Timing and Success of Breeding in Pelagic Cormorants

Of the 6 cormorant species that inhabit the North Pacific, the Pelagic Cormorant (Phalocrocorax pelagicus) is the smallest and most widely distributed (Hobson 1997). Pelagic Cormorants breed from Baja California in the eastern Pacific north to the Bering and Chukchi Seas, and south across the western Pacific to southern China. North America supports an estimated one-third of the global population of 400,000 birds (Johnsgard 1993), with the bulk breeding in Alaska. Campbell and others (1990) estimated that only about 4200 pairs bred in British Columbia, mostly at small and widely dispersed sites centered within the Strait of Georgia. Information on the population status of Pelagic Cormorants in British Columbia is limited. Populations appear to have declined over recent decades in the Haida Gwaii archipelago (Fraser and others 1999), although there is some question about the reliability of the baseline data (Harfenist and others 2002). More conclusive evidence suggests that dramatic declines have occurred in the southern Strait of Georgia (by about 50% since 1987; Chatwin and others 2002), and along the west coast of Vancouver Island (by about 85% since 1969; Carter and others 2007). The declines have been attributed to disturbance at breeding colonies by humans and Bald Eagles (Haliaeetus leucocephalus), and reductions in prey availability due to oceanographic change (Chatwin and others 2002; Carter and others 2007). Persistent contamination and mortality due to oiling and fisheries practices might also be contributing factors (Harris and others 2005; Smith and Morgan 2005). There is at present very little information on the timing and success of Pelagic Cormorant breeding at colony sites in British Columbia, especially for regions outside of the Strait of Georgia (Drent and others 1964; Harris and others 2005). This information is pertinent to understanding causes of regional population change, and given declining population trajectories, there is a need for basic demographic measures. Triangle Island is situated about 40 km west of Cape Scott off the northwestern tip of Vancouver Island. This site supports one of British Columbia’s largest Pelagic Cormorant breeding aggregations, comprising over 400 breeding pairs in 1989 (Rodway and others 1990). Here we report on a 6-y study (2003– 2008) of the timing and success of breeding in Pelagic Cormorants at Triangle Island, through a period of extreme oceanographic variation along the British Columbia coast (Mackas and others 2007). This report constitutes the most comprehensive account of the biology of the species at this comparatively pristine site, and should provide a baseline against which future changes can be measured. Field crews collected data on Triangle Island (UTM: Zone 09, 0494480E 5634395N, WGS84) from late March until late August in each year between 2003 and 2007, and from mid-May until mid-August in 2008. Beginning on arrival in the spring, we logged systematic, detailed observations of all cormorant nests situated within the borders of a small cliff-face plot (approximately 25 m) established in 2002 on Puffin Rock, a small islet adjoined to the main island at low tide. Our viewing blind was situated about 50 m from and 2 m above the highest nest on the plot, and provided a good view into all nests when adults stood up. For all active nests, we recorded initial laying dates, hatching dates, and dates when a chick