8 USGS/BRD/ITR-2000-0012 



(Boekelheide et al. 1990). Replacement rates of 52% 

 and 40% have been reported in the United Kingdom 

 and Alaska, respectively (Birkhead and Hudson 1977; 

 Byrd et al. 1993; Murphy 1995). Eggs laid and lost 

 early in the season are more likely to be replaced than 

 those laid later (Uspenski 1958; Tuck 1961; Gaston 

 and Nettleship 1981; Harris and Birkhead 1985; 

 Boekelheide et al. 1990). The interval between loss and 

 replacement is 14-15 days (range, 13-23 days; 

 Boekelheide et al. 1990; Murphy 1995). On the South 

 Farallon Islands, more than 50% of replacement eggs 

 were laid in only 3 years (i.e., 1973, 1977, and 1981) 

 between 1972 and 1983 (Boekelheide et al. 1990; Table 

 1 . 1 ). In the latter 2 years, high loss of first eggs occurred 

 early in the season and favorable conditions for relaying 

 extended late into the season. 



Incubation and Hatching 



Both members of a breeding pair incubate the egg. 

 Incubating birds typically face inwards toward the face 

 of vertical cliffs, upslope, or rock walls (Figure 1 .3), 

 which allows for effective incubation conditions on 

 small breeding sites on narrow ledges or within larger 

 groups of densely-breeding birds on flat surfaces. In 

 addition, incubating and brooding postures reduce 

 jostling between neighboring birds and limit access by 

 avian predators of murre eggs and chicks (e.g., gulls 

 and corvids). Adults will retrieve eggs that roll a short 

 distance away from breeding sites and birds occasionally 

 incubate an egg which is not their own (Tschanz 1959). 



The incubation period shows little annual variation in 

 Newfoundland (Verspooret al. 1987), but the incubation 

 period was shorter for later eggs in Alaska (Murphy 

 1995). From 1973 through 1983, murre incubation 

 periods at the South Farallon Islands averaged 32-33 

 days but ranged from 26 to 39 days (Boekelheide et al. 

 1990). Similar incubation periods were found for 

 common murres in Alaska (Murphy 1 995) and for thick- 

 billed murres in arctic Canada (Gaston and Nettleship 

 1 98 1 ). Based on chromosome analyses, a female to male 

 ratio of 25: 17 was found for chicks at about day 20 but 

 this ratio did not differ significantly from equality (Parker 

 etal. 1991). 



Chick Hearing and Colony Departure 



Common murre chicks are constantly attended by 

 at least one parent at the breeding site. During the first 

 few days after hatching, the chick must be brooded 

 constantly by an adult until it is able to thermoregulate. 

 Murre chicks, on rare occasions, are brooded and fed by 

 a nonparent adult or "helper," often a failed breeder 

 from a neighboring site whose relationship to the chick 

 and parents is unknown (Tschanz 1968, 1979; Birkhead 

 1977a; Birkhead and Nettleship 1984; Wanless and 

 Harris 1985). Tuck (1961) noted that the greatest source 

 of chick mortality was exposure during the first 6 days 

 of life. Murre chicks develop a specialized plumage 

 and onset of thermoregulation occurs by 10 days 

 (Johnson and West 1975). Older chicks are not brooded 

 constantly but attending adults protect the chick from 



Table 1.1. Reproductive success of first and replacement clutches of the common murre on the South Farallon Islands, California, 1 972-1 983 

 (from Boekelheide et al. 1 990; see Figure 1 .5). a 



"Codes: HS, hatching success (percent of eggs laid that hatch); Lost (i.e., disappeared before hatch); Addled (i.e.. eggs that fail to 

 hatch although incubated for at least 32 days); FS, fledging success (i.e., percent of chicks hatched that depart from the colony); 

 Breeding success (i.e., number of fledglings per breeding site). 



b Mean percent values for 1972-82 (n = 1 1 years) excluded data in 1983 during severe El Nino conditions; however, total n values 

 included 1983 data (n = 12 years). 



