COMPARATIVE DEMOGRAPHY OF NEW-WORLD POPULATIONS OF THRUSHES (TURDUS SPP)

This analysis uses information obtained primarily from museum collecti ons to estimate demographic parameters of populations of thrushes (Tur dus spp.) from throughout the Western Hemisphere. Adult survival rates were estimated from the proportions of individuals >1 yr old in museu m collections, assuming constant population size, discrete reproductiv e seasons, unbiased collecting, and maturity at 1 yr old. The effects of relaxing each of these assumptions were examined. The resulting bia ses 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), whic h has a standard error of [(AI)/(A + I)(3)](1/2), where A is the numbe r of adults in a collection and I is the number of immatures. In Turdu s, 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 approximate to 1 yr old. The present ana lysis included 8653 specimens from 30 populations of 19 species from A laska to southern Patagonia. Additional data tabulated from museum spe cimens included numbers of juveniles, evidence of breeding, worn pluma ge, and molt. Estimated annual survival rates varied from a mean of 0. 56 in temperate North American populations, to 0.68 in subtropical Sou th American populations, 0.76 in lowland tropical populations, and 0.8 0 and 0.85 in tropical montane populations in Central and South Amer i ca, respectively. Samples of adults and immatures restricted to the pe riod immediately prior to the breeding season gave results that were i ndistinguishable from samples summed over the entire year. Survival ra te was most strongly correlated with the difference between maximum an d minimum mean monthly temperatures (r = -0.84). Thus, survival is inv ersely related to seasonality of temperature. An index to fecundity (J ) was estimated by the frequency of juvenile specimens (generally <2 m o old) relative to those of adults in museum collections. This index w as inversely related to annual adult survival. A relative index of pre reproductive (generally Ist-yr) survival (S-a) was calculated as the a nnual adult mortality [1 - E(S)] divided by J. Estimated S,was indepen dent of estimated S. Calculations of annual fecundity (F) from field s tudies on breeding of selected species supported both of these observa tions. These results, which are consistent with conclusions based upon broader comparisons among birds, suggest several hypotheses: (1) adul t 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 f eedbacks from adult population size and, possibly, to environmental fa ctors correlated with other factors that independently affect adult su rvival; and (3) Ist-yr survival, age at maturity, or both, also are af fected by density-dependent factors.