Title

Stopover Behavior and Age-Specific Ecology of Neotropical Passerine Migrant Landbirds During Autumn Along the Northern Coast of the Gulf of Mexico

Date of Award

1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

First Advisor

Frank R. Moore

Advisor Department

Biological Sciences

Abstract

Neotropical passerine migrant landbirds were studied in coastal Alabama during four fall migration seasons (late August-late October), 1990-1993, to look for age-specific differences during stopover. In a study of migrants at three latitudinally separated sites in eastern North America, I found adult Red-eyed Vireos (Vireo olivaceus) preceded immatures at all sites while immature Magnolia Warblers (Dendroica magnolia) preceded adults at all sites. Age-dependent timing of passage varied by year and site for Swainson's Thrushes (Catharus ustulatus), American Redstarts (Setophaga ruticilla), and Common Yellowthroats (Geothlypis trichas). There was little evidence of differential rate of migration for Swainson's Thrushes and Magnolia Warblers. I studied six species to look for age-specific differences in arrival mass, stopover length, and rates of mass change during stopover. Adult Swainson's Thrushes, White-eyed Vireos (Vireo griseus), and American Redstarts were significantly heavier than juveniles upon arrival. The reverse was true for Gray Catbirds (Dumetella carolinensis) and Magnolia Warblers. None of the species showed age-related differences in stopover length or rates of mass change. Flight range estimates indicated that average mass AHY Swainson's Thrushes, White-eyed Vireos, American Redstarts, and HY Magnolia Warblers had sufficient fat stores to complete a trans-Gulf flight whereas average mass HY Swainson's Thrushes, White-eyed Vireos, American Redstarts, and AHY Magnolia Warblers carried insufficient fat stores to cross the Gulf. Free-ranging, color-banded American Redstarts showed no age-related differences in foraging rate or pattern. However, redstarts did show significant differences in microhabitats used while foraging. Density-dependent social dominance apparently constrains young redstarts during stopover because they foraged more frequently in the outer portions of slash pines (Pinus elliottii) on days when densities of conspecifics were highest. Use of slash pine habitats, habitats which are frequented by migrating raptors, likely exposes young migrants to increased predation risk. These results indicate that intraspecific age-dependent differences in stopover biology are apparent in a variety of migratory species, and thus should be addressed when considering the behavior and ecology of migratory birds.