Attendance and effects of disturbance on coastal Common Murre colonies at Point Reyes, California

We surveyed six Common Murre (Uria aalge) colonies in the vicinity of Eleph ant Seal Cove Beach (ESCB), Point Reyes National Seashore, Marin County., C alifornia from 1998 through 1997 to investigate the effects of disturbance on local murre populations in the wake of a small shipwreck, and to establi sh seasonal and diurnal attendance patterns of coastal murre populations. F ollowing the shipwreck in 1995, one colony (Cliff Rock West) was abandoned, and three others (Flattop Rock, Mid Rock, and East Rock) experienced reduc ed productivity due to disturbance and predation by Brown Pelicans (Pelecan us occentalis), Western Gulls (Larus occidentalis), and Common Ravens (Corv us corax). A fifth colony, Face Rock, farthest from the grounding site, was apparently not affected. Murres attended Cliff Rock West only early in 199 6, and not at all in 1997. A new colon, formed on another seastack (Northwe st 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 atten dance fluctuated widely until the onset of egg-laying in early May, but the n stabilized and increased somewhat until July, when numbers began to drop at the end of the fledging period. Both air temperature and the upwelling i ndex (UI) were significantly correlated with murre attendance in May and Ju ne during the incubation and nestling periods. Mean attendance increased 18 -30% at these colonies between 1996 and 1997. Diurnal attendance in 1996 an d 1997 varied both within and between colonies and days, yet there was a si gnificant downward trend in numbers throughout the day as well as an effect of colony disruption by other marine species. Given that seasonal and diur nal attendance patterns of Flattop, East, and Mid Rocks in 1996 and 1997 ap pear similar, both between years and to other studies, we assume that these colonies recovered quickly following the 1995 shipwreck. However, interspe cific interactions and predation continue to affect diurnal attendance patt erns and productivity of these colonies.