Hamer and Nelson 



Chapter 4 



Nesting Chronology 



Table 2 Number of Marbled Murrelt young observed and estimated to have fledged from nests in North America in 10-day intervals and listed 

 by state and province. Records (n = 86) were used only if the error in estimating the fledging date was <8 days. The total length of the breeding 

 period is listed under each state or province. The date displayed is the beginning of each 10-day period 



Explanations for the presence of two distinct breeding 

 periods include: (1) small sample sizes, (2) variations in the 

 timing of breeding of murrelets between years (as suggested 

 by Carter and Erickson 1992), or (3) variation in oceanic and 

 environmental conditions that promote breeding within these 

 two distinct periods. Small sample sizes may not adequately 

 explain this phenomenon, because it occurs only in the 

 southern portions of the murrelet range, with sample sizes 

 very similar to regions to the north. If the between-year 

 variation in the timing of breeding was responsible for this 

 trend, some young would have been expected to fledge over 

 a period of years within the 1- to 2-week gap between 

 breeding periods. A distinct proportion of the population 

 nesting during each period is also possible. It is not clear 

 what environmental or biological agents might cause this to 

 occur. However, the most likely explanation is that some 

 proportion of murrelets may attempt to lay a second clutch 

 within the same breeding period. It is also possible that pairs 

 with failed nests attempt to renest. The longer breeding 

 season available for the murrelet in Oregon and California 

 may make renesting more likely than in the northern regions 

 of the range. The gap in breeding chronology may give 

 females enough time to develop a new egg and select a 

 different nest site. The shorter incubation and nestling period 

 in the Marbled Murrelet, when compared to that of other 

 species, such as the puffins and auklets, may make double 

 brooding more feasible. 



Egg replacement is a regular occurrence in ledge-nesting 

 Alcids (Johnsgard 1987). Tuck (1960) estimated that 44 

 percent of Thick-billed Murres (Uria lomvia) pairs lost at 

 least one egg during a 32-day period, with 30 percent of 

 pairs laying one replacement egg; 1 1 percent laid two 

 replacements, and the remaining 3 percent deserted or did 

 not lay again. For higher arctic forms there are probably no 

 opportunities to renest because of a short breeding season 

 (Johnsgard 1987). The Xantus' Murrelet (Synthliboramphus 

 hypoleucus) may lay two broods because egg laying has 

 been observed until July, and Murray and others (1983) 



found evidence of occasional egg replacement in this species. 

 Both Sealy (1975a) and Gaston (1992) found no evidence of 

 replacement clutches in the Ancient Murrelet (5. antiquus). 

 If renesting and egg replacement does occur in the Marbled 

 Murrelet, it will affect the interpretation of inland survey 

 data and at-sea census results and population modeling for 

 this species. 



Breeding Phenology Dates 



From the information presented above, we propose the 

 following dates for the breeding phenology of the murrelet 

 by state and province (fig. 3). In California, we estimated 

 that the total breeding season lasted approximately 170 days. 

 The first breeding period was 103 days long while a possible 

 second period was 87 days long. The breeding periods were 

 separated by 8-11 days. Incubation commenced 24 March 

 and ended 13 August. The nestling period began 23 April 

 and ended 9 September. 



In Oregon, incubation was estimated to begin on 26 

 April and last until 25 August (fig. 3). The nestling period 

 was estimated to begin 26 May and end on 21 September. 

 The total breeding season length was 21 days shorter than 

 that in California and was approximately 149 days long. The 

 two possible periods of breeding activity were separated by 

 only 6 days. The earliest recorded fledging date in Oregon is 

 of a nestling observed to fledge from a nest on 22 June 1993 

 (Nelson, pers. obs.). 



North of California and Oregon, the length of the breeding 

 season was more restricted (fig. 3) (table 2). In Washington, 

 the breeding season might appear shorter because of the 

 smaller sample of breeding records used to predict fledging 

 dates. However, it is probably similar to that found in British 

 Columbia. Incubation was estimated to begin 26 April and 

 end 30 July. The nestling period began 26 May and ended on 

 27 August. The total length of the breeding season was 1 24 

 days long, 25 days less than Oregon. The earliest fledging 

 record is a nestling observed to fledge on 22 June 1993 

 (Ritchie, pers. comm.). The latest record is that of a nestling 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



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