Western Snowy Plover 

 (Charadrius alexandrinus nivosus) 



While less well studied than the least tern 

 colonies, the snowy plover breeding 

 population at Tijuana Estuary is observed 

 annually during least tern censusing (Copper, 

 pers. comm.). In addition, periodic statewide 

 surveys are conducted by Point Reyes Bird 

 Observatory biologists, who have been 

 studying this species throughout the western 

 U.S. and Baja California for nearly 2 decades. 

 Both sources of information reveal that the 

 snowy plover population at the estuary has 

 undergone a significant decline, reflecting the 

 downward trend in the entire coastal 

 population that prompted the recent proposal 

 for Federal listing. In 1978, biologists 

 counted 37 adult snowy plovers at the mouth 

 of the Tijuana River; a decade later, the 

 population was down to 21 adults, and by 

 1991, only 4 adults were counted during the 

 spring survey (Page and Stenzel 1981, Page 

 et al. 1991). E. Copper (pers. comm.) 

 reports that while approximately 100 nesting 

 pairs were typically seen nesting on beaches 

 and in the dunes north and south of the river 

 mouth, in 1990 only 10 pairs were observed. 



Least Bell's Vireo 

 (Vireo bellii pusillus) 



The least Bell's vireo, one of four 

 subspecies of Bell's vireo occurring in the 

 U.S., is a state and federally endangered 

 species. Although historically vireos occupied 

 habitat along the length of the Tijuana River, 

 the nesting population was reduced to 5 pairs 

 by 1986, including a single pair within the 

 boundaries of the refuge (RECON 1988). 

 Since that time, the population has made a 

 steady comeback, increasing to 9 pairs and 4 

 unpaired males in 1990 (Kus 1990b), and 

 then nearly doubling to 15 pairs and 8 

 unpaired males in 1991 (Kus 1991). Most of 

 the population expansion has occurred west of 

 Hollister Street towards the estuary. 

 Currently, the Tijuana River vireo population 

 is the fifth largest in San Diego county. 



The increase in the vireo population at the 

 Tijuana River is in part the result of an 



effective cowbird removal program 

 implemented in recent years. Parasitism of 

 vireo nests has been virtually eliminated 

 since 1990, and nesting success has been 

 consistently high (Kus 1990b; 1991). With 

 proper management, the Tijuana River has the 

 potential to support one of the largest vireo 

 populations in California, making it a critical 

 site in efforts to promote the recovery of this 

 endangered bird. 



Studies of individually recognizable color- 

 banded vireos indicate that part of the 

 breeding population at the Tijuana River 

 consists of dispersers from natal sites at the 

 Sweetwater and San Diego Rivers (Kus 

 1990b, 1991). The Tijuana River has also 

 been documented as a stopover site used by 

 migrating vireos (Kus 1990b), and may be an 

 important link in the chain of riparian 

 woodlands connecting southern California with 

 the vireo's wintering grounds in Baja 

 California. 



The salt marsh bird's beak 



Cordylanthus maritimus s sp. maritimus) 



Salt marsh bird's beak is endangered with 

 extinction. Only 5 of its 12 historic sites 

 still support the species. Ti|uana Estuary has 

 several colonies that reappear in most years, 

 but densities and seed production are highly 

 variable. Among the more puzzling attributes 

 is the occasional failure of a colony to 

 germinate, with reappearance after a year or 

 two. The timing and amount of spring rainfall 

 are possible controls. 



This endangered hemiparasitic annual 

 plant has been the subject of studies of its 

 ecological requirements and potential for 

 restoring historic populations (Fink and 

 Zedler 1989). Following field and laboratory 

 experimentation, seed from Tijuana River 

 National Wildlife Refuge (NWR) was sown at 

 the nearby Sweetwater Marsh NWR (along San 

 Diego Bay) in 1991. Several patches grew 

 and reproduced (PERL, studies in progress), 

 indicating potential for reestablishment of a 

 viable population. However, it is doubtful 

 that the number of seeds produced was as great 

 as the number planted (B. Fink, pers. comm.). 



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