Date of Award

2008

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Zoology

Advisor

Rebecca L. Holberton

Second Committee Member

Kenneth P. Able

Third Committee Member

William Glanz

Abstract

Energetic condition influences migratory decisions made by songbirds along their migration and at stopover sites. Over-water or coastal flights can be dangerous for landbirds if they are not able to rest and refuel, but are often the routes of shortest travel distance. In this study, I performed trials on migratory songbirds in the Gulf of Maine region to investigate how various components of energetic condition relate to the likelihood that an individual will initiate migratory flight and the direction that it will choose to fly during both spring and fall migration. I used release tests and measurements of fat (energy stores), plasma triglycerides (indicating fattening trajectory), size-corrected body mass, and within-day changes in body mass to investigate whether any of these measures of condition relate to the decisions migrants must make about when and where to go. My results indicate that the amount of fat a bird has at the time of release, and within-day changes in body mass influence the decision to initiate a flight during both spring and fall migration in the Gulf of Maine region. Additionally, in Blackpoll Warblers {Dendroica striata) during spring migration, size-corrected mass was higher in birds that initiated flights than those that did not. The amount of fat a bird had at the time of release was related to the preferred direction of migrants during fall migration but not during spring migration. Within-day changes in body mass and plasma triglyceride levels were not related to orientation decisions made by migrants in either spring or fall migration. Further, size-corrected mass was not related to the directional decisions of Blackpoll Warblers during spring migration. During spring migration, I investigated differences between species by comparing flight initiation and orientation decisions between Blackpoll Warblers and all other species tested in the season ("nonblackpolls"). Blackpoll Warblers were more likely to initiate flights compared to "nonblackpolls". Additionally, blackpolls oriented north-northwest while all other birds had a westerly orientation. Based on the results from my study, it appears that migrant songbirds on stopover rely on information about their energetic condition when making departure decisions. Only some of the variables tested were related to orientation decisions, however, thus indicating that information about energetic condition may be more strongly related to decisions about when to fly rather than where to fly. While there were some similarities in the relationships between migratory behavior and energetic condition between spring migration and fall migration, differences may be due to the relative nearness of spring birds to the destinations compared to those in fall species (or breeding destination) may also be related to these flight initiation and orientation decisions.

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