Post-breeding ecology in the Prothonotary Warbler: Evaluating trade-offs between breeding, molt, and migration phenology
Brian Robert Tsuru, MS
Advisor: Christopher M. Tonra
Migratory birds must balance a constantly shifting web of physiological and behavioral processes throughout their annual life cycles. These processes, and the life history stages they comprise, are linked to one another in ways we are still only beginning to understand. Furthermore, the need to understand these links across the annual cycle grows ever more pressing in an era of global anthropogenic upheaval. In particular, the post-breeding period is a potentially critical, yet largely understudied, period of the annual cycle of migratory birds. For many birds, this transitional period involves three key life history stages: cessation of parental care, completion of prebasic molt, and preparation for departure on fall migration. Individuals may be forced to trade off among these stages in various ways to complete these demanding processes in a relatively short period of time. Such trade-offs, which each have their own costs and benefits, should become most apparent in situations where a protracted breeding season overlaps or otherwise interacts with subsequent stages of the annual cycle, leading to conflicts between these life history events. My research explored trade-offs made by migratory birds during the post-breeding period, focusing on the costs and benefits of these trade- offs, the ways they manifest empirically in migratory birds, and what they might mean for migratory bird populations across the full annual cycle, particularly in the context of ongoing anthropogenic threats.
I describe a synthesis of hypotheses and predictions relevant to the post-breeding period, asking how migratory birds might manage the potentially conflicting demands imposed on them during this time, in the first chapter of the thesis. I first review each component of the post-breeding period and the energetic and temporal constraints associated with each. I then describe the potential strategies by which individuals could balance these constraints among the component stages. I developed this framework around three possible and non-mutually exclusive hypotheses that reflect the consequences of trade-offs made between life history stages: increased energetic requirements, reduced feather quality, and/or phenological delays. I review the limited studies that have addressed these hypotheses and discuss both the needs and challenges of simultaneously testing all three. I conclude by describing the potential carry-over effects these strategies may have on survival and future reproductive success, and advocate for more focus on this aspect of migratory bird life histories. Carry-over effects may interact with constraints imposed by external stressors, such as climate change, and may substantially influence the fitness costs and benefits of these trade-offs in ways that ultimately influence population growth. This framework will help other researchers pull back the curtain on this important but little-studied transitional period of the annual cycle and make predictions for how it may be impacted in an ever-changing global environment.
The second chapter of the thesis assesses empirical evidence suggesting the occurrence of multiple post-breeding trade-offs in the Prothonotary Warbler (Protonotaria citrea), a long-distance migratory songbird and species of conservation concern in Ohio and across North America. I monitored color-banded Prothonotary Warblers in central Ohio through their breeding and post-breeding activities, recording the timing of major events such as spring arrival, nest fledging, and the cessation of post- fledging care. I tracked 40 radio-marked adults throughout the post-breeding period to determine the timing of their migratory departure from the site. I also assessed the timing of prebasic molt in a smaller number of individuals (n = 17) during my second field season, finding that many (n =10) breeding birds overlapped molt with parental care, indicating an energetic trade-off between breeding and molt. I fit a series of linear models to explore the impacts of breeding timing and investment on a) probability of overlapping breeding and molt and b) on fall migratory departure dates, testing for evidence of multiple plausible trade-offs among breeding, molt, and migration. I found that individual birds caring for offspring later into the season were more likely to overlap their life history stages. I also found that later-breeding individuals showed delayed migratory departure dates, indicating a phenological trade-off between breeding and fall migration. Analysis of the pathways by which these trade-offs manifest also indicates that male and female warblers manage the competing demands of the post-breeding period in slightly different ways.
My results are the first to indicate that adult Prothonotary Warblers make multiple trade-offs between breeding, molt, and migratory phenology in the post-breeding period, and are among a small number of studies to that consider these trade-offs at all, let alone simultaneously, in migratory songbirds. By elucidating the nature of these trade-offs, my work adds to our knowledge of how breeding season events can produce carry-over effects affecting individual birds in subsequent stages of the annual cycle.