. The ecology of Tijuana Estuary, California : a national estuarine research reserve. Estuarine ecology -- California Tijuana River Estuary. ). 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 Ashfi
. The ecology of Tijuana Estuary, California : a national estuarine research reserve. Estuarine ecology -- California Tijuana River Estuary. ). 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 ). 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
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