Sex-, age-, and size-related differences in the distribution of migrating landbirds in the northern coast of the Gulf of Mexico
Date
2020
Authors
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Publisher
University of Delaware
Abstract
The goal of my thesis was to increase knowledge on broad-scale demographic and physiological distribution patterns in forest landbird migrants at stopover sites along the northern Gulf of Mexico coast in both the spring and autumn. With mist-netting and transect survey data collected from six research sites located in Alabama and Louisiana for spring and autumn migratory seasons, I rigorously sampled this well- known migration system to test for relationships between intrinsic bird traits such as age, sex, body size and energetic condition with two extrinsic geographical position factors: longitude and distance from the coast, and also analyzed data on a temporal scale. I supplemented my analysis of intrinsic bird traits and extrinsic geographical factors with spatio-temporal distribution patterns of migrant abundances. I analyzed data for physiological and demographic traits in two separate chapters. ☐ In Chapter 1, I analyzed mist-netting and survey data to test the relationships between bird body size, specifically wing chord and fat free mass, and condition, to distance and longitude from the coast. Specifically, I used the mist-netting data to test for intraspecific differences in migrant body size, while I used both survey and mist- netting data to test for interspecific differences in body size. I compared my results to a previous study that examined interspecific differences in geographic patterns related to bird size along the Gulf of Mexico. My thesis expands the analysis from the previous study by including intraspecific differences in bird distributions. While my results indicated a statistically significant intraspecific increase in wing chord with greater distance from the coast as predicted, the differences were likely too small to be biologically meaningful. Furthermore, interspecific patterns showed birds were decreasing in size with increasing distance from the coast. Re-analysis of the data from the previous study revealed a similar trend when not controlling for confounding factors like plant and landscape composition. Thus, confounding factors are most likely responsible for the unexpected results observed in my interspecific analysis. Further analysis is needed to understand the relationship of wing chord to distance from the coast for this study, particularly incorporating measures of habitat. ☐ Chapter 2 focuses on relationships between age and sex with longitude and distance from the coast, using strictly mist-netting data. I found evidence of a stronger clockwise loop migration for younger birds compared to older birds. Young birds utilize routes most likely driven by endogenous cues and may compensate less for wind drift than adults who have a better map sense. I also did not observe higher proportions of young birds at coastal sites during the autumn, as observed by Ralph (1981), but I did find higher proportions of younger birds at coastal sites during the spring. Overall, age differences were stronger for the spring season than the expected autumn season. Furthermore, sex differences were only observed on a temporal scale in the spring, with males preceding females on the northern coast of the Gulf of Mexico.
Description
Keywords
Age, Gulf of Mexico, Migration, Physiological traits, Sex, Stopover sites