Population Monitoring of California Least Terns and Western Snowy Plovers

The Applied Animal Ecology Division of CRES continues to coordinate long-term monitoring efforts essential to understanding and abating population declines in local threatened and endangered shorebirds.

The California least tern population estimates for 2004 were the second highest recorded since monitoring began in 1969, with 6,268 to 6,561 estimated breeding pairs establishing 7,937 nests at 44 locations statewide. This represents a 7percent decrease in the minimum estimated number of breeding pairs from 2003, but it is a 44 percent increase from 2002 and a 25 percent increase over the previous peak of 4,672 pairs in 2001. The nesting colony at Camp Pendleton continues to be the largest in the state, accounting for 22 percent of the breeding pairs and 18 percent of the nests. Despite the increased nesting attempts, least tern productivity statewide was low this season. Chick mortality, possibly related to lack of availability of prey fish, was the primary factor limiting reproductive sucess. Other reported causes of mortality included predation, nest abandonment, and human activity, including loss of chicks to vehicles.

The nesting population of western snowy plovers at Camp Pendleton had 212 plover nests, with a conservative estimate of 100 to 130 fledglings, the highest recorded nest total to date. In contrast, the nesting plover population at the San Diego Bay sites appears to have declined, fledging a limited number of young.

In South San Diego Bay, a pair of elegant terns successfully nested at Chula Vista Wildlife Reserve for the first time. Their large colony at the South San Diego Bay saltworks had at least 1,020 nests established this season. This species of concern is limited to five nesting colonies in the world. Numbers of royal terns were also relatively high, with 38 nests established. Other seabird colonies included at least 496 black skimmer nests, 313 Caspian tern nests, and 275 Forster's tern nests.