Xantus' Murrelet, Sythliboramphus hypoleucus I: Status and Conservation.
Santa Barbara Island is home to the largest colony of Xantus' Murrelets (Sythliboramphus hypoleucus) in the United States, constituting 25% of the world's population with its 1,000 to 3,000 breeding birds (Carter et al. in press; Sydeman et al. 1998). In the late winter or early spring of each year, murrelets return to their nest sites in the rock crevices and shrubs scattered along the cliffs of the Channel Islands. Two to three weeks after they begin visiting sites, murrelets lay two eggs, approximately eight days apart (Murray et al. 1983). The first egg is left alone, vulnerable to predation, until the second egg is laid. Murrelets then begin incubating, with incubation shifts of approximately three days. After 34 days, two chicks hatch in close succession despite the difference in laying date. Because parents are not tied to the nest waiting for a second chick to hatch, synchronous hatching allows the family to leave one to two nights after hatching, with the chicks still downy and unable to fly. Presumably, the parents lead their young to foraging grounds, where the chicks grow and achieve independence.
Biologists at Channel Islands National Park have been monitoring the arrival and departure of the murrelets, and tracking their reproductive success, since the early 1980s, with assistance from Point Reyes Bird Observatory biologists from 1993 to 1999. Two study plots are established on Santa Barbara Island, one in habitat dominated by rock crevices, and the other dominated by Eriophyllum nevinii, a low shrub that provides shelter to nesting murrelets. At the beginning of each breeding season, marked nest sites within these plots are checked for the presence of eggs and/or incubating birds. Sites are monitored throughout the season to determine the fate of the eggs, and data are used to generate occupancy, clutch initiation, hatching success, and productivity estimates. Annual estimates indicate that timing of breeding varied widely from year to year, probably reflecting food availability at the start of the breeding season (Hunt and Butler 1980). Reproductive success has also fluctuated annually, due to a combination of predation on eggs and adults and variation in food supply in any given year (Murray et al. 1983; Drost and Lewis 1995; Sydeman et al. 1998).
The murrelets' nocturnal habits, concealed nests, and the inaccessibility of much of their nesting habitat (i.e. cliffs) have made population size difficult to estimate. However, rough population estimates indicate that the size of the murrelet population on Santa Barbara Island declined by 30 to 50% from 1977 to 1991 (Sydeman et al. 1998). In fact, two scenarios from a recent population dynamics model (viability analysis) indicate that if the current trend continues, the island population will be at a critically low level in 20 years (Sydeman et al. 1998). The decline is a major concern, not only for the Santa Barbara Island population, but for the worldwide population. There are likely fewer than 10,000 individuals in the world; this estimate includes a total of 3,000 to 7,000 breeding birds in the Southern California Bight, including the Santa Barbara Island population (Carter et al. in press), and 2,225 to 6,700 birds (including breeding and/or roosting birds) on the islands off of Baja California, Mexico (Keitt 1999).
In addition to having a small population, Xantus' Murrelets have a limited breeding distribution, nesting on offshore islands from Point Conception to central Baja California (Drost and Lewis 1995). Recent radio-telemetry work indicates that murrelets in the Southern California Bight concentrate in particular regions of the Bight to forage (Whitworth et al. 2000). This limited distribution makes the murrelet population especially vulnerable to at-sea threats such as catastrophic oil spills, chronic oil pollution, military operations, organochlorine pollution, incidental by-catch from commercial fisheries, and attraction to bright lights on ships and platforms (Carter et al. in press).
Changes in oceanographic conditions, including El Nino/Southern Oscillation Events and large-scale regime shifts (alternating periods of predominantly cold and predominantly warm water temperatures), which may affect the murrelet's food supply, have also been implicated in affecting the viability of the murrelet population (MacCall 1996; McGowan et al. 1998; Sydeman et al. 1998). Little is known about murrelets foraging habitats, though available evidence indicates that they rely primarily on larval fish, including anchovy, saury, and rockfish (Hunt et al. 1979; Hunt and Butler 1980). They may also rely heavily on zooplankton in some years. An 80% decline in zooplankton off southern California from 1951 to 1993, and the concurrent increase of 1.6 [degrees] C in sea surface temperature in the Southern California [light, indicate a decline in primary and secondary productivity that may be detrimental to murrelets (Roemmich and McGowan 1995). Fluctuations in fish populations have also been linked to the alternation of warm and cold temperature regimes in the marine environment (MacCall 1996).
In the colonies, murrelets suffer loss from natural and introduced predators. Predators on adult murrelets include Barn Owls (Tyto alba) and Peregrine Falcons (Falco peregrinus). Barn Owl predation on Santa Barbara Island has ranged from 16 to 165 adult murrelets, sometimes increasing with crashes in the deer mouse (Peromyscus maniculatus) population (Drost 1989; Wolf et al. 2000). Peregrine Falcons historically nested on the Channel Islands, but were reportedly absent from all of the islands by the 1950s, if not earlier (Kiff 1980). Currently, Peregrine Falcons are beginning to re-occupy the Channel Islands, and their return, and subsequent predation on adult birds, may have a negative impact on the murrelet population. Feral cats have been removed from the Channel Islands, but are still a problem on some of the islands off Baja California (Keitt 1999). In addition, depredation of eggs and chicks by introduced black rats (Rattus rattus) and native island deer mice can be substantial. Rats have probably negatively affected the murrelet population on Anacapa Island, California and several of the populations off of Baja California (Drost and Lewis 1995; Keitt 1999). Deer mice regularly prey on eggs on Santa Barbara Island, consuming an average of 46% of all eggs produced (Sydeman et al. 1998). In addition to predators, murrelets may suffer from human activities in the southern part of their range, where colonies and fishing villages coincide (Keitt 1999).
In recent years, murrelets have faced an additional threat; the California squid fishery has been rapidly expanding near their nesting sites in the Channel Islands. Squid fishermen work close to shore, using generators to run bright lights intended to draw the squid to the surface of the water. The lighting may affect murrelets by attracting them to the boats, where they may run into structures or become entangled; disrupting social behaviors in staging areas near the island; illuminating habitat, making the small, nocturnal birds vulnerable to predation by owls; or disrupting nesting behavior by causing increased egg neglect or abandonment (Sydeman and Nur 1999). In addition, lights may disorient chicks trying to reunite with their parents after leaving the nest site.
The Xantus' Murrelet is designated as a "Species of Management Concern" by the U.S. Fish and Wildlife Service due to the small population size, limited distribution, and known threats listed above. In 2000, the Pacific Seabird Group, an international society of professional seabird biologists, initiated a petition aiming to upgrade the status of the Xantus' Murrelet to "threatened" under the Endangered Species Act of 1973. Measures to protect and restore this species, including removal of non-native predators from nesting islands, education of humans living and working near colonies, and possible management of native predators should continue or be considered while more information is gathered on population size, trends, and demography throughout the species' range. More information on foraging habits and important foraging areas is also necessary in order to monitor the effects of possible increased pressure from commercial fisheries and long-term changes in the marine environment on prey populations.
Carter, H.R., Whitworth, D.L., Takekawa, J.Y., Keeney, T.W., and Kelly, P.R. In press. At-sea threats to Xantus' Murrelets (Synthliboramphus hypoleucus) in the Southern California Bight. In D. Browne, H. Chaney, and K. Mitchell (eds.), Fifth Channel Islands Symposium, Camarillo, CA, Minerals Management Service.
Drost, C.A. 1989. Predation and population cycles on a southern California island. Master's thesis, University of California, Davis.
Drost, C.A. and D.B. Lewis. 1995. Xantus' Murrelet (Synthliboramphus hypoleucus). In A. Poole and F. Gill (eds.), The Birds of North America, No. 164. The Academy of Natural Sciences, Philadelphia, PA and The American Ornithologists' Union, Washington, D.C.
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Keitt, B.S. 1999. Status of the Xantus' Murrelet (Synthliboramphus hypoleucus) on the islands of Baja California, Mexico. Unpublished Report. Island Conservation and Ecology Group, Davenport, CA.
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MacCall, A.D. 1996. Patterns of low-frequency variability in fish populations of the California Current. California Cooperative Fisheries Investigations Report 37:100-110.
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Roemmich, D. and J. McGowan. 1995. Climatic warming and the decline of zooplankton in the California Current. Science 267: 1324-1326,
Sydeman, W. and N. Nur. 1999. A review of the status and threats to Xantus' Murrelets throughout its range in Baja California, Mexico and the United States. In N. Nur, W.J. Sydeman, D. Girman, T.B. Smith, and D. Gilmer, Population status, prospects, and risks faced by two seabirds of the California Current: the Ashy Storm-Petrel, Oceanodroma homochroa, and Xantus' Murrelet, Synthliboramphus hypoleucus. Final report to the USGS Biological Resources Division. Point Reyes Bird Observatory, Stinson Beach, CA.
Sydeman, W.J., N. Nur, and P. Martin. 1998. Population viability analyses for endemic seabirds of the California marine ecosystem: the Ashy Storm-Petrel (Oceanodroma homochroa) and Xantus' Murrelet (Synthliboramphus hypoleucus). Final report to USGS Biological Resources Division, Species At Risk Program, Washington D.C. Point Reyes Bird Observatory, Stinson Beach, CA.
Whitworth, D.L., J.Y. Takekawa, H.R. Carter, S.H. Newman, T.W. Keeney, P.R. Kelly. 2000. Distribution of Xantus' Murrelet Synthliboramphus hypoleucus at sea in the Southern California Bight, 1995-97. Ibis 142: 268-279.
Wolf, S., J.E. Roth, and W.J. Sydeman. 2000. Population size, phenology, and productivity of seabirds on Santa Barbara Island, 1999. Final Report to Channel Islands National Park, Ventura, CA. Point Reyes Bird Observatory, Stinson Beach, CA.
Jennifer E. Roth Point Reyes Bird Observatory, Marine Science Division, 4990 Shoreline Hwy., Stinson Beach, CA 94970; 415-868-1221 x22; firstname.lastname@example.org
William J. Sydeman Point Reyes Bird Observatory, Marine Science Division, 4990 Shoreline Hwy., Stinson Beach, CA 94970; 415-868-1221 x19; email@example.com