(Egretta caerulea), tricolored heron (Egretta 

 tricolor), reddish egret (Egretta rufescens) 

 and black skimmer (Rynchops niger). The 

 black skimmer began nesting at the salt ponds 

 of south San Diego Bay in 1976, and the 

 population has grown rapidly since then 

 (Unitt 1984). Tijuana Estuary is one of the 

 few sites outside of San Diego Bay that 

 skimmers use for foraging. 



Channels are important foraging habitats 

 for a variety of birds from other habitats. 

 Belding's Savannah sparrows rely on tidal 

 creek and channel edge habitats for feeding. 

 Clapper rail foraging behavior has been 

 discussed in Section 3.3. Feeding studies of 

 California least terns (Minsky 1984; Atwood 

 and Minsky 1983) document feeding in 

 nearshore waters, as well as estuarine 

 channels and bay habitats. Preferred fishes 

 include northern anchovy, topsmelt, and 

 jacksmelt (Atherinops californiensis). 

 Breeding adults catch and feed these small fish 

 (4-9 cm long) to the chicks. The young begin 

 to fly at about 20 days of age and the fledglings 

 develop foraging skills in calm, protected 

 waters. "Even estuarine and freshwater 

 localities that are distant from active nesting 

 sites . . . may be heavily used by least terns 

 during post-fledgling dispersal; loss or 

 disturbance of such areas may reduce the 

 survivorship of dependent young" (Atwood and 

 Minsky 1983). 



3.7 INTERTIDAL FLATS 



The conspicuous species of the sandflats 

 and mudflats are the shorebirds that feed and 

 rest there during low tide (Figure 3.25). 

 Most of their invertebrate food species were 

 discussed in Section 3.6.2. Many of the prey 

 animals are distributed from the subtidal 

 channels to the lower limit of the salt marsh. 



Four invertebrate species that are 

 characteristic of exposed flats are the 

 California horn snail, the yellow shore crab, 

 the fiddler crab and the lined shore crab 

 (Figure 3.25). There are no quantitative data 

 on any of these at Tijuana Estuary. We know 

 only that the horn snails can be extremely 

 abundant (hundreds to thousands per square 



meter), and that both horn snails and crabs 

 are important foods for the clapper rail 

 (Jorgensen 1975). It is likely that all these 

 species were negatively affected by estuarine 

 closure in 1984. Large numbers of empty 

 horn snail shells were collected from the 

 mudflat adjacent to the inland lagoons in 

 1984; only an occasional live individual was 

 found. Lined shore crabs were found dead and 

 floating in the hypersaline water during 

 1984. 



In a study of habitat utilization and feeding 

 strategies of shorebirds at Tijuana Estuary, 

 Boland (1981) found that intertidal mudflats 

 and sandflats were used by many more species 

 and individuals than any other habitat type. 

 He observed that the species that fed in these 

 habitats appeared to partition their activities 

 among different sediment depths and water 

 depths (Figure 3.26). Waders, such as the 

 greater yellowlegs, fed on items in the water 

 column and on the sediment surface. Long- 

 billed sandpipers (e.g., marbled godwit) 

 probed deep into the sediment, often wading 

 deep into the water. Short-billed sandpipers 

 like the western sandpiper probed less deep 

 and remained near the edge of the water; while 

 plovers (e.g., semipalmated plover) fed on the 

 surface of moist-to-dry sediments. 



Boland concluded that the length of leg and 

 bill determined the feeding position of each 

 species, and that the community was composed 

 of species that differed in leg and bill lengths. 

 Such different morphologies should reduce 

 competitive interference by reducing overlap 

 in where the birds feed. However, as Boland 

 pointed out, tides constantly change the depth 

 of water so theat longer and shorter legged 

 birds may take foods from the same spots but 

 at different times. Thus, they cannot avoid 

 exploitative competition when food supplies 

 are limiting. Boland (1988) has tested these 

 ideas through gut analyses of eight shorebird 

 species and quantitative analyses of 

 invertebrate food availability at Morro Bay in 

 central California. He found that food was 

 limiting during winter and spring, and that 

 exploitative competition occurred among many 

 of the species-even among species that had 

 different morphologies (e.g., dunlin, willet, 

 and marbled godwit). 



58 



