Untangling the role of glucocorticoids and territory use on nest success in a free-living songbird

Abstract

Glucocorticoids (GC) are hormones that mediate many physiological and behavioral traits and, consequently, play a role in an organism’s reproduction and survival. Researchers have attempted to use baseline and stress-induced GC concentrations as a proxy for fitness, however, the relationship between GCs and fitness is highly variable within and between different species. For this reason, additional environmental factors, such as food availability or habitat alteration, are being included in these studies to better understand how GCs influence fitness. In this study, we aim to understand how intra-individual (co)variation in GC concentrations affect nest success in a free-living songbird, including measurements of territory use (size and distribution) as an environmental factor. First, we measure GC repeatability to determine the level of variation among and within individuals and how repeatability relates to nest success (Chapter 1). We use the well-known measure of repeatability introduced by Lessells and Boag (1987), in addition to a new metric, profile repeatability, which measures the variability of the entire stress response (baseline and stress-induced) within an individual. Second, we measure territory size and distribution in relation to plasma and feather GC levels, and relate these measurements with nest success (Chapter 2). We found evidence for repeatability in stress-induced GCs but not baseline, as well as high variation in within-individual stress responses. Territory size was not related to nest success, but we found that territories were spatially distributed with higher levels of stress-induced GCs found in more central territories. Lastly, covariation in GCs were not related to nest success. These results add evidence to discourage researchers from assuming individuals have stable GC profiles and we urge future studies to measure within and among individual GC repeatability. Further investigation into additional territory metrics will help us to better understand the biological influence of territory use on nest success, in the hope of eventually identifying direct links between GCs and fitness.