Spatial survival analysis accounts for female-biased breeding dispersal and provides realistic estimates of true annual survival in migratory warblers

Abstract

Breeding dispersal—between-season change in breeding location—is usually female-biased in birds and creates problems in accurately estimating annual survival, as conventional Cormack-Jolly-Seber (CJS) survival models cannot discriminate between mortality and undetected emigration. Recently, spatial CJS (s-CJS) models have been developed that use data on breeding dispersal within a population to account for undetected emigration and provide corrected estimates of true annual survival, a development that promises to advance avian conservation initiatives that require accurate estimates of annual survival. Using a 14-yr dataset on a color-banded population of Setophaga citrina (Hooded Warbler) in northwest Pennsylvania, I examined female-biased breeding dispersal and performance of a s-CJS model in estimating true annual survival of females and males. I also compared my findings to published literature on other migratory North American warblers, a group with many species of high conservation concern. Breeding dispersal in the S. citrina study population is strongly female-biased, with median dispersal distances of 151 m for females (n = 227) and 51 m (n = 336) for males. Although most individuals disperse short distances, the observed pattern of breeding dispersal within the study site was best modeled using a heavy-tailed Cauchy dispersal kernel, a model that indicates the presence of a substantial tail of undetected long-distance dispersal, particularly in females. Using the Cauchy model, s-CJS analysis yielded realistic estimates of S. citrina true annual survival, 0.61 for both sexes, and resolved ambiguities evident in much lower estimates of apparent annual survival, 0.45 for females and 0.54 for males, derived from conventional CJS analysis. Because long-distance breeding dispersal is widespread in migratory warblers and especially frequent in females, analyses of warbler survival should employ s-CJS methods whenever possible, as estimates of apparent annual survival derived from conventional CJS methods will in most cases poorly approximate true annual survival.