Bald eagle (Haliaeetus leucocephalus) subadult demography
Demography of West Coast Wintering Subadult Bald Eagles Based Upon Frequency of Plumage Classes 1979-1981.Jorma Jyrkkanen, 7 March 1985.
Abstract:
Demographic parameters lx, qx and ex were computed for subadult Bald Eagles (Haliaeetus leucocephalus) based on frequency of four subadult and one adult plumage classes (J1, J2, J3, J4, A) (After Gerrard et al. 1978) from within-year shrinkage on the assumption that it represented between year mortality.
From the four year mean, these plumage classes formed for J1, J2, J3, J4/20.57%, 15.5%, 10.3%, 4.95% respectively of the sample population. Survivorship (lx) varied from year to year.
Lowest life expectancy (ex) was for J1 at 2.87 years while the maximum was for J4 at 21.6 remaining years. The mean ex from four years of sampling statistics were ej1, ej2, ej3, ej4/4.42, 4.7, 5.81, 10.58 years remaining respectively. Real reproductive rate Rr was (1979, 1980, 1981, 1982/1.166, 1.09, .8893, .453) coinciding with years of high, lean, very lean and undetermined salmon carcass availability.
These results suggest a large decline of juveniles younger than recruit age probably due to competition over late winter food resources with selection improving the life expectancy outlook for survivors. Production cutbacks following lean years suggests juveniles are sacrificed during lean years to conserve the adult breeding population and older juveniles for recruitment. Older juveniles have proven their ability to become effective hunters and social creatures by virtue of their survival.
Key Words:Bald eagle, Haliaeetus leucocephalus, British Columbia, Squamish, Coastal, Demography, survival, life-expectancy, real reproductive rate, mortality, frequency, plumage class.
© 1985 Jorma Jyrkkanen
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