3.9). The estuary is a part of the Pacific 

 Flyway used by millions of birds traveling 

 between Arctic and sub-Arctic breeding sites 

 and lower latitude wintering sites. Tijuana 

 Estuary is one of a dwindling number of 

 stopover sites used by migrants to "re-fuel" 

 during their long journey, and it supports 

 large populations of over-wintering birds 

 that depend on intertidal habitats for food. 



Recent censuses conducted at Tijuana 

 Estuary (Kus and Ashfield 1989) and 

 throughout the Pacific Flyway (Warnock et al. 

 1989, Page et al. 1990) have documented 

 that the number of migratory waterbirds at 

 the estuary peaks in the fall, and is an order 

 of magnitude greater than the number present 

 in spring, by which time most birds have 

 departed for the breeding grounds. Only a 

 handful of species nest at Tijuana Estuary 

 during summer. 



Seasonal and longer-term differences in 

 abundance are not the only types of temporal 

 variability characterizing the Tijuana 

 estuary waterbird community. Uses change 

 on a diurnal basis, as tidal conditions deter- 

 mine the availability of feeding and resting 

 sites. Kus and Ashfield (1989) conducted 

 regular surveys of the estuary's birds 

 between October and April 1989. In their 

 transects, they found that the number of birds 

 was consistently higher during high-tide 

 surveys than during low tides, with the most 

 pronounced differences observed for small and 

 large waders and among the waterfowl (Figure 

 3.29). This pattern indicated movement 

 between areas in response to changing tidal 

 conditions; during high tides, birds were 

 concentrated at roosting sites within the 

 surveyed areas, while during low tides, birds 

 dispersed to feeding locations within and 

 outside of the census area. 



Although Kus and Ashfield could not 

 document the extent to which movement 

 between feeding and roosting areas involved 

 sites outside of the Tijuana Estuary, it is 

 likely that such movement between wetlands 

 is an important determinant of local 

 population size. Moreover, it suggests that 

 regional populations of wetland-dependent 

 birds may be limited by the availability of 

 certain critical habitats found at only a few 



3500 T 



3000 -- 



2500 -- 



2000 - 



1500 



\M gulls 

 EH herons 

 H vaterbird3 

 O large wader3 



I 3mall wader3 



1000 



500 



Tide: LHLHLHLHLHLH 

 Day: 7 14 22 29 5 1 2 20 27 4 9 1 5 24 



Month : October 



November December 



Figure 3.29. Variability in bird use by tidal 

 condition. Data are numbers of individuals 

 observed during low (L) and high (H) tides, 

 by category of bird, showing that both the 

 numbers of birds and the types of birds show 

 temporal variation. Data are from Kus and 

 Ashfield (1989). 



67 



