|Eared Grebe. ©Peter LaTourrette www.birdphotography.com|
From 1999 through the present, PRBO has been conducting studies in collaboration with the U.S. Geological Survey and the San Francisco Bay Bird Observatory on the use of these salt ponds by birds. One of the forces driving our research has been the March 2003 sale of over 15,000 acres of South Bay salt ponds, owned by Cargill Salt, to a conglomerate of private foundations and state and federal agencies, to be restored to some mix of tidal marsh and salt pond habitat. As an initial step in attempting to understand how the anticipated restoration might affect the Bay's bird populations, we looked at bird use of South Bay salt ponds on high and low tides during the winter months of 1999-00 and 2000-01. The results of this study were recently published in the journal Waterbirds (Vol. 25, Special Publication 2).
During the study, we looked at behavior and habitat use of birds in the ponds and examined the effects of tide cycle, pond salinity, and pond area on bird use. In surveys of salt ponds in the South Bay from September 1999 to February 2001, we recorded 75 species of waterbirds totaling over a million bird-use-days on high tide. Shorebirds and dabbling ducks were the most abundant groups of birds using the salt ponds. Ponds at high tides held higher numbers of birds than the same ponds on low tides, mainly because some bird species move out to feed on tidal mudflats on the low tides. Considerable numbers of birds fed (and roosted) in the salt ponds on high and low tides, although this greatly varied by species.
|Figure 1. Click on image for larger view..|
Two things strongly influence the use of salt ponds by waterbirds: pond salinity and pond depth. Pond salinity affects prey diversity and abundance in the ponds, and pond depth determines accessibility of prey in ponds for different bird species. While we are still studying the effects of pond depth on waterbirds, during this study we found that waterbird numbers and diversity were significantly affected by the salinity of ponds.
Lower-salinity ponds have a higher diversity of vertebrate and invertebrate prey for birds to feed on, and thus they attract a higher diversity of birds. Most fish species do not do well after waters reach salinities greater than about 40 parts per thousand (ppt); many invertebrate species cannot survive then, as well. Thus, as shown in Figure 1, the abundance of fish-eating birds like cormorants and grebes (excluding Eared Grebes) quickly decreases in higher-salinity ponds.
However, as diversity of invertebrate prey decreases with increasing salinity levels, competition and predation pressure decrease on the remaining invertebrates that can tolerate high salinities. As a consequence, these few invertebrate species, notably brine flies, brine shrimp, and water boatman, occur in huge numbers in medium- to very high-salinity ponds. These ponds become favored foraging sites for many waterbird species, such as shorebirds (Figure 2), dabbling ducks, and Eared Grebes, that feed on the abundant invertebrates. After ponds reach salinities of about 200 ppt, even the hardiest of prey cannot live well in them, and bird use drops off rapidly except for roosting and breeding around the edges.
This study has led to a better understanding of how birds use San Francisco Bay's salt ponds. It will help guide restoration of much of this habitat back to tidal marsh while retaining the habitat characteristics of the salt ponds that contribute the most to maintaining viable waterbird populations in the bay.