Comparative demography and behavioral comparison of the blue-gray gnatcatcher (Polioptila cerulea) and the cerulean warbler (Setophaga cerulea)
The Neotropical-migratory Cerulean Warbler (Setophaga cerulea) is one of North America's strongest declining songbirds. Alterations of breeding ground habitat are a potential primary cause of decline, versus wintering and migration habitat alterations being a primary factor driving population decline. Evaluating the influence of these hypotheses on population trends could reallocate the limited resources of conservation efforts. For the Cerulean, two studies show that annual adult migration survival reflects the range expected for non-declining species (40-60% winter/migration survival), whereas annual reproductive output is very low throughout its breeding range. This suggests a strong role for events on the breeding grounds in its population decline. As an independent test of this qualitative model, annual survival and reproduction were measured in a non-declining Blue-gray Gnatcatcher (Polioptila cerulea). Annual survival of the Blue-gray Gnatcatcher was predicted to be similar to that of the Cerulean, but reproduction of the Gnatcatcher to be significantly higher. Results from two field seasons (April-August 2011 and 2012) of Blue-gray gnatcatcher data were rigorously compared to six field seasons of data on the Cerulean warbler collected by Christopher M. Rogers (May-June 2003-2008). All statistical measures that were assessed regarding were contrary to original predictions. Cerulean warblers had higher nest success, equal nest output per successful nest, a higher number of female fledglings per female within the study populations and a higher rate of survival over wintering and migration. It is speculated that there was a great influence of drought on the two field seasons of Blue-gray gnatcatcher data collection.