NESTING SUCCESS AND BREEDING BIOLOGY OF CERULEAN WARBLERS IN MICHIGAN

Abstract

The Cerulean Warbler (Dendroica cerulea) is a Nearctic-Neotropical migratory bird species that has declined significantly over the long-term. Poor reproductive success may be an important factor contributing to the observed decline, but reproductive output has been measured for very few breeding populations. From 2003 to 2005, I intensively monitored 22–23 breeding territories/year in each of two large forest habitats in southwestern Michigan: oak- (Quercus spp.) hickory (Carya spp.) (2003: Barry State Game Area) and black locust- (Robinia pseudoacacia) black cherry (Prunus serotina) (2004–2005: Fort Custer U.S. Army Michigan National Guard Reservation). I also gathered descriptive data on nonsong vocalizations and age of territorial males. I describe four distinctive call notes, by sex, including the social and environmental contexts in which they were used. Using two independent methods of aging, there was a strong preponderance of after-second-year males at both study sites. Only 9 (n = 7 nests), 12 (n = 14), and 30 (n = 25) fledglings were produced during the 2003, 2004, and 2005 breeding seasons, respectively. Nest heights were the highest recorded for this species (mean = 19–20 m). During the same period, male reproductive success was 0.30, 0.32, and 0.80 male fledglings/breeding male and 0.60, 0.63, and 1.58 fledglings/breeding pair. Productivity estimates, not thought to be self-sustaining, were even lower than those of a well-studied Cerulean Warbler population in southern Ontario. Thus, reproductive output was low in two geographic regions—representing three different forest types—in the northern portions of the Cerulean Warbler's breeding range. The preponderance of after-second-year males at the Michigan study sites and in southern Ontario suggests a need for regional models of Cerulean Warbler population dynamics.