A. Status of the Spectacled Eider - U.S. Fish and Wildlife Service

of older females: (1) older females in Common Eiders have larger mean clutch sizes (Baille
and Mime 1982); and (2) mean clutch size has increased significantly on the YKD (Stehn et al.
1993). Although this increase in clutch size could be a density dependent response to reduced
population size (Hario and Selin 1988), we would be encouraged to gather evidence on the
proportion of new nesters if we had survival data of the quality of Coulson‘s.
Of course, we do not currently have sufficient data on Spectacled Eiders to estimate survival
rates. It is therefore reasonable to ask whether, given the range of survival rates for Common
Eiders, we expect to see differences in values (e.g., the proportion of first-time nesters), that
would allow hypothesis testing with a high probability of correctly detecting a difference. The
approach taken is similar to the exercise presented above. Parameter values for the first four
parameters listed above are chosen from plausible distributions for those values based on
Common Eider data where the population growth rate was thought to be near stable (r = 0).
Because the null hypothesis is that the population is stable (r = 0), the final parameter, firstyear
survival rate, is solved to yield this growth rate. The alternate hypothesis is that one of
the parameters decreased to give a plausible rate of decline for the YKD (chosen from the
post-model distribution for the ground plot surveys in Appendix I). By decreasing adult
survival, first-year survival and fertility separately, we can calculate distributions of what
values these parameters would need to be to have been the sole cause of the decline.
Comparison of these distributions then helps us assess whether demographic research will be
likely to be able to eliminate hypotheses concerning the proximate cause of decline. For
example, if we knew thatjuvenile survival was the primary cause of the decline, research
priorities would be shifted to finding the cause for the reduction in juvenile survival.
To incorporate uncertainty in the four demographic parameters, each is drawn randomly from
distributions intended to cover the possible range of values for that parameter when
r = 0. For Common Eiders, demographic parameters each have quite large ranges. Because
our understanding of how these parameters might represent Spectacled Eiders is rudimentary,
we chose to represent most probability distributions with a triangular distribution. The
triangular distributions are defined by the minimum and maximum values for the parameter
found in the literature and have a maximum probability at the mean between the extreme
values. Estimates of mean clutch size from Spectacled Eiders in Alaska are used for the premodel
distribution (minimum = 4.05, maximum = 5.92, maximum probability of triangular
distribution = [minimum+ maximum]/2 = 4.985). This minimum is the 1965-1976 mean
(4.688) less 2 standard deviations (0.3176). The maximum is the 1986-1992 mean (5.104)
plus 2 standard deviations (0.406) (Stehn et al. 1993). Data for the proportion of adult
females nesting are: mean = 0.753 (Milne 1974), mean = 0.78 (Coulson 1984), 0.78-0.90
(Baillie and Milne 1982). Based on these values we used a triangular distribution: minimum
= 0.5; maximum = 1.0; maximum probability = 0.75. Estimates of age-specific proportion
mature are scarce but support a gradual onset of maturity between the ages of two and five
(Reed 1983; Baille and Milne 1982). Estimates from Common Eider populations that do not
show evidence of a decline (some were increasing at low rates usually