BIOLOGY AND CONSERVATION OF THE COMMON MURRE 7 



of the colony, prior to developing regular attendance 

 during the pre-egg stage (Parrish 1995). 



Daily attendance patterns vary depending on the 

 year, time of year, weather, stage of the reproductive 

 cycle, and location of breeding colony. Variation mainly 

 reflects the amount of time spent on the colony for 

 breeding-related purposes versus time spent at sea. In 

 Newfoundland, incubation shifts of common murres 

 averaged 17 h, whereas daytime brooding shifts 

 averaged 4 h and overnight brooding shifts averaged 

 12 h (Verspoor et al. 1987). Incubation and brooding 

 shift durations did not differ between males and females, 

 females incubated more at night than males, breeding 

 pairs did not change over at night, and breeding pairs 

 did not change over at the same time each day. However, 

 shifts during chick rearing and feeding rates varied 

 between years in response to differences in food 

 availability (Birkhead and Nettleship 1987c; Verspoor 

 et al. 1987). On the South Farallon Islands during 1971- 

 72, attendance was consistently higher in the morning 

 than midday throughout winter and spring prior to 

 breeding and peaked again in the evening (Boekelheide 

 et al. 1990), although some sites were occupied all day. 

 In winter, murres were usually absent on days with high 

 winds or heavy rain, which is a pattern also observed in 

 the United Kingdom (Birkhead 1978a). Attendance by 

 thick-billed murres in arctic Canada appears linked to 

 barometric pressure but high winds are associated-with 

 the approach of low pressure systems (Gaston and 

 Nettleship 1981). Murres rarely visited colony sites on 

 the South Farallon Islands on days with large swells, 

 which were often associated with the passage of storm 

 systems when air pressure changed radically 

 (Boekelheide et al. 1990). 



Timing of Breeding 



In California, the breeding season generally occurs 

 between late April and early August when prey are 

 abundant in the California Current upwelling system 

 (Briggs et al. 1988; Ainley et al. 1990; Boekelheide et 

 al. 1990; Tyler et al. 1993). On the South Farallon 

 Islands, first egg dates for 1972-83 were 26 April-23 

 May with mean dates for first eggs 9 May-9 June 

 (Boekelheide et al. 1990). The timing of breeding in 

 Oregon is similar to that in California with egg laying 

 starting in early to mid-May and peaking in late May to 

 early June (Scott 1973; R. W. Lowe, unpublished data). 

 Breeding occurs somewhat later in Washington and 

 British Columbia with egg laying occurring from late 

 May to mid-August, reaching a peak in early July 

 (Jewett et al. 1953; Vallee and Carter 1987; Campbell 

 et al. 1990; Rodway 1990; Parrish 1995). The later 

 breeding phenology in Washington and British 



Columbia may be partly related to a later availability of 

 abundant prey resources than farther south in the 

 California Current upwelling system (Tyler et al. 1993; 

 Wahl et al. 1993; Murphy and Schauer 1994). 



Colony departure begins in late June or early July 

 in most years at the South Farallon Islands, but did not 

 occur until late July in 1983 during severe El Nino 

 conditions (Boekelheide et al. 1990; Takekawa et al. 

 1990). Peak departure typically occurs in early to 

 mid-July. Colony departure begins in late June and 

 continues during July in Oregon (Bayer et al . 1 99 1 ). At 

 Yaquina Head, Oregon, peak fledging occurred in late 

 July from 1969 to 1971 (Scott 1973). At Tatoosh Island, 

 Washington, colony departure occurred from late July 

 to early September 1991 (Parrish 1995). At Triangle 

 Island, British Columbia, colony departure begins in 

 mid- August and peaks in late August to early September 

 (Vallee and Carter 1987; Rodway 1990). 



Egg Laying 



The breeding site typically is located in a 

 depression or crack in rock, guano, or soil, and can be 

 bordered by adjacent rocks (Figure 1.3). On occasion, 

 birds breed in small caves, under boulders, or under 

 ledges. The common murre has a large single medial 

 brood patch and incubates the single egg on the bare 

 substrate or, at times, between and on top of its feet. No 

 nest is built, although small stones, feathers, or other 

 materials can be present at breeding sites. The large egg 

 (about 108 g or about 1 1% adult weight; Mahoney and 

 Threlfall 1981; Nettleship 1996; Gaston and Jones 

 1998) is often brightly colored (blue or green; at times 

 white), marked with variable amounts of dark streaks or 

 blotches, and has a strongly-pointed ("pyriform") shape. 

 The pointed shape of murre eggs is often cited as an 

 apparent adaptation for breeding on narrow rock cliff 

 ledges by reducing egg loss from accidental 

 displacement. These eggs roll in a tight circle compared 

 with more ovate egg shapes, especially during the late 

 incubation period when the embryo is partly developed 

 (Tschanz et al. 1969). Even so, many murre eggs roll 

 away from breeding sites, sometimes accounting for most 

 breeding failures (Tuck 1961). Other research suggests 

 that breeding-site characteristics and incubating 

 behavior also are important adaptations preventing egg 

 loss (Ingold 1980; Harris and Birkhead 1985; Birkhead 

 and Nettleship 1987b). 



A replacement egg will be laid by some females, if 

 the first egg is lost. Replacement eggs are about 5-10% 

 lighter than first eggs (Mahoney and Threlfall 1981; 

 Gaston and Jones 1998). Replacement eggs were laid 

 by 32% of Farallon murres that lost first eggs 



