Distribution and Abundance of Double-crested Cormorants Nesting in the Interior of California, 2009–2012

Abstract

As part of an 11-state inventory, we censused the Double-crested Cormorant (Phalacrocorax auritus) in the interior of California from 2009 to 2012, using a combination of aerial, ground, and boat surveys. An estimated 8791 pairs breeding in the interior of the state in 2009–2012 exceeded the 7170 pairs estimated in 1998–1999. In both periods, cormorants were breeding in 9 of 11 ecoregions, but three-fourths were at one site—Mullet Island at the Salton Sea in the Sonoran Desert ecoregion (abandoned in 2014). The ecoregions with the next highest proportions were the Sacramento Valley, San Joaquin Valley, and Modoc Plateau. The apparent increase in numbers and colony sites since 1999—consistent with the pattern through much of western North America—reflects the (short-lived) increase in numbers at the Salton Sea, an increasing number of colonies and breeding pairs in the Central Valley, and slightly better coverage on the recent surveys. Because of practical survey constraints and limited data to date, evidence of change in numbers of Doublecrested Cormorants breeding in the interior of California between 1998–1999 and 2009–2012 is inconclusive. Plans for monitoring will need to take into account the effects of substantial annual variation in numbers, which may be associated with large fluctuations in cormorants’ prey base, short-term cycles of drought and flood, shifts of nesting cormorants into or out of the interior of California, and the expectation of greater environmental fluctuations with continuing climate change. The factors most likely to limit the number of cormorants breeding in the interior of the state are habitat loss or alteration (particularly from reallocation of water for human needs), disease, human disturbance, and the long-term effects of climate change.