Sex-based Differences in Winter Body Condition of Rusty Blackbirds in Acorn vs. Pecan Habitats
SEX-BASED DIFFERENCES IN WINTER BODY CONDITION OF RUSTY BLACKBIRDS IN ACORN VS. PECAN HABITATS
Patti Newell1, Sara H. Schweitzer1, and Russell Greenberg2
1 D.B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602 2 Smithsonian Migratory Bird Center, Washington, D.C.
Abstract: The Rusty Blackbird (Euphagus carolinus) has experienced an 85-95% overall decline continent-wide, with accelerated rates of population decline since the 1970s. Reasons for the decline remain unclear, but loss and modification of winter wetland habitats and associated carry-over effects to breeding grounds are likely principal factors. Winter habitats selected by Rusty Blackbirds include mast-bearing, bottomland trees and shrubs, and sources of invertebrates, small fish, and amphibians. We propose that tree mast provides lipids and carbohydrates needed by Rusty Blackbirds to endure extreme winter temperatures, and to accumulate body fat, amino acids, and other physiological needs; thus, Rusty Blackbirds foraging in habitats with abundant mast will be in excellent condition during winter and upon arrival at the breeding ground. With mist nets, we captured 57 Rusty Blackbirds in winter 2007 and 89 in winter 2009. We recorded age category, sex, mass, and lengths of wing chord, tail, and tarsus. A body condition index was based on a regression analysis of mass relative to lengths. Body condition was best explained by sex (male or female) and system (acorn or pecan habitat), determined from results of a generalized linear mixed model, where males in pecan habitat were in best winter condition. To reduce competition between males and females for pecan habitat and improve winter condition of females, we must provide more habitat with consistent and abundant mast available during winter.