOBB, we counted 745 nests on our single survey on 20 June, only 6% lower than the peak count of 796 on 18 June 1999 (Rauzon et al. 1999; Table 8). However, given increasing populations of Double-crested Cormorants in the San Francisco Bay area over the past two decades (Table 5), numbers of nests at the SFOBB likely increased between 1999 and 2002 such that a greater decrease may have occurred between 2002 and 2003. The distribution of nests was also similar to 1999 surveys, with 79% and 81% of the nests occurring between piers E7 and E10 in 2003 and 1999, respectively (Table 8).

The total number of RSRB nests in 2003 (n = 632) was 11% higher than peak counts in 2000 (n = 569), also reflecting little or no impacts from sources of potential human disturbance related to bridge repairs. We counted 632 nests on our single survey date on 20 June, 398 nests in cords (including 6 nests on scaffolding) and 234 nests under the bridge (Table 9). Rauzon et al. (2000) reported that their single day high counts were 377 nest for the cords (3 June) and 192 nests under the bridge (16 June), corresponding to 6% and 22% lower counts than in 2003, respectively. As noted earlier in this report, single day counts of cormorant colonies often underestimate total breeding population size. In 2000, peak counts corresponded to 85% of the total number of breeding pairs over the season (n = 669). Our 2003 cord count is also likely a slight underestimate since nests in the cords are difficult to count during boat surveys. In 2003, nests in the cords were counted from boat by observing birds in incubating posture with their tails or heads

23 visible, or by observing chicks, with appropriate spacing assumed between nests. Earlier surveys at the RSRB counted cord nests from the roadbed (i.e., above nests), where nest structures and birds can be counted directly without error (Stenzel et al. 1995, Rauzon et al. 2000). At SFOBB, cormorants do not nest in cords and previous surveys have been conducted entirely from below the bridge.

Growth of the RSRB colony and lower numbers of nests at SFOBB colony may indicate that small numbers of cormorants shifted from the SFOBB to the RSRB, possibly in response to higher levels of human disturbance at the SFOBB. However, birds may also shift to or from other colonies in the San Francisco Bay area that are farther away. Monitoring of population sizes of all colonies in the San Francisco Bay area should continue annually to document how Double-crested Cormorants respond to construction activities at these bridges. In particular, we do not expect that all Double- crested Cormorants will merely switch from the existing SFOBB to the new SFOBB. Instead, we expect that the largest colony in central California at the SFOBB will be affected to some degree, with movements of birds to other colonies. The recent formation of and increases at five new colonies in the San Francisco Bay area may already signal movements of birds from bridge colonies to other colonies, possibly prior to and during construction activities at the SFOBB and the RSRB. However, the ability of most Double-crested Cormorants to apparently withstand major noise and human disturbance at the SFOBB in 2003 was remarkable and may signal that many may remain at the SFOBB during the construction period and possibly colonize nest platforms on the new SFOBB. For a full assessment of cormorant population changes in the San Francisco Bay area, it will be necessary to count archived annual aerial photographs for non-bridge colonies.

ACKNOWLEDGEMENTS

Funding for 2003 surveys was provided to Humboldt State University by the U.S. Fish and Wildlife Service (Region 1, Office of Migratory Birds and Habitat Programs, Portland, Oregon), through the efforts of M. Naughton. Collaborative efforts by the U.S. Fish and Wildlife Service (San Francisco Bay National Wildlife Refuge Complex) were funded by the Apex Houston Trustee Council (U.S. Fish and Wildlife Service, National Oceanic and Atmospheric Administration, and California Department of Fish and Game). Funding for Southern California surveys and photograph counting in 2001 were provided to Humboldt State University by the U.S. Geological Survey (Western Ecological Research Center, Dixon, California), through the efforts of D. Orthmeyer. Funding for flight support in 2003 was provided by the California Department of Fish and Game (Office of Spill Prevention and Response, Sacramento, California) and U.S. Navy (Naval Air Warfare Center, Weapons Division, Point Mugu, California) through the efforts of P. Kelly and S. Schwartz, respectively. Printing this report was paid for in part by Apex Houston Trustee Council.

In 2003, aerial photographic surveys were safely and efficiently conducted with expert piloting by W. Burnett and L. Heitz (California Department of Fish and Game, Air Services, Sacramento), and D. Federson and P. O’Brien (Aspen Helicopters, Inc.,

24 Oxnard, California). J. Buffa, C. Caurant, E. Creel, J. Irwin, H. Knechtel, J. Mason, and E. Nelson assisted with aerial photographic surveys. N. Jones, J. Koepke, T. Poitras, A. Robinson, and K. Bixler assisted with counting aerial photographs of central California colonies for the Common Murre Restoration Project. Aerial surveys in 1979-80 were conducted by A. L. Sowls, A. R. DeGange and others. Aerial surveys in 1989-91 were assisted by J. E. Takekawa, M. W. Parker, E. Nelson, D. L. Jaques, C. S. Strong, L. Vicenzio and others.

Aerial surveys in northern and central California in 2003 were conducted under permit number GFNMS/MBNMS-2000-003-G provided by the Gulf of the Farallones and Monterey Bay National Marine Sanctuaries. Aerial surveys in southern California were conducted under a permit provided by the Channel Islands National Marine Sanctuary. J. Y. Takekawa (U.S. Geological Survey, Western Ecological Research Center, Vallejo, California) loaned us a motor and inflatable boat for surveys of bridge colonies. Assistance with surveys of bridge colonies was provided by R. Slabaugh (California Department of Transportation) and S. Granholm (LSA Associates). Administrative assistance for this work was provided by B. Clueit and R. Okey (Humboldt State University). Channel Islands National Park assisted the California Institute of Environmental Studies with field logistics at Anacapa Island, and L. Harvey assisted 2001 and 2003 Anacapa data analyses with MBHP funding. Close-up photographs during ground monitoring of northern California colonies in 2003 were taken with a digital camera provided by B. Clueit. Unpublished data and other information were provided by S. Allen, R. Brown, M. Colwell, T. Keeney, J. Kelly, D. Murphy, M. W. Parker, M. Rauzon, G. Smith, C. Strong, and J. E. Takekawa.

We appreciate valuable comments on this report provided by M. Naughton (U.S. Fish and Wildlife Service), and comments on sections related to the San Francisco Bay bridge colonies by M. Melandry, R. Slabaugh, A. Hitt (California Department of Transportation), and M. Rauzon.

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30 Table 1. Summary of dates and personnel for aerial photographic surveys in northern, central, and southern California in 2003.

Region Date Photographers Data Recorder Pilot Northern California 3 June H. Carter, G. McChesney J. Buffa W. Burnett 11 June P. Capitolo, G. McChesney E. Nelson L. Heitz Central California 28 May P. Capitolo, G. McChesney J. Irwin L. Heitz 29 May P. Capitolo, G. McChesney C. Caurant L. Heitz 30 May P. Capitolo, G. McChesney H. Knechtel L. Heitz 2 June H. Carter, G. McChesney J. Mason W. Burnett 4 June H. Carter, G. McChesney J. Buffa W. Burnett 6 June H. Carter, G. McChesney H. Carter, G. McChesney W. Burnett Southern California 29 April P. Capitolo, H. Carter P. Capitolo, H. Carter W. Burnett 19 May P. Capitolo, R. Young H. Carter D. Federson 20 May P. Capitolo, H. Carter H. Carter D. Federson 21 May P. Capitolo, H. Carter E. Creel P. O’Brien 16 June P. Capitolo, H. Carter E. Creel D. Federson

31 Table 2. Summary of numbers of nests and colonies of Brandt’s Cormorants in northern, central, and southern California in 1975-80, 1989-91, and 2001-03. Percentages of the California total appear in parentheses. Nests Active Colonies Known Coastal Section 1975-80 1989-91 2001-03 Colonies 1975-80 1989-91 2001-03 Northern California 8,669 7,046 5,397 43 35 32 30 (32) (19) (20) (38) (31) (31) Central California 15,150 17,514 15,566 67 33 39 39 (56) (46) (58) (35) (38) (40) Southern California 3,016 13,130 5,755 48 25 31 28 (11) (35) (22) (27) (30) (29) Total California 26,835 37,690 26,718 158 93 102 97

Table 3. Summary of numbers of nests and colonies of Double-crested Cormorants in northern, central, and southern California in 1975-80, 1989-91, and 2001-03. Percentages of the California total appear in parentheses. Nests Active Colonies Known Coastal Section 1975-80 1989-91 2001-03 Colonies 1975-80 1989-91 2001-03 Northern California 610 1,392 2,500 27 10 17 19 (65) (32) (39) (59) (44) (45) Central California 118 1,866 3,287 25 3 16 17 (13) (43) (51) (18) (41) (41) Southern California 208 1,113 690 9 4 6 6 (22) (25) (11) (24) (15) (14) Total California 936 4,371 6,477 61 17 39 42

32 Table 4. Total numbers of nests of Brandt’s and Double-crested Cormorants at colonies in northern California in 1979-80, 1989, and 2003 (Sowls et al. 1980; Carter et al. 1992; this study). Dashes (-) indicate that nesting by that species has not been documented at that location. Brandt’s Cormorant Double-crested Cormorant COLONY CCN USFWSCN 1979-80 1989 2003 1979-80 1989 2003 Hunter Rocks DN-414-02 325-002 340 211 0 - - - Prince Island DN-414-03 325-003 0 141 215 225 459 323 Castle Rock DN-414-06 325-006 1,100 1,132 1,0341 0 0 136 Tolowa Rocks DN-414-07 325-007 - - - 0 5 0 Whaler Island, Crescent City BW DN-414-09 325-045 20 0 0 - - - Unnamed Small Rocks DN-414-10 325-046 - - - 11 1 0 Rock R DN-412-01 325-048 70 98 24 - - - Sister Rocks DN-412-02 325-009 21 51 0 - - - Last Chance Rock DN-412-04 325-049 - - - 2 0 0 False Klamath Rock DN-412-07 325-010 178 324 218 42 13 68 Radar Station Rocks DN-412-09 325-051 - - - 10 60 72 Flint Rock Head DN-412-10 325-011 - - - 0 11 0 White Rock DN-412-11 325-012 120 186 109 0 31 33 High Bluff South DN-412-12 325-061 0 0 59 0 0 0 Split Rock DN-412-13 325-062 0 23 0 - - - Redding Rock HU-412-01 325-013 79 112 30 - - - Sea Gull Rock HU-410-05 325-017 - - - 68 54 21 Scotty Point-Megwil Point HU-410-06 325-053 1 0 0 - - - Sea Lion Rock HU-410-07 325-018 - - - 0 37 20 White Rock HU-410-08 325-019 - - - 0 0 0 Green Rock HU-410-09 325-020 108 28 36 - - - Flatiron Rock HU-410-13 325-023 770 469 254 - - - Blank Rock HU-410-14 325-024 26 46 0 - - - Pilot Rock HU-410-16 325-026 20 0 7 0 4 0 Little River Rock HU-410-19 325-035 - - - 50 226 141 Arcata Bay Sand Islands HU-404-01 325-066 - - - 0 0 809 Old Arcata Wharf HU-404-02 325-067 - - - 170 119 70 Humboldt Bay Duck Blinds2 HU-404-04 325-065 - - - 0 39 0 Teal Island HU-404-05 325-069 - - - 0 0 365 False Cape Rocks HU-402-01 325-040 200 360 178 0 373 52 Sugarloaf Island HU-402-02 325-041 100 133 26 16 114 53

33 Table 4 continued. Brandt’s Cormorant Double-crested Cormorant COLONY CCN USFWSCN 1979-80 1989 2003 1979-80 1989 2003 Steamboat Rock HU-402-03 325-042 247 211 165 - - - Three Brothers and Hair Seal Rocks HU-400-01 325-055 55 26 7 - - - Anderson Cliffs ME-394-03 379-013 157 77 34 - - - Mistake Point-Big White Rock ME-394-04 379-014 0 0 22 - - - Soldier Frank Point ME-394-06 379-016 0 0 2 - - - Rockport Rocks ME-394-07 379-001 0 385 77 - - - Cape Vizcaino ME-394-08 379-002 1,540 772 889 - - - Kibesillah Rock ME-392-04 379-004 175 30 15 - - - Newport Rocks ME-392-05 379-005 0 87 47 - - - Goat Island Area ME-390-01 379-006 620 494 476 - - - Devil’s Basin ME-390-08 379-007 150 119 25 - - - Cavanaugh Cove to Gunderson Rock ME-390-09 379-030 2 0 0 - - - Casket Rock ME-390-11 379-009 165 32 39 - - - White Rock ME-390-13 379-010 224 226 239 - - - 333 Point ME-390-14 379-032 180 0 0 - - - Fish Rock Cove ME-384-09 404-022 9 0 0 - - - Fish Rocks ME-384-10 404-003 9 96 184 - - - Gualala Point Island SO-384-01 404-004 620 237 132 - - - Russian Gulch SO-382-08 404-033 - - - 0 0 40 Russian River Rocks SO-382-09 404-005 62 23 0 16 176 119 Arched Rock SO-382-11 404-006 200 326 218 - - - Gull Rock SO-382-13 404-035 325 0 0 0 0 34 Shell-Wright Beach Rocks SO-382-14 404-036 - - - 0 0 16 Bodega Rock SO-380-02 404-008 675 558 361 - - - Sonoma-Marin County Line MA-380-01 404-040 6 0 0 - - - Dillon Beach Rocks MA-380-02 404-009 95 8 0 0 6 16 Bird Rock MA-380-04 404-010 0 25 275 - - - Hog Island MA-380-06 404-068 - - - 0 0 112 TOTAL 8,669 7,046 5,3971 610 1,392 2,500 1 Includes 20 unidentified cormorant nests that were probably BRCO. 2 Structures used for nesting no longer exist. 3 Correction to Carter et al. (1992, 1996) which reported 0 nests.

34 Table 5. Total numbers of nests of Brandt’s and Double-crested Cormorants at colonies in central California in 1979-80, 1989-1991, and 2003 (Sowls et al. 1980; Carter et al. 1992; this study). Dashes (-) indicate that nesting by that species has not been documented at that location. Brandt’s Cormorant Double-crested Cormorant COLONY CCN USFWSCN 1979-80 1989-90 2003 1979-80 1989-91 2003 Point Reyes MA-374-01 429-001 1,200 692 439 - - - Point Resistance MA-374-03 429-024 75 21 26 0 0 0 Millers Point Rocks MA-374-04 429-002 97 52 29 - - - Double Point Rocks MA-374-05 429-003 129 82 125 - - - Bird Island MA-374-09 429-007 0 0 0 - - - North Farallon Islands SF-FAI-01 429-051 ND 73 98 - - - South Farallon Islands SF-FAI-02 429-052 8,025 7,610 6,801 90 475 439 Lobos Rock and Land’s End SF-374-02 429-029 40 53 99 - - - Seal Rocks SF-374-03 429-009 38 0 0 0 0 0 Lake Merced SF-374-04 429-118 - - - 0 0 125 San Pedro Rock SM-372-02 429-013 0 0 0 - - - Devil’s Slide Rock and Mainland SM-372-03 429-014 11 3 263 - - - Pillar Point1 SM-372-04 429-030 - - - 0 0 0 Seal Rock Cliffs1 SM-372-06 429-032 25 17 0 - - - Martins Beach1 SM-372-07 429-033 20 0 0 - - - Año Nuevo Island SM-370-04 429-023 0 2 969 - - - El Jarro Point-Davenport SC-370-02 429-050 118 140 36 - - - SFB-NA-02 404-051 - - - 25 153 2 Knight Island SFB-SN-01 404-053 - - - 0 65 200 North San Pablo Bay Radar Target SFB-SN-03 404-055 - - - 0 20 15 Northeast San Pablo Bay Beacon SFB-SN-04 404-056 - - - 0 4 2 Wheeler Island SFB-SN-09 405-001 - - - 0 27 126 Donlon Island1 SFB-SA-01 405-002 - - - 0 28 0 Richmond-San Rafael Bridge SFB-CC-12 429-061 - - - 0 389 632 San Francisco-Oakland Bay Bridge E. SFB-AL-05 429-071 - - - 0 465 745 Lake Merritt SFB-AL-33 429-119 - - - 0 0 80 Alviso Plant, Pond Nos. A9 and A10 SFB-SR-04 430-005 - - - 0 0 39 Dumbarton Bridge Power Towers SFB-SM-11 429-121 - - - 0 0 0 Bair Island Power Towers SFB-SM-10 429-120 - - - 0 0 191 San Mateo Bridge & P.G.&E. Towers SFB-SM-07 429-107 - - - 0 76 105 Yerba Buena Island1 SFB-SF-07 429-038 0 4 0 - - -

35 Table 5 continued. Brandt’s Cormorant Double-crested Cormorant COLONY CCN USFWSCN 1979-80 1989 2003 1979-80 1989 2003 Alcatraz Island SFB-SF-11 429-036 0 0 589 - - - Terrace Point-Point Santa Cruz SC-364-04 454-041 0 0 54 - - - Monterey Harbor MO-362-01 454-043 0 0 247 - - - Cannery Row MO-362-02 454-044 0 0 155 - - - Bird Rock MO-362-03 454-006 670 1,205 829 - - - Cypress Point MO-362-22 454-062 0 0 0 - - - The Lone Cypress MO-362-21 454-061 0 0 194 - - - Guillemot Island Area MO-362-06 454-023 0 252 0 - - - Pinnacle Point Area MO-362-07 454-007 100 0 0 - - - Bird Island MO-362-09 454-009 2,100 2,796 1,345 - - - Yankee Point MO-362-11 454-024 0 0 872 - - - Soberanes Point South MO-362-15 454-027 0 0 33 - - - Rocky Point MO-362-17 454-028 0 0 27 - - - Bench Mark-227X MO-362-18 454-029 0 0 0 - - - Castle Rocks and Mainland MO-362-19 454-010 9 341 333 - - - Hurricane Point Rocks MO-362-20 454-011 222 13 0 - - - Cooper Point and Islands MO-360-03 454-031 36 17 0 - - - Pfeiffer Point MO-360-04 454-032 3 0 19 - - - Grimes Point MO-360-06 454-033 8 89 16 - - - Lafler Rock and Mainland MO-360-07 454-034 6 107 24 - - - Torre Canyon Rocks MO-360-08 454-013 62 0 0 - - - Partington Ridge North MO-360-10 454-014 455 78 138 3 2 0 McWay Rocks MO-360-11 454-015 10 48 0 - - - Partington Ridge South MO-360-12 454-035 0 7 0 - - - Anderson Canyon Rocks MO-360-13 454-016 0 146 0 0 2 0 Burns Creek Rocks MO-360-14 454-017 174 147 0 - - - Dolan Rock MO-360-16 454-018 50 0 0 - - - Square Black Rock MO-360-18 454-019 0 0 0 - - - Gamboa Point MO-360-19 454-053 0 0 28 - - - Lopez Rock MO-360-21 454-020 0 52 0 - - - Rockland Landing North MO-360-23 454-037 8 3 57 0 11 9 Plaskett Rock MO-354-07 477-002 0 386 13 - - - Cape San Martin MO-354-08 477-003 340 341 148 0 1 0 Unnamed Rock MO-354-09 477-004 0 125 0 - - -

36 Table 5 continued. Brandt’s Cormorant Double-crested Cormorant COLONY CCN USFWSCN 1979-80 1989 2003 1979-80 1989 2003 Redwood Gulch Rock MO-354-12 477-005 239 169 0 - - - Seastack South of Redwood Gulch MO-354-13 477-018 0 0 67 - - - Unmapped Island MO-354-14 477-019 0 14 11 - - - Ragged Point Lodge Colony SL-354-01 477-022 0 0 68 - - - 3 Rocks SL-354-03 477-023 0 0 65 - - - La Cruz Rock SL-354-04 477-006 0 308 0 - - - Piedras Blancas Island SL-352-01 477-007 600 1,194 612 - - - Morro Rock and Pillar Rock SL-352-07 477-026 0 53 438 0 10 41 Fairbank Point SL-352-08 477-044 - - - 0 138 406 Point Buchon SL-350-02 477-009 0 0 86 - - - Unnamed Rocks SL-350-03 477-010 0 0 143 - - - Pup Rock and Adjacent Mainland SL-350-04 477-028 0 595 0 - - - Lion Rock SL-350-05 477-011 50 126 602 - - - Diablo Rock & Adjacent Mainland SL-350-06 477-029 106 7 0 - - - Diablo Cyn Nuclear Power Plant S. SL-350-07 477-030 50 0 0 - - - Pecho Rock SL-350-09 477-032 74 146 0 - - - Shell Beach Rocks SL-350-13 477-035 - - - 0 0 130 Point Arguello SB-342-04 501-011 0 0 10 - - - Rocky Point SB-342-05 501-012 0 0 0 - - - TOTAL 15,150 17,514 15,566 118 1,866 3,287 1 Colony not surveyed in 2003, but assumed empty (see text); 2 Visual estimate (no photographs were taken).

37 Table 6. Total numbers of nests of Brandt’s Cormorants and Double-crested Cormorants at colonies in southern California in 1975- 77, 1991, and 2001 (Sowls et al. 1980; Carter et al. 1992; this study). Dashes (-) indicate that nesting by that species has not been documented at that location. Brandt’s Cormorant Double-crested Cormorant COLONY CCN USFWSCN 1975-77 1991 2001 1975-77 1991 2001 Sandpiper Pier Foundation SB-342-07 502-029 0 0 24 - - - Point Bennett SB-SMI-01 501-014 27 996 293 - - - Castle Rock SB-SMI-02 501-005 916 1,962 280 - - - Richardson Rock SB-SMI-03 501-026 0 6 8 - - - Northwest San Miguel Island SB-SMI-04 501-027 0 11 0 - - - Harris Point-Cuyler Harbor SB-SMI-06 501-015 5 1 50 - - - Prince Island SB-SMI-07 501-004 907 1,601 601 75 230 140 Hoffman Point Area SB-SMI-08 501-029 0 5 0 - - - Bay Point Area SB-SMI-09 501-016 0 2,548 0 - - - Crook Point-Tyler Bight SB-SMI-11 501-031 0 0 224 - - - Sandy Point SB-SRI-01 501-033 Present1 86 64 - - - Northwest Santa Rosa Island SB-SRI-02 501-034 Present1 371 457 - - - Brockway Point Area SB-SRI-03 501-035 Present1 0 0 - - - Cañada Verde-Santa Rosa I West Base SB-SRI-04 501-035 Present1 919 436 - - - Carrington Point SB-SRI-05 501-037 Present1 703 181 - - - North Becher's Bay SB-SRI-06 501-038 0 0 48 - - - Sierra Pablo Area SB-SRI-09 501-040 0 1 83 - - - South Point Area SB-SRI-11 501-042 0 1 0 - - - Bee Rock Mainland Area SB-SRI-13 501-044 0 1 70 - - - Fraser Point SB-SZI-01 502-015 Present2 24 0 - - - West Point Area SB-SZI-02 502-016 Present2 464 349 - - - Profile Point Area SB-SZI-03 502-017 Present2 24 0 - - - Cueva Valdaze Area SB-SZI-04 502-018 Present2 16 0 - - - Coche Point Northeast SB-SZI-11 502-024 0 7 0 - - - Cavern Point Area SB-SZI-12 502-025 0 12 0 - - - Scorpion Anchorage SB-SZI-13 502-026 0 1 0 - - - Scorpion Rocks SB-SZI-14 502-010 - - - 0 0 0 San Pedro Point Area SB-SZI-15 502-027 0 5 0 - - - Gull Island SB-SZI-22 524-001 67 723 385 - - - Anacapa Island – West VE-ANI-01 502-007 1 24 3063 66 360 3304 Anacapa Island – Middle VE-ANI-02 502-008 2 1 95 - - -

38 Table 6 continued. Brandt’s Cormorant Double-crested Cormorant COLONY CCN USFWSCN 1975-77 1991 2001 1975-77 1991 2001 Anacapa Island – East VE-ANI-03 502-009 0 0 74 --- Shag Rock SB-SBI-01 524-007 0 2 0 0 4 0 Santa Barbara Island SB-SBI-02 524-008 51 252 385 7 320 105 Sutil Island SB-SBI-03 524-009 76 30 26 60 172 66 Vizcaino Point South VE-SNI-01 524-022 0 0 726 - - - Vizcaino Point East VE-SNI-02 524-023 145 2,313 0 - - - Cormorant Rock Area VE-SNI-07 524-028 50 0 540 - - - White Bluffs VE-SNI-08 524-068 0 0 15 - - - Castle Rock LA-CLI-01 524-029 0 0 1 - - - Bird Rock LA-CLI-02 524-030 0 0 144 - - - Northwest Harbor to Wilson Cove LA-CLI-03 524-031 5 0 0 - - - Pyramid Cove LA-CLI-04 524-032 2 0 0 - - - Lost Point South LA-CLI-06 524-034 0 20 26 - - - Mail Point South LA-CLI-07 524-035 4 0 0 - - - Seal Cove Area LA-CLI-08 524-036 13 0 0 0 0 17 Ship Rock LA-CAI-03 524-039 0 0 0 0 0 0 Bird Rock LA-CAI-04 524-010 0 0 0 - - - La Jolla SD-324-04 545-009 3 0 17 - - - South San Diego Bay Saltworks SD-322-04 545-027 - - - 0 27 32 TOTAL 3,016 13,130 5,755 208 1,113 690 1 700 nests were found between Sandy Point and Carrington Point. 2 42 nests were found between Fraser Point and Cueva Valdaze. 3 278 nests were counted in ground/boat surveys. 4 Count is from ground/boat surveys. 5 17 nests counted in ground/boat surveys.

39 Table 7. Summary of new Brandt’s and Double-crested Cormorant nesting areas in northern and central California. Colonies in italics are newly-described seabird colonies. New subcolonies are described for other colonies. Other nesting areas not described in Carter et al. (1992) are noted in Carter et al. (1996) or McChesney et al. (1998a,b, 1999, 2001). Colony CCN USFWSCN SC # Species Description

Mistake Point-Big White Rock ME-394-04 379-014 7 BRCO Offshore rock immediately E of and adjacent to SC 1

Hog Island MA-380-06 404-068 1 DCCO Trees on Hog Island in Tomales Bay

Lake Merced SF-374-04 429-118 1 DCCO Trees at Lake Merced in San Francisco

Lake Merritt SFB-AL-33 429-119 1 DCCO Trees at Lake Merritt in Oakland

Dumbarton Bridge Power Towers SFB-SM-11 429-121 - DCCO Power towers on N side of bridge; no nests in 2003; photos from previous years not yet counted Bair Island Power Towers SFB-SM-10 429-120 1 DCCO Power tower on N side of Steinberger Slough

Bair Island Power Towers SFB-SM-10 429-120 2 DCCO Power tower on S side of Steinberger Slough

San Mateo Bridge & P.G.&E. Towers SFB-SM-07 429-107 6-11 DCCO Consecutive power towers E of SC 5

Cypress Point MO-362-22 454-062 - BRCO Offshore rock and mainland; no nests in 2003; photos from previous years not yet counted The Lone Cypress MO-362-21 454-061 1 BRCO Mainland area between Sunset Point & Midway Point

Bird Island MO-362-09 454-009 10 BRCO Rock SE of SC 2; connected to mainland at low tide; see also Carter et al. (2000) Yankee Point MO-362-11 454-024 1 BRCO Mainland area adjacent to Yankee Point Rock; N and S sides of cove Rocky Point MO-362-17 454-028 4 BRCO Rock adjacent to mainland between SC 2 and 3; below restaurant

Lafler Rock and Mainland MO-360-07 454-034 4 BRCO Mainland N of SC 2

Partington Ridge North MO-360-10 454-014 6 BRCO Offshore rock between SC 3 and 4; see also Carter et al. (2000)

Anderson Canyon Rocks MO-360-13 454-016 8 BRCO Second offshore rock S of SC 7; no nests in 2003; occupied in 1994 - see Carter et al. (2000) Unnamed Rocks SL-350-03 477-010 16 BRCO Rock adjacent to mainland SC 1

40 Table 8. Distribution of Double-crested Cormorant nests by pier range at the San Francisco-Oakland Bay Bridge East colony, California, in 1999 and 2003. Data from 1999 are from Rauzon et al. (1999). Double-crested Cormorant Nests Pier Range 18 June 1999 20 June 2003 E3-E4 0 1 E4-E5 25 27 E5-E6 34 34 E6-E7 53 42 E7-E8 115 135 E8-E9 420 329 E9-E10 110 121 E10-E11 20 21 E11-E12 8 19 E12-E13 8 6 E13-E14 1 10 E14-E15 2 0 Total 796 745

Table 9. Distribution of Double-crested Cormorant nests by pier range and bridge structures at the Richmond-San Rafael Bridge colony, California, in 2003. Double-crested Cormorant Nests Pier Range North Cord South Cord Beams Gussets/Plates Travelers Total 51-52 22 1 0 7 7 37 52-53 41 10 0 7 24 82 53-54 571 28 5 10 34 134 54-55 51 42 19 2 46 160 55-56 55 29 5 5 35 129 56-57 36 26 0 12 16 90 Total 262 136 29 43 162 632 1 Includes 6 nests on scaffolding.

41 Table 10. J correction factors at Double-crested Cormorant colonies, subcolonies, or plots, and for all areas combined, in northern California in 2003. Seasonal Nest Aerial Survey J Correction Colony Area Total Nest Total Factor Radar Station Rocks Subcolony 2 (all) 4 3 1.33 Radar Station Rocks Subcolony 4 (plot) 6 6 1.00 White Rock Low east plot 23 14 1.64 White Rock High south plot 22 19 1.16 Sea Gull Rock Sea Gull Rock (all) 23 21 1.10 Total 78 63 1.24

42

Figure 1. Brandt’s Cormorant nests in a close-up aerial photograph of Prince Island, Del Norte County, California, 11 June 2003. Photo 03-CA-23-32.

43

Figure 2. Double-crested Cormorant nests in a close-up aerial photograph of Prince Island, Del Norte County, California, 11 June 2003. Compared to Figure 1, note the wider spacing between nests and the obvious presence of chicks in most nests due to earlier breeding phenology of Double-crested Cormorants. Sticks are also obvious in nests when the slide is projected. Photo 03-CA-23-32.

44

E9

E8

Figure 3. San Francisco-Oakland Bay Bridge East and construction activities for the new east span. Piers E8 and E9 indicate the center of the Double-crested Cormorant colony in 2003 (see text). Photo taken from the San Francisco Chronicle, 11 January 2004, used with permission. Photographer: Darryl Bush.

45

Figure 4. The San Francisco-Oakland Bay Bridge, 20 June 2003. The barge and pile- driver shown here are located at the east end of the Double-crested Cormorant colony.

52 51

Figure 5. The Richmond-San Rafael Bridge, 20 June 2003. Buoys mark a boat exclusion zone beginning between piers 51 and 52.

46 BRCO HS DCCO

LE DCCO

Figure 6. Location of study plots for ground monitoring of Brandt’s (BRCO) and Double-crested cormorants (DCCO) at White Rock, Del Norte County, California, 9 May 2003. HS = High South study plot; LE = Low East study plot.

Figure 7. Close-up digital photograph of the Low East Double-crested Cormorant study plot at White Rock, Del Norte County, California, 3 June 2003.

47 APPENDIX A. Numbers of Common Murres at Castle Rock National Wildlife Refuge in 2003.

We conducted an aerial photographic survey of Castle Rock National Wildlife Refuge, Del Norte County, California, on 11 June 2003. Standard methodology was used to take and count aerial photographs (Carter et al. 2001). With MBHP funding, PJC and RJY pieced together 117 close-up photographs and derived a total raw count of 62,504 Common Murres. By multiplying this raw count by a k correction factor of 1.67 to account for some breeding birds not attending the colony and attendance by non-breeding birds during the survey (Carter et al. 2001), we derived a breeding population estimate of 104,382 breeding murres (rounded to 104,400 breeding murres or 52,200 breeding pairs). To analyze possible patterns of population growth or decline on certain parts of the island, we also determined subtotals of the total raw count for the west portion of the island and for six distinct subareas on the east portion of the island. We re-inspected archived 1989 photographs and dotting sheets to generate similar subtotals for comparison.

In 2003, we derived raw total counts and subarea counts similar to those from 1989 (Carter et al. 1992; Table A-1). Numbers were slightly higher for east island subareas and slightly lower for the in 2003 than in 1989. Overall estimated numbers of breeding Common Murres were remarkably similar between 1989 (107,673; rounded to 107,700) and 2003 (104,400). This survey has validated that numbers of Common Murres were relatively high at Castle Rock in 2003, providing valuable information for the U.S. Fish and Wildlife Service (Humboldt Bay National Wildlife Refuge), which manages this colony. However, it is necessary to count additional photographs from other years to better determine whether or not this colony has: a) declined since 1982; b) recovered since 1997-99; or c) has been relatively stable since 1982 with large differences in estimates related to difficulties with photographing and counting photographs of this large colony (Takekawa et al. 1990, Carter et al. 1992, 2001, Jaques and Strong 2001). Excellent photographs were obtained in 2003 and in 1998-99, but coverage was incomplete in 1997 and of lower quality in earlier years. Personnel counting 2003 photographs also had greater recent training and experience than those counting 1997-99 photographs. Several years of photographs are available for counting to better assess potential problems (1985, 1987-88, 1990, 1993-96, 2000-02; see Carter et al. 2001).

The amount of vegetated area on the southeast portion of the island in 2003 was noticeably reduced compared to 1989 photographs, and murres and Brandt’s Cormorants nested in these denuded areas. Double-crested Cormorants were first noted nesting at Castle Rock in 1997 in vegetation on the southeastern rim of the island (Jaques and Strong 2001) and are likely contributing to the loss of vegetation through nest construction. Double-crested Cormorants nested only on bare slopes and cliffs in 2003, but utilize terrestrial vegetation in nest construction while Brandt’s and Pelagic Cormorants use primarily marine algae in nest construction. Reduction of vegetated areas may benefit Common Murres by opening up preferable nesting habitat, but may have significant negative impacts on burrow-nesting seabirds at Castle Rock. The

48 Leach’s Storm Petrel (Oceanodroma leucorhoa) colony at Little River Rock has undergone a dramatic decline following the growth of the Double-crested Cormorant colony there and the subsequent denuding of the island of vegetation and soil (Carter et al. 1992). Aleutian Canada Geese (Branta canadensis leucopareia) use Castle Rock as a staging area in spring and are also likely contributing to the loss of vegetation. In 1999 as many as 27,000 geese used Castle Rock for night-roosting and foraging (Jaques and Strong 2001).

Table A-1. Raw whole-colony and subcolony counts of Common Murres at Castle Rock National Wildlife Refuge, California, from aerial photographic surveys in 1989 and 2003. Area 1989 2003 Southeast Corner 9,921 13,604 Northeast Cliffs 249 10 North Central 1,074 2,564 Southwest Veg. Group 38 103 South Central Ridge 417 398 East Island Subtotal 11,699 16,679 West Island 52,776 45,825 Castle Rock Total 64,475 62,504

49 APPENDIX B. Numbers of Caspian Terns from aerial photographic surveys at Arcata Bay Sand Islands and Knight Island in 2003.

Current studies are underway at various locations in Washington, Oregon, and California by the U.S. Geological Survey (Oregon Cooperative Fish and Wildlife Research Unit, Oregon State University, Corvallis, Oregon), with partial funding from MBHP, to better assess distribution, reproductive success, diet and movements of Caspian Terns (Roby et al. 2003a,b,c). Our surveys of four coastal tern colonies in 2003 (Table B-1) should help facilitate future monitoring of colonies with aerial photographic surveys by helping to develop correction factors for relating numbers of birds to numbers of nesting attempts (Colwell et al. 2003, Roby et al. 2003c). To date, we have counted two tern colonies which co-occurred with Double-crested Cormorant colonies.

Our aerial survey count of terns at Arcata Bay Sand Islands differed considerably from weekly ground counts by Colwell et al. (2003). Ground counts of Sand Island were conducted at high tide from April through August from 350 m northeast of the colony on Little Sand Island. A high count of 97 terns occurred on 11 May, much lower than the aerial survey count of 262 on 3 June. We believe most of this difference is due to the complete and unobstructed view of the tern colony possible from aerial photographs. The entire tern colony likely could not be seen well from Little Sand Island (Colwell et al. 2003). However, the aerial survey took place at a lower tide than ground counts, and more terns may have been present due to more available roosting area (Figure B-1). We could not determine what proportion were in intertidal habitats, but most or all terns appeared to be in potential nesting habitats. We did not attempt to determine nest counts of terns since the terns do not build a nest structure that is visible in aerial photographs (Figure B-2).

Gill and Mewaldt (1983) reported that Caspian Terns first colonized the Humboldt Bay area in the 1960s, followed by a decrease from 150 pairs in 1964 to 27 pairs in 1969, and none were known to nest at Arcata Bay Sand Islands from 1969 to 1989 (Sowls et al. 1980, Carter et al. 1992; S. W. Harris, pers. comm.). Gill and Mewaldt (1983) incorrectly stated that 20 pairs were present in 1979 (S. W. Harris, pers. comm.). Recolonization has occurred since 1989. Caspian Terns first colonized the San Francisco Bay area in the 1920s and expanded to five colonies by 1981 (Gill and Mewaldt 1983), including Knight Island. In 1990, 38 nests and 133 birds were counted at Knight Island (Carter et al. 1992).

Table B-1. Numbers of Caspian Terns (CATE) counted from aerial photographic surveys at Arcata Bay Sand Islands and Knight Island in 2003.

Colony CCN USFWSCN Date Time CATE Arcata Bay Sand Islands HU-404-01 325-066 3 June 14:31-14:35 262 Knight Island SFB-SN-01 404-053 29 May 11:15-11:25 321 Brooks Island Area SFB-CC-17 429-065 29 May 11:49-12:01 Incomplete Salinas River Mouth MO-364-06 454-060 29 May 15:00-15:02 Not Counted

50

Figure B-1. Overview aerial photograph of Arcata Bay Sand Islands, 3 June 2003. Dashed line indicates the extent of area where Caspian Terns were counted (see Figure B-2). Photo 03-GJM-20-25.

Figure B-2. Close-up aerial photograph of Arcata Bay Sand Islands, 3 June 2003. Photo 03-HRC-44-22. This photograph was used to count 183 Caspian Terns; 79 terns were counted from two other photographs.

51