sites, such as high-tide refuges. This 

 question is currently under investigation by 

 Kus. 



From the above discussion, it is clear that 

 many waterbirds are not confined to 

 particular habitat types, but rather use a 

 mosaic of habitats for feeding, roosting and 

 nesting. Spatial variation in waterbird 

 distribution is a function of abiotic factors, 

 such as tidal conditions, as well as biotic 

 factors, including competition and predation. 

 Species that typically occur at Tijuana 

 Estuary differ in the extent to which such 

 factors influence their distribution. For 

 example, Kus and Ashfield found that pelicans, 

 grebes, cormorants and most waterfowl 

 species were restricted to intertidal flats and 

 channels, while most gulls, terns, and herons 

 used several habitat types (Table 3.8). 



Both Boland (1981) and Kus and Ashfield 

 (1989; Table 3.8) observed shorebirds in a 

 wide variety of habitats, and noted that with 

 the exception of the common snipe, every 

 species they studied occurred in intertidal 

 areas. Although many shorebird species used 

 several different habitats, the distribution of 

 individuals was not uniform across all habitat 

 types. Boland (1981) consistently found the 

 highest densities of all but the sanderling in 

 intertidal flats and channels; likewise, Kus 

 and Ashfield (1989) observed that the 

 majority of large and small waders seen 

 during low-tide surveys occurred in those 

 habitats (Figure 3.30). 



Kus and Ashfield also noted that the 

 distribution of birds across habitat types 

 varied as a function of tidally-mediated 

 movement between habitats. Small waders 

 typically occurred in large numbers in the 

 sandy dune habitat during high tides, moving 

 to the intertidal areas to feed during low tides 

 (Figure 3.31). Large waders exhibited a 

 similar, but less dramatic, shift between 

 habitats (Figure 3.32). Proportionately 

 more large waders than small waders 

 remained in intertidal areas during high 

 tides, probably because their larger size and 

 longer legs enabled them to do so. 



Habitat 



□ Sand flats 

 Dunes 



Small Waders 



100 -\ 



80 - 



60 - 



40 - 



20 - 



I 



I 



Beach 

 Salt marsh 

 Intertidal flats 



, , | ' i ' i'i'i  ' ! -  i ' i 'i   ' |V i 'i 1 1 '■' i V |  |V |  . 



Large Waders 



100 -i 



80 - 



I 



I 



60 - 



40 - 



20 - 



I 



I    (  ■■■■[ 



r- <M CO 



" 



i. 



 



%: 



| 



I 



I '■' I '  ' I ■■' I ' '"I 1 " 1 "! 

 ^ in id s co oi o 



t- CM CO •"tf- 



Figure 3.30. Percent of all individuals seen 

 in each of five habitat types during low tide 

 surveys. Sample dates (x axis) are biweekly 

 beginning October 7 and ending April 8, 

 1988. 



69 



