3.10.2 Species of Special Concern 



Tijuana Estuary provides habitat for some 

 24 rare, threatened or endangered species 

 (Table 3.9). Without exception, these species 

 have undergone precipitous declines in recent 

 decades because of the loss and degradation of 

 wetland habitat. The occurrence of these 

 plants and animals at Tijuana Estuary bears 

 testimony to the role that the estuary plays in 

 protecting regional and global biodiversity. 



Many of the sensitive species at the 

 estuary are monitored by the U.S. Fish and 

 Wildlife Service and other biologists, and a 

 few are targets of research in wetland 

 restoration. The following sections 

 summarize the status of selected species. 



Belding's Savannah Sparrow 

 (Ammodramus sandwichensis beldingi) 



The population of Belding's Savannah 

 sparrows at Tijuana Estuary has grown 

 steadily since surveys were initiated in the 

 mid-1 980's. In 1985, 81 pairs nested in the 

 middle marsh areas north of the Tijuana River 

 (White 1986). Censuses in 1973 and 1977 

 indicated that there were 100 and 95 nesting 

 pairs, respectively, throughout the entire 

 estuary, including the previously unsurveyed 

 marsh south of the river. By 1986, the 

 population had grown to 225 pairs (Zembal et 

 al. 1988). Kus and Ashfield (1989) report 

 that between 299 and 320 pairs occurred 

 throughout the estuary in the spring of 1989; 

 the population underwent further increase 

 during the following year and was estimated at 

 between 289 and 397 pairs in 1990 (Kus 

 1990a). Much of the population growth in 

 recent years is the result of expansion into 

 the salt marsh in the southwestern portion of 

 the estuary. 



Data collected during statewide censuses of 

 Belding's Savannah sparrows reveal that 

 Tijuana Estuary supports at least 10 percent 

 of the state's population (Massey 1979; 

 Zembal et al. 1988). Only 3 of the 31 

 wetlands surveyed in the last statewide effort 

 (1986) supported larger populations. 



California Least Tern 

 (Sterna antillarum browni) 



The number of nesting pairs of least terns 

 at Tijuana Estuary fluctuates widely from 

 year to year, as at other nest sites that are 

 unprotected from predation and human 

 disturbance. Adult terns breeding for the 

 first time do not show a clear preference for 

 their natal colonies but tend to return to a site 

 once they have bred (Massey and Atwood 

 1984). Estimates of the number of nesting 

 pairs have ranged from 21 to 72 in the past 

 10 years (Table 3.10), with population 

 trends at the estuary paralleling those seen in 

 the statewide population (E. Copper, pers. 

 comm.). 



Factors such as winter mortality have not 

 been examined but could help to explain 

 annual variations in tern numbers. The 1984 

 closure of Tijuana Estuary to tidal flushing 

 may have adversely affected recruitment. In 

 1984, about 50 pairs nested but no fledglings 

 were observed. The stagnant channel water 

 and corresponding reduction in fish and 

 invertebrate populations may have con- 

 tributed to nesting failure (E. Copper, pers. 

 comm.). 



Table 3.10. California least tern nesting data, 

 1982-1991 (from Least Tern Recovery 

 Team, unpubl., E. Copper, unpubl., and Patton 

 1991). Prs = pairs. 



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