insectivorous animals. Many animals in 

 upland communities are attracted to riparian 

 woodlands for access to water and shade, 

 particularly during the rainless southern 

 California summers. The riparian zone also 

 serves as a natural corridor linking together 

 adjacent ecosystems and facilitating movement 

 of animals between them. In these ways, the 

 presence of riparian habitat significantly 

 enriches regional biodiversity beyond what 

 could otherwise be supported. 



One of the summer visitors to the Tijuana 

 River is the least Bell's vireo (Vireo bellii 

 pusillus), a small migrant that nests in dense 

 stands of herbaceous and shrubby plants. Once 

 abundant throughout the central valley and 

 other low elevation riverine valleys, the 

 species is now one of California's most 

 endangered birds. Population declines have 

 been precipitous, occurring largely in the last 

 four decades, and have accompanied the loss 

 and degradation of riparian habitat throughout 

 the state. Additionally, brood parasitism by 

 the brown-headed cowbird (Molothrus ater), 

 a species native to the eastern United States, 

 has reduced nesting success to the point that 

 the state's current vireo population consists 

 of only a few hundred pairs (Franzreb 1989). 

 The vireo is now largely confined to San Diego 

 County and northern Baja California. 



Least Bell's vireos depart from their 

 wintering grounds in the Cape region of Baja 

 California in early spring, arriving at the 

 Tijuana River in late March. Males establish 

 territories and attract females with their 

 distinctive song: cheedle-cheedle chee? 

 Cheedle-cheedle chew! The pair then con- 

 structs a small cup-shaped nest out of leaves 

 and grass, typically placing it about one meter 

 above the ground in dense willow or mulefat 

 vegetation. Females generally lay three to 

 four eggs, which hatch after about two weeks; 

 two weeks later, the nestlings leave the nest 

 and become independent. Vireo pairs may nest 

 up to four times in a season, depending on 

 their success, before departing for wintering 

 grounds in September (Kus 1990b, 1991). 



Few riparian plant species are listed as 

 rare or endangered; rather, the entire 

 riparian community is endangered as a result 

 of human activities including agriculture, 



flood control projects and channelization, 

 grazing, sand and gravel extraction, road 

 construction, and residential and commercial 

 development. Riparian habitat has been 

 vanishing from the landscape of California at 

 such a pace that today less than 10 percent of 

 the woodlands in existence at the time of the 

 Gold Rush remain, and those are but 

 fragmented remnants. Fortunately, riparian 

 vegetation appears to be resilient and the 

 potential for habitat restoration is high. 

 However, ecologists have much to learn about 

 how natural riparian ecosystems function. 

 Until scientists are convinced that man can 

 restore degraded environments to functioning, 

 self-sustaining ecosystems, protection of 

 existing habitat must be the conservation 

 priority. 



3.10 DYNAMICS OF FEATURED SPECIES 



3.10.1 Spatial and Temporal Patterns of 

 Habitat Use by Waterbirds. 



Waterbirds, i.e., shorebirds, herons, 

 rails, ducks, pelicans, grebes, gulls and 

 terns, are the most conspicuous animals of the 

 estuarine environment. Valued by the public 

 for the educational and recreational 

 opportunities they provide, water-associated 

 birds also are a common focus of scientific 

 studies investigating the health, structure and 

 functioning of coastal wetlands. 



Perhaps more than any other group of 

 organisms, waterbirds reflect the dynamic 

 nature of the estuarine environment. The 

 large and diverse group of birds using Tijuana 

 Estuary exhibits spatial as well as temporal 

 variability in abundance, distribution, and 

 activity in response to changing physical 

 conditions. Patterns in the bird use of the 

 estuary not only reveal the importance of 

 particular habitats for feeding, roosting, or 

 nesting, but also illustrate the inter- 

 relatedness of the various habitat types in 

 their value to wildlife. 



The most observable change in the use of 

 Tijuana Estuary by birds is the seasonal 

 variation in species richness and abundance of 

 migratory shorebirds and waterfowl (Table 



66 



