Interactions of Pigeon Guillemots and Rhinoceros Auklets with the marine environment

Abstract

Pigeon Guillemots (Cepphus columba) and Rhinoceros Auklets (Cerorhinca monocerata) are seabirds with a widespread year-round distribution throughout the Salish Sea, where they have a long history of study. They have been identified as indicators, i.e., Puget Sound Vital Signs, by the Puget Sound Partnership. The purpose of a Vital Sign is to act as a benchmark measure of the ecological health of Puget Sound and to guide recovery goals. Despite their status as indicators, major gaps exist in our knowledge of their relationship to the marine environment, including the relationship between marine conditions and their demography, and characteristics of their foraging habitat. These knowledge gaps limit their utility as indicators. In this thesis, I address some of these gaps using data from Protection Island in the Salish Sea. In Chapter 1, I developed hierarchical models in a Bayesian framework to understand the relationships between Pigeon Guillemot reproductive outcomes and oceanographic conditions. I considered the influence of multiple indicators of oceanographic conditions across different temporal scales relative to the breeding season to learn how the temporal occurrence of these conditions influences Pigeon Guillemot breeding outcomes. Pigeon Guillemot reproductive success, defined as the probability of a nest fledging at least one chick, was positively correlated with the North Pacific Gyre Oscillation (NPGO) index. Higher NGPO values are indicators of increased upwelling in the northeast Pacific and higher marine productivity. Guillemot reproductive success did not appear to be influenced by local marine conditions including sea surface temperature or chlorophyll-a concentration, or by the broader conditions described by the Pacific Decadal Oscillation. The lack of influence of any other covariate considered may be explained by the Pigeon Guillemots’ generalist nature, in particular their ability to prey switch depending on the abundance or quality of prey species available to them, which may buffer them against variability in marine conditions. My results provide a better understanding of how the Pigeon Guillemot population at Protection Island responds to marine conditions and informs their use as a Puget Sound indicator species. In Chapter 2, I conducted preliminary analyses of the behavior-specific movements of Pigeon Guillemots and Rhinoceros Auklets nesting at Protection Island. I tracked individuals during the breeding seasons of 2022 and 2023 and used these location data to fit discrete-time hidden Markov models to characterize behavioral states of both species, which have very different foraging and provisioning strategies during the breeding season. I used step length and turn angle to model movements of tracked birds across resting, transiting, and foraging states. Fitted models largely conflated movements associated with resting and foraging states, and predicted that foraging behavior occurred on or near Protection Island. These findings suggest the importance of fixing the resting state in these models, given that we know when and where resting, or stationary, behavior is primarily occurring, such that the models are only distinguishing transit and foraging states. I discuss future directions for this analysis. The utility of Pigeon Guillemots and Rhinoceros Auklets as indicators of environmental conditions in the Salish Sea is contingent on our understanding of how they respond to and use their habitat. Together, these chapters explore aspects of their breeding ecology and behavior and suggest processes through which we can productively view their relationship with the environment in the Salish Sea.