BREEDING STATUS AND DIET TRENDS OF TWO CALIFORNIA LEAST TERN COLONIES IN THE SAN FRANCISCO BAY

ABSTRACT RESULTS DISCUSSION The two largest colonies of the California least tern (Sternula antillarum browni) in the Since the colony’s inception, the These two successful San Francisco Bay area, Alameda Point and Hayward, are located 10 miles apart. The Alameda Point population has in- least tern colonies lo- Alameda Point colony, on the former Naval Air Station, Alameda, has grown at an aver- creased yearly in the number of cated in San Francisco age rate of 9.2% per year since its inception in 1976. Long-term breeding success at Al- least tern pairs and nests. How- Bay estuary, the northern ameda has averaged 0.86 fledglings per breeding pair. Breeding success increased ever, the colonies productivity has end of the range for this through the 1980s, declined through the 1990s and the early 2000s, and increased again varied. It increased through the species have several simi- in the last five years. The Hayward colony, located on an island created from dredge ma- 1970s, 1980s and then declined larities and differences terials in 2001 has been enhanced with additional substrate materials, vegetation man- through the 1990s and early (see Table 1). For ex- agement, social attraction devices, and predator management. Least terns began ap- 2000s. 2005 was the peak year ample the Alameda site is pearing at this island in 2003, and successful breeding attempts have been observed with 424 breeding pairs. Depreda- in a more urban setting, since 2006. This colony has grown at an average rate of 51.5% per year. Long-term tion events by burrowing owls are while Hayward is se- breeding success has averaged 0.87 fledglings per breeding pair, despite lower success attributed to the decrease in 2006. cluded along the shore- rates in 2008 and 2009. Dropped fish have been collected from both colonies, and sil- The average annual growth rate line. The Alameda popu- Figure 2. The average annual growth rate of 9.2% for the Alameda versides (family Atherinopsidae) are the dominant dropped prey at both sites. Hayward for this colony is 9.2% per year Point California least tern colony. lation has declined in terns forage mainly on nearshore species in the shallow marsh waters near their breed- (Figure 2). Despite the population recent years, while the Figure 4. Twenty-five years of dropped fish data for the Alameda Point California ing site; Alameda terns forage on a greater variety of species found in both shallow and decline in recent years, the breed- Hayward colony has in- least tern colony representing 19 different families with the Atherinopsidae being deeper waters of the Central and South Bay. ing success and productivity of the creased. This could be the most abundant followed by Engraulidae and Clupeidae. While Atherinopsids remain the dominant colony is improving. The nesting 80 explained by terns breeding 70 dropped prey, northern anchovy (Engraulis density for this colony ranges pairs moving from Alameda to mordax) and surfperches have declined since from 1-153 nests per ha (0.5-62 60 total nests other sites. Alameda the 1990s. per ac) (Figure 6). 50 productivity is on the 40 rise, possibly due to California Gull depredation at the fewer predators and STUDY AREA 30 Hayward site resulted in complete more energy-rich fish in The Alameda Point colony is situated on the colony failure in 2005 and near 20 the diet. Hayward pro- runway complex of the former Naval Air Sta- colony failure in 2006. Starting in 10 ductivity is stabilizing, tion, Alameda, California (Figure 1). The air sta- the 2007 nesting season a suggesting that this could tion closed in 1997; is now known as Alameda predator/gull management effort be due to the island 2005 2006 2007 2008 2009 2010 Point and is owned by the United States Navy. started and the colony increased Year reaching its carrying ca- The tern breeding site is 9.7 ac. The Hayward dramatically. 2009 was the peak Figure 3. The average annual growth rate of 51.5% for the Hayward Shoreline pacity or the spread of a Shoreline colony is located within the Hayward year with 70 breeding pairs and 80 California least tern colony. noxious weed [mayweed Regional Shoreline marsh complex, Hayward, nests. This colony has a 51.5% av- chamomile (Anthemis California (Figure 1) and is operated by the East erage annual growth rate (Figure 3). The nesting density for this colony ranges from 33-333 nests cotula L.)] which limits Figure 5. Five years of dropped fish data for the Hayward Shoreline Bay Regional Park District. The least tern island per ha (13-133 per ac) (Figure 6). tern nesting space by California least tern colony representing 6 different families with the is 0.6 ac in size. Figure 1. Atherinopsids being the most abundant followed by Fundulidae. The locations of the Alameda Point and Hayward confining them to just a Shoreline Least tern colonies. They are the two Based on twenty-five years of dropped fish data for the Alameda colony 19 different families have few open areas. Alam- largest colonies north of San Luis Obispo County been collected of which three have been the most common. Atherinopsidae are the most abun- METHODS and are 23 km appart. eda terns have a more dant family followed by Engraulidae (northern anchovy) and Clupeidae (pacific herring) (Figure diverse diet than Hay- Data collection to determine nest distribution, 4). The northern anchovy was frequently collected during most of the 1980s and 1990s, has de- ward, which can be ex- chronology of nesting, and reproductive suc- clined in the 2000s, with the exception of 2009. The opposite appears true for pacific herring Similarities plained by the different cess has primarily been accomplished using the which became more abundant in the 2000s, especially in 2008. Based on five years of dropped foraging habitats they Type 1 method (active monitoring inside colony • Successful breeding colonies • fish data for the Hayward colony, 6 different families have been collected of which Atherinopsidae utilize. Herring and an- by permitted biologists) and Type 2 methods Substrate are the most abundant fish (Figure 5). Freshwater fishes, like killifishes (Fundulidae) are common • Predator and site management chovy are both rich in (passive monitoring outside colony by staff or at this site due to the brackish water marsh, as compared to the open Bay environment available lipids and could be con- volunteers). These types of monitoring allows • Use of Volunteers to the Alameda terns. sidered a favorable fish for quantitative comparisons to be made Differences to terns and may help among sites and years. Alameda Point Hayward Shoreline explain the increased breeding success at the Prior to 2000, dropped fish were collected op- More urban surroundings More secluded Alameda colony. More portunistically during the breeding season and Declining population – Increasing population – years of data will help us identified to the lowest taxonomic category Emigrating to other northern CA Predator managemen t? colonies? see if this trend contin- possible. From 2000-2010, dropped fish were Alameda terns? Increasing productivity – Stable productivity – ues. Figure 6. Nesting density for the Alameda Point and Hayward Shoreline California collected during each in-site nest survey. Each More predator deterrents? Carrying capacity? least tern colonies. specimen was identified to the lowest taxo- More anchovy in diet? nomic level possible. More diverse diet – Less diverse diet – AUTHORS Foraging in Bay Foraging in ponds/ marshes DAVID L. RIENSCHE MEREDITH L. ELLIOTT SUSAN H. EUING Three other nesting species Nine other nesting species East Bay Regional Park District PRBO Conservation Science U.S. Fish and Wildlife Service Presented to Wildlife Resource Analyst Marine Ecology Division Alameda Point 2011 Annual Conference Certified Wildlife Biologist® Senior Scientist, Biologist 950 Peralta Oaks Court, 3820 Cypress Drive, #11 P.O. Box 159, of the Western Section Table 1. P.O. Box 5381 Petaluma, CA 94954 USA Alameda, CA 94501 USA of The Wildlife Society Similarities and differnces for the Alameda Point and Oakland, CA 94605 USA [email protected] [email protected] February 8-11, 2011 Riverside, CA Hayward Shoreline California least tern colonies. A recently hatched California least tern chick. California least tern with a silverside (family Atherinopsidae). [email protected]

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