Attendance and Effects of Disturbance on Coastal Common Murre Colonies at Point Reyes, California

Abstract

-We surveyed six Common Murre (Uria aalge) colonies in the vicinity of Elephant Seal Cove Beach (ESCB), Point Reyes National Seashore, Marin County, California from 1995 through 1997 to investigate the effects of disturbance on local murre populations in the wake of a small shipwreck and to establish seasonal and diurnal attendance patterns of coastal murre populations. Following the shipwreck in 1995, one colony (Cliff Rock West) was abandoned, and three others (Flattop Rock, Mid Rock, and East Rock) experienced reduced productivity due to disturbance and predation by Brown Pelicans (Pelecanus occidentalis), Western Gulls (Larus occidentalis), and Common Ravens (Corvus corax). A fifth colony, Face Rock, farthest from the grounding site, was apparently not affected. Murres attended Cliff Rock West only early in 1996, and not at all in 1997. A new colony formed on another seastack (Northwest Rock) after murres left the Cliff Rock West colony in 1996. However, no eggs were observed at Northwest Rock in 1996 and this colony was abandoned early in 1997. Seasonal attendance among colonies on Flattop Rock, Mid Rock, and East Rock followed a similar trend in both 1996 and 1997. Murre attendance fluctuated widely until the onset of egg-laying in early May, but then stabilized and increased somewhat untilJuly, when numbers began to drop at the end of the fledging period. Both air temperature and the upwelling index (UI) were significantly correlated with murre attendance in May and June during the incubation and nestling periods. Mean attendance increased 18-30% at these colonies between 1996 and 1997. Diurnal attendance in 1996 and 1997 varied both within and between colonies and days, yet there was a significant downward trend in numbers throughout the day, as well as an effect of colony disruption by other marine species. Given that seasonal and diurnal attendance patterns of Flattop, East, and Mid Rocks in 1996 and 1997 appear similar, both between years and to other studies, we assume that these colonies recovered quickly following the 1995 shipwreck. However, interspecific interactions and predation continue to affect diurnal attendance patterns and productivity of these colonies. Received 5 April 1998, accepted 11 October 1998, final acceptance 27 March 1999. Waterbirds 22(1): 130-139, 1999 Seasonal and diurnal attendance has been examined for a number of Common Murre populations (Lloyd 1975; Birkhead 1978a; Slater 1980; Harris et al. 1983; Hatch and Hatch 1989; Takekawa et al. 1990). During the incubation and nestling periods, the number of birds on a colony may be influenced by ambient weather; numbers may be lower during periods of high winds (Birkhead 1978a; Gaston and Nettleship 1981) or warm temperatures (Murphy and Schauer 1994). Few studies have investigated attendance patterns in relation to disturbance. Aside from human disturbance, disruption to murre colonies may be caused by predators and competitors; such effects have been recorded in studies of murre behavior, survivorship, productivity, and the costs and benefits of coloniality (Birkhead 1977; Spear 1993; Sydeman 1993; Parrish 1995; Gaston and Elliot 1996; Schauer and Murphy 1996). On 10June 1995, the vessel Wayfarer, a 42' Grand Banks pleasure-craft, ran aground ne r Elephant Seal Cove Beach, Point Reyes Headlands, Marin County, California (Fig. 1). As a result of the wreck, engine oil, debris, and approximately 200 gallons of diesel fuel were released into coastal waters (Point Reyes National Seashore Case/Incident Record #950136). During the event, a U.S. Coast Guard helicopter was dispatched to the scene to rescue the one-man crew.