Doctoral Dissertations

Date of Award

5-2002

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geography

Major Professor

Kenneth H. Orvis

Committee Members

Carol P. Harden, Sally P. Horn, Daniel Simberloff

Abstract

Successful migration for Neotropical migratory birds depends on an abundance of stopover habitat for resting and refueling between nightly migration flights. The goal of the research reported in this dissertation is to analyze the regional distribution and quality of stopover habitat best suited to forest migrants: nocturnally migrating passerine songbirds (Class Aves, Order Passeriformes, Suborder Passeres) that require forested landcover. I test the hypothesis that patch-level measures of forest quality define the overall suitability of any stopover, and that forest migrants are able to judge stopover suitability while still aloft at the end of their nightly flight. In this study, (1) examine the geography of patch-level measures of forest habitat integrity; (2) examine how stopover use by forest passerines during spring migration reflects those measures; and (3) model possible migration pathways across an eastern United States map of present-day stopover suitability as identified in steps and 2, using migration models defined by fixed orientation headings and fixed nightly flight distances.

I began by analyzing landcover data for the southeastern United States at 30 resolution and calculating and mapping variety of integrity measures for patches of remaining forest. Working with citizen scientists, then surveyed 128 forest patches scattered across the Southeast during spring migration 1999 to quantify the relationship between migrant abundance and the patch-level measures of forest integrity. My results indicate that forest migrants prefer unfragmented, dense forest areas with relatively open understories, little coniferous canopy cover, and intact riparian forest habitat; they avoid human-altered habitats, particularly stopovers surrounded by agricultural landcover or within areas of high road density.

Using the survey results, mapped stopover quality for the eastern United States as continuous surface of 14 km² hexagons representing the average condition of stopovers within each. This mapping allowed me to model migration between hexagons, simplifying the computational challenge while still keeping reasonable picture of actual landscape condition. Results show that migration defined by fixed-orientation, fixed distance nightly flights between stopovers is reasonable, and still remains somewhat viable in parts of the eastern United States. Model runs highlight migratory pathways that are relatively intact, and pathways where human-altered habitats disrupt migration. Longer nightly flight distances (representing birds in better physical condition) increase the chance of success, particularly along northeast-bound paths; modeled migrants flying 190-200 kilometers per night are successful along most routes, but success declines with flight distance.

Conservation and management require clear understanding of the regional distribution of stopover habitat. This dissertation provides an initial geographic analysis that can serve as solid framework for the organization of local-scale research and conservation. If local programs are to succeed in conserving migrant bird populations and habitats, local goals and priorities must reflect the regional needs of migrants across the length and breadth of migratory pathways.

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