Beissinger 



Chapter 37 



Population Trends Projected from Demographic Analyses 



Table 3 Predicted rates of annual change for Marbled Murrelet populations based on likely combinations of 

 demographic rates based on three different scenarios of juvenile recruitment and nesting success measured in the 

 field, and two levels of adult survival from comparative analysis. Lambda, the expected growth rate of the population, 

 was virtually unaffected by changes in age of first breeding 



Table 4 Sensitivity of lambda to changes in the Leslie matrix elements for the Marbled 

 Murrelet based on the three different fecundity scenarios for an age of first breeding of 

 3 years. See Table 3 for values used in each of the fecundity scenarios 



Discussion 



Model Parameter Estimates 



There are a number of sources of uncertainty in the 

 parameter estimates that may have affected model outcomes. 

 Lambda was most sensitive to changes in adult survivorship 

 (table 4), which is typical for potentially long-lived birds 

 like the murrelet. Estimates of survival have the greatest 

 uncertainty, since they were not derived from field data but 

 instead were based on comparative analyses of allometric 

 models. Nevertheless, there are reasons for confidence in the 

 estimates evaluated. Survivorship is often strongly related to 

 both body size and reproductive effort in birds (e.g., Gaillard 

 and others 1989, Saether 1988), and this trend was also 

 strong in the Alcidae (Nur 1993). The range of annual 

 survivorship values for adults evaluated in the model (0.85- 

 0.90) included more than two standard errors for the upper 

 bound of the prediction from the regression, which should 

 encompass > 95 percent of the variation in potential mean 

 estimates. Higher annual survival rates (0.90-0.94) are typical 

 only for three species of auks with body masses exceeding 

 600 g (Nur 1993; De Santo and Nelson, this volume), three 

 times the size of the murrelet. Survivorship ranges from 

 0.75-0.88 for seven alcid species with medium and small 

 body sizes (< 600 g); only the Atlantic Puffin had annual 

 survival rates routinely above 0.90. 



It is likely that annual survivorship for Marbled Murrelets 

 will be among the upper range of values evaluated in this 

 model (e.g., 0.87-0.90), because the murrelet' s inherently 

 low reproductive rate (1 egg per nesting attempt) requires 

 high survivorship for populations to grow. On the other 

 hand, because the murrelet' s unusual life history strategy of 

 nesting in old growth forests often far from the sea, it probably 

 faces higher mortality risks than other seabirds. Field studies 

 to determine survival rates are needed, and are becoming 

 more feasible as marking and telemetry techniques are 

 perfected for this bird (Quinlan and Hughes 1992; Priest and 

 Burns, pers. comm.). 



All measures of fecundity from field data for the Marbled 

 Murrelet appear to be low. Arguably the most complete 

 measures of fecundity were derived from juvenile ratios based 

 on extensive at-sea censuses corrected for the date of census 

 in relation to the timing of fledging (table 2, fig. 2). Extensive 

 at-sea censuses conducted recently have universally produced 

 low percentages of juvenile birds (table 2). Such low ratios 

 indicate poor reproductive success that could be due to high 

 nest failure rates from predation (Nelson and Hamer, this 

 volume b), or a low proportion of adults attempting to breed, 

 perhaps because they are unable to find suitable nest sites. 

 Some portion of the low reproductive success could have 

 been due to El Nino effects on food supplies. Although there 

 is ample evidence that El Nino affects nesting success of 



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



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