Hamer and Nelson 



Chapter 4 



Nesting Chronology 



the same geographic area and surveys commenced before 

 the fledging period of the breeding season. Therefore, only 

 one datum was used for each boat survey in each year, 

 indicating the first fledging date for that season. These 

 observations made up a small portion of the records used. 



Birds were assumed to be juveniles when a grounded 

 juvenile was reported, and a plumage description was not 

 provided. Fortunately, the use of the word "juvenile" to 

 describe a young bird that had lost its downy plumage was 

 consistent throughout the literature. We assumed that grounded 

 juveniles were recently fledged individuals. The majority of 

 records of grounded juveniles included descriptions of 

 remaining down on the back and head and the presence of an 

 egg tooth, confirming recent fledging. 



Records of eggs collected or found and of incubating 

 adults were not used because the development period of the 

 embryo was unknown. But the presence of a postovulatory 

 follicle, unshelled egg in the oviduct, or mature follicles 

 from collected females indicated that egg laying would 

 occur in 1-3 days (Sealy 1974). Therefore, egg-laying dates 

 were estimated for these records by adding 2 days to the 

 collection date and then estimating the fledging date by 

 adding 58 days. 



Sealy (1974) obtained 12 breeding records in British 

 Columbia. He collected murrelet specimens in weekly 

 intervals during the breeding season between 30 April and 

 10 August. He examined the size of the brood patch in male 

 and females and the size and maturation of the largest 

 follicle in each ovary of females to estimate the timing of 

 egg laying, incubation, and chick rearing. Observations of 

 adults carrying fish in their bills at dusk were used to 

 estimate the hatching dates of eggs. The first fledglings 

 observed on the water were used as an indication of the 

 earliest fledging dates. For our summary, we used only 

 records from Sealy in which the size and maturation of the 

 ovarian follicles of females enabled an accurate estimate of 

 the egg-laying date, and two cases in which juveniles were 

 first observed at sea. Observations of brood patch 

 development and fish-carrying behavior were not used 

 because the accuracy of these methods in estimating the 

 nesting stage of the Marbled Murrelet is unknown. 



Carter and Erickson (1992) reviewed the breeding 

 chronology of the murrelet in California by examining inland 

 records of downy young and grounded juveniles, molt 

 conditions of museum specimens, and records of juveniles 

 observed at sea from 1892 to 1987. Carter and Erickson used 

 28 days for a nestling period and 30 days for an incubation 

 period to estimate breeding chronology. In addition, 41 inland 

 records of downy young and fledgling murrelets from 1918 

 to 1986 were summarized in North America by Carter and 

 Sealy (1987b). Records of grounded nestlings and juveniles 

 from these studies, in which an accurate fledging date could 

 be estimated, were used in this analysis. 



Fledgling dates were estimated using 86 breeding records 

 from California (n = 25), Oregon (n = 13), Washington (n = 

 13), British Columbia (n = 23), and Alaska (n = 12) (table 



1). Records used for this analysis included observations of 

 the presence of a postovulatory follicle (n = 9) or unshelled 

 egg in the oviduct of collected females (n = 2), known egg- 

 laying dates (n = 2), known egg-hatching dates (n = 5), 

 observations of young on nests (n = 4), grounded chicks (n = 

 9), grounded fledglings (n = 35), juveniles observed to fledge 

 (n = 10), and dates that juveniles were first observed at sea 

 from marine census studies (n = 9). Fledging dates for a 

 large proportion of the records (67 percent) could be estimated 

 accurately because the egg-laying date or the hatching date 

 was known, the young were accurately aged on the nest, a 

 grounded fledgling was recorded, or a nestling was actually 

 observed to fledge (n = 56). 



Results 



Records of the earliest and latest breeding records of the 

 Marbled Murrelet were all collected from California and 

 Oregon. The earliest fledging record in North America was 

 of a grounded downy chick discovered in Big Basin State 

 Park in central California on 20 May 1989. The chick was 

 estimated to be at least 2 weeks old (S.W. Singer, pers. 

 comm.). The next fledging was a nestling observed to fledge 

 on 7 June 1992 from a nest also in Big Basin State Park 

 (S.W. Singer, pers. comm.) (fig. 1). A record also exists of a 

 grounded murrelet in Big Basin State Park on 18 May 1984, 

 but it was not clear whether the bird was an adult or juvenile 

 (S.W. Singer, pers. comm.). The next four earliest fledging 

 dates, from 12 June to 17 June, were all from Big Basin and 

 Portola State Parks in central California (Anderson 1972; 

 Carter and Erickson 1992; S.W. Singer, pers. comm.). 



The latest fledging date was a record of a fledging found 

 on 21 September 1987 in a parking lot in the town of Siletz, 

 Oregon (Heinl 1988, Nelson and Peck, in press). The next 

 four latest fledging dates, from 30 August to 9 September, 

 were all recorded from California and Oregon (Carter and 

 Erickson 1992, Erickson and Morlan 1978, Jewett 1930, 

 Nelson and others 1992, Singer and Verardo 1975). 



The number of young observed or estimated to have 

 fledged for all North American records was summarized for 

 each 10-day period during the breeding season. Fledging 

 rates increased rapidly from 6 June to 19 July, and peaked by 

 the 10-day period beginning 19 July (fig. 1). A possible 

 second peak in the number of young fledged was evident for 

 the 10-day period beginning 1 8 August, with a rapid decrease 

 in the number of young leaving nests in late August and 

 early September. Egg laying and incubation began 24 March 

 and ended 25 August, with the nestling period beginning 23 

 April and ending with a fledging record on 2 1 September, a 

 breeding period of 182 days. 



An analysis of the cumulative number of young fledged 

 in North America for each 10-day period was used to predict 

 the percentage of total juveniles that would be observed in 

 the marine environment during different periods of the 

 breeding season. This analysis also demonstrated the broad 

 nesting chronology of the Marbled Murrelet (fig. 2). The 



52 



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



