The Neotropical-migratory Cerulean Warbler (Setophaga cerulea) is one of North America's strongly declining songbirds. The three hypotheses of alteration in breeding, wintering, and migration habitat, potentially explain declines. Distinction between these hypotheses could possibly reallocate resources of current conservation efforts. For the Cerulean, two studies show that annual adult migration survival reflects the range expected for non-declining species (0.4-0.6), whereas, annual reproductive output is very low throughout its breeding range. This suggests a strong influence for events on the breeding grounds affecting its population decline. As an independent test of this qualitative model, annual survival and reproduction are being measured in a non-declining, small-bodied, blue, insectivorous Neotropical migrant, the Blue-gray Gnatcatcher (Polioptila caerulea).