COMPARATIVE DEMOGRAPHY OF NEW WORLD POPULATIONS OF THRUSHES (TURDUSSPP.)

Abstract

This analysis uses information obtained primarily from museum collections to estimate demographic parameters of populations of thrushes (Turdus spp.) from throughout the Western Hemisphere. Adult survival rates were estimated from the proportions of individuals >1 yr old in museum collections, assuming constant population size, discrete reproductive seasons, unbiased collecting, and maturity at 1 yr old. The effects of relaxing each of these assumptions were examined. The resulting biases were found to be small compared to differences between populations, and in several cases, they were opposite to the conventional wisdom that survival rate increases towards the equator. The annual survival rate of adults (S) is estimated without bias by E(S) = A/(A + I), which has a standard error of [(AI)/(A + I)3]½, where A is the number of adults in a collection and I is the number of immatures. In Turdus, adult and immature birds can be distinguished by the appearance of the secondary coverts, the immature forms of which are retained until the first postnuptial molt at ≈1 yr old. The present analysis included 8653 specimens from 30 populations of 19 species from Alaska to southern Patagonia. Additional data tabulated from museum specimens included numbers of juveniles, evidence of breeding, worn plumage, and molt. Estimated annual survival rates varied from a mean of 0.56 in temperate North American populations, to 0.68 in subtropical South American populations, 0.76 in lowland tropical populations, and 0.80 and 0.85 in tropical montane populations in Central and South America, respectively. Samples of adults and immatures restricted to the period immediately prior to the breeding season gave results that were indistinguishable from samples summed over the entire year. Survival rate was most strongly correlated with the difference between maximum and minimum mean monthly temperatures (r = −0.84). Thus, survival is inversely related to seasonality of temperature. An index to fecundity (J) was estimated by the frequency of juvenile specimens (generally <2 mo old) relative to those of adults in museum collections. This index was inversely related to annual adult survival. A relative index of prereproductive (generally 1st-yr) survival (Sa) was calculated as the annual adult mortality [1 − E(S)] divided by J. Estimated Sa was independent of estimated S. Calculations of annual fecundity (F) from field studies on breeding of selected species supported both of these observations. These results, which are consistent with conclusions based upon broader comparisons among birds, suggest several hypotheses: (1) adult survival in Turdus populations is determined by physical conditions during the nonbreeding season, particularly various ecological effects of low temperature; (2) fecundity is sensitive to density-dependent feedbacks from adult population size and, possibly, to environmental factors correlated with other factors that independently affect adult survival; and (3) 1st-yr survival, age at maturity, or both, also are affected by density-dependent factors.