Dossman Thesis

Stopover Departure and Movement Behaviors of Migratory Songbirds

Bryant Dossman, MS (2015)
Advisors:  Stephen Matthews and Paul Rodewald

Thesis

Billions of migratory songbirds worldwide make energetically costly migrations between breeding and overwintering areas. Their small body size, and limited fuel storage capacity, requires that these migrants make punctuated stops along their migratory route to rest and refuel. With timing of arrival on the breeding grounds directly influencing reproductive success, migratory birds exhibit a suite of behaviors during stopover that are thought to minimize the overall time spent on migration. These behaviors and the needs of migrants to rest and refuel, make the distribution and quality of stopover habitats a critical concern for bird conservation. 

Both endogenous (sex, age, energetic condition) and exogenous factors (habitat quality, food, geographical location) have been shown to influence stopover behaviors. I sought to identify the primary drivers of stopover departure decisions and their effect on movement behavior for two migratory passerines, the American redstart Setophaga ruticilla and the yellow-rumped (myrtle) warbler Setophaga coronata coronata within the western Lake Erie basin of the United States (Ohio, Michigan) and Canada (southern Ontario). Experimental manipulation of condition revealed that condition had a strong negative influence on stopover duration and a positive relationship to movement rates of these two migrant species.

A broader sample of 124 radio-marked birds demonstrated that stopover departure was influenced by condition and age, with adult (after-second-year) birds exhibiting greater daily probability of departure than juveniles (second-year) birds. Weather conditions also had a significant influence on stopover duration with increasing tailwinds being associated with a greater daily probability of departure. Interestingly, wind was also an important driver of landscape level movement patterns. Individuals that departed with favorable tailwinds took a more direct route across the lake, which likely increased energetic and temporal efficiencies, and could have cumulative effects across the entire migratory period. 

Stopover departure timing and movement behavior are shown to be complex and governed by a variety of endogenous and exogenous factors. Ultimately, individual responses are likely driven by the constraints imposed on them. Given the ultimate importance of arrival timing on the breeding grounds, individuals are likely making behavioral decisions that facilitate a more rapid and timely migration by minimizing stopover periods. Condition upon arrival was associated with early departure (shorter stopover duration), but departure under ideal weather conditions might have a substantially larger impact on the overall rate of migration. As a consequence, individuals that are better able to make `optimal’ decisions (presumably adults) are likely able to realize shorter stopovers and arrive earlier on the breeding grounds. 
Although habitat quality and other exogenous factors influence stopover, my study highlights how weather at the time of departure is an important determinant of stopover duration. With a changing climate, increasing variability in weather, and rapid land development, it is imperative to gain a holistic understanding of the factors that influence the timing of migration and ultimately reproductive success on the breeding grounds.