Photos courtesy:
R.Aaron Raymond & the Essential Image Source Foundation

Xantus's Murrelet Monitoring

In the fall of 2001, the National Park Service and the Island Conservation and Ecology Group began eliminating black rats from Anacapa Island in one of the largest seabird restoration projects on the West coast of North America. The primary species that stands to benefit from this project is a rare species of Alcidae (the bird family which includes puffins and murres) called the Xantus’s Murrelet that was recently listed as threatened by the state of California and is being considered for listing as threatened under the federal Endangered Species Act. The Xantus’s Murrelet is a small black and white seabird whose global population numbers less than 10,000. It is roughly 8 inches in length and has a slender beak and short tail. Murrelets breed only on a few islands off the coast of Southern California and Baja California, where populations over the last century have been severely depleted by introduced species such as feral cats and black rats, leaving the fate of the species in a precarious position.

In 2000, the Anacapa Island Xantus’s Murrelet Monitoring Team (composed of researchers from the Channel Islands National Marine Sanctuary, California Institute of Environmental Studies, Humboldt State University, and Hamer Environmental) began a program to determine a baseline for murrelet population size and breeding success as well as a method of measuring population changes after the rat eradication. The program utilizes techniques such as nest monitoring, nocturnal spotlight surveys, and radar monitoring. Spotlight surveys were conducted to measure the numbers of individuals attending at-sea congregations near breeding colonies. Due to the inaccessibility of many breeding sites, spotlight surveys were found to be the best method of direct counting in order to provide an index on the overall size of the population. Spotlights surveys entail going out in an inflatable boat and scanning the water along the shoreline to determine the number of murrelets within a given region.

Initial results from the murrelet monitoring program have been encouraging, as no rat-depredated nests have been found on Anacapa since completion of the rat eradication in fall 2002. Murrelets hatched a higher proportion of their eggs in 2003 and 2004 compared to years when rats where present on the island. Breeding effort (the number of murrelet nests found) in 2003 was the highest since the study began, although murrelets nested in later and in lower numbers in 2004, probably due to poor foraging conditions around the islands. Several more years of spotlight surveys will be needed to begin to detect increases in the population because Xantus’s Murrelets, like other Alcidae, exhibit delayed sexual maturity, low reproductive rates and a high degree of natal philopatry. These traits contribute to what will likely be a slow recovery for the population.

In 2005, seabird biologists completed the sixth year of Xantus’s Murrelet nest monitoring on Anacapa Island.  Biologists measured the number of nests and breeding success prior to (2000-02) and after (2003-05) THE ERADICATION OF RATS.  The Murrelets’ difficult nesting habitats made these studies quite challenging (nests are hidden in small crevices in steep, rocky cliffs or sea caves), but the monitoring has clearly demonstrated the benefits of rat removal.  Overall, the number of murrelet nests found increased 81% in 2003-05.  Most notably, no murrelet nests have been destroyed by rats since 2002.  In previous years, rats destroyed more than half of all nests.  Rat eradication has greatly improved the prospects for survival of this colony, but ongoing monitoring is needed to document the continued recovery of Xantus’s Murrelets at Anacapa Island.