Beissinger 



Chapter 37 



Population Trends Projected from Demographic Analyses 



Table 2 Surveys of the ratios of juveniles to after-hatch-year birds (adults and subadults) for Marbled Murrelets 

 during the breeding season conducted from boats cruising at a variety of distances from shore. The percentage of 

 juveniles (Pet. juv.) was adjusted for the timing of the survey (survey period) by using the cumulative frequency of 

 fledging dates (fig. 2) to estimate an adjusted percentage of juveniles (Adj. pet. juv.) for the end of the nesting season 



The adjusted ratios of young-of-the-year murrelets to 

 after-hatch-year birds were generally low, although there 

 was considerable variation among juvenile ratios (tables 1 

 and 2). The most reliable ratios for estimating murrelet 

 fecundity would come from at-sea surveys which covered 

 long distances (>20 km) or large areas and surveyed close to 

 shore (< 500 m) as well as farther away in order to have a 

 better chance of encountering clumps or groups of juveniles. 

 To the best of my knowledge, only two data sets fulfill both 

 requirements - total area counts in Clayoquot Sound, British 

 Columbia and surveys off the coast of central Oregon (table 

 2). Both studies had seasonally adjusted juvenile ratios around 

 4-5 percent, so I chose to use 5 percent as a realistic estimate 

 of fecundity. Although Ralph and Long's (this volume) 

 surveys indicate that juvenile ratios may be as low as 2 

 percent, their transects did not consistently extend closer 

 than 800 m from shore and may have underestimated the 

 true ratio. Likewise, the 15 percent ratios from onshore 

 counts appear to greatly overestimate the proportion of 

 juveniles because the vast majority of adults would have 

 been too far from shore to be detected (Ralph and Miller, 

 this volume). However, onshore counts do suggest that the 5 

 percent estimate of fecundity could be too low if at-sea 

 surveys had missed many juveniles. Thus, I also evaluated 

 optimistic estimates of adjusted juvenile ratios of 10 percent, 

 twice the realistic value and similar to corrected nesting 

 success derived below. 



Fecundity might also be estimated from studies of nesting 

 success, but this is more difficult to do for the murrelet. A 

 total of 22 nests have been found in the Pacific Northwest 

 see table 2 of the study by Nelson and Hamer (this volume 

 b). Only 36 percent of the murrelets successfully fledged 

 young. This would yield an estimate of 0.36 young produced 

 per nesting pair (since murrelets can fledge only 1 young), or 

 0. 1 8 female young per nesting female, assuming half of the 

 young fledging would be males based on the sex ratio found 

 by Sealy (1975a). 



This value overestimates fecundity for two reasons. First, 

 many nests were found after the young had hatched. This 

 would greatly overestimate overall nesting success because 



murrelet nests often fail (>50 percent) in the egg or early 

 stages of chick-rearing before they are likely to be detected 

 see table 3 of the study by Nelson and Hamer (this volume 

 b). The true number of female chicks fledging per female 

 may be closer to 0.15. Second, it is unlikely that all females 

 would attempt to nest every year and a significant proportion 

 of the population (5-16 percent) may be nonbreeders (Hudson 

 1985). Third, the estimate of fecundity for the post-breeding 

 model assumes that the young have safely reached the ocean. 

 The long flight from the nest to the ocean can be expected to 

 be hazardous for nestlings as exemplified by grounded young 

 birds that have been found (Carter and Erickson 1 992, Rodway 

 and others 1992). Thus, to arrive at a fecundity value, the 

 true number of female young per nesting female (0.15) 

 would have to be corrected by multiplying it by: (1) the 

 estimated proportion of adult birds nesting (averaged from 

 the estimates of Hudson cited above to yield 0.9); (2) the 

 proportion of young that survive from fledging to until the 

 time of census (anybody's guess, but 0.9 might be a reasonable 

 estimate); and (3) the number of nesting attempts per pau- 

 per year which is assumed to be 1 (Hamer and Nelson, this 

 volume a). This would result in a fecundity value around 

 0.12, similar to average estimates from onshore juvenile 

 ratios (table 1). 



Estimating Survivorship 



Nur (1993) found that the annual probability of survival 

 for adults (P 2 ) was positively related to body size for 10 

 species of alcids. Similar data are presented in figure 1 of De 

 Santo and Nelson (this volume). Adult survivorship ranged 

 from about 0.75-0.77 for small-bodied Least Auklets (Aethia 

 pusilla) and Ancient Murrelets (Synthliboramphus antiquus) 

 to 0.91-0.94 for large-bodied Atlantic Puffins (Fratercula 

 arctica), and Common and Thick-billed murres (Uria aalge 

 and U. lomuia). Nur also found that adult survivorship was 

 negatively related to annual reproductive effort (clutch size 

 times broods per year) after controlling for the effects of 

 body size. Together these two variables accounted for 72 

 percent of the variation in annual survivorship among the 10 

 species. Nur then derived a multiple regression model to 



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



389 



