Degree

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

Document Type

Dissertation

Abstract

Understanding how species, populations, and individuals cope with variation in foraging conditions is essential for predicting species responses to environmental change. Guanay cormorants (Leucocarbo bougainvillii) and Peruvian boobies (Sula variegata) are seabirds endemic to the Northern Humboldt Current System (NHCS), their populations supported by its unusually high productivity and abundance of anchoveta (Engraulis ringens), a key forage fish. However, El Niño Southern Oscillation (ENSO) can significantly impact anchoveta abundance; therefore, the populations, survival rates, and breeding success of these seabirds are proposed to be tightly linked to ENSO conditions. Although both species depend on anchoveta, their responses to ENSO will likely differ due to their varying foraging strategies and plasticity in diet and foraging behaviors. Most existing studies focus on these birds’ foraging ecologies during breeding; as such, much less is known about their foraging ecologies during the nonbreeding season. Alternative methodologies, used alongside traditional techniques, can help bridge this gap in our knowledge.

This research aims to advance this area of knowledge by combining both traditional and novel approaches –– diet, GPS, multi-tissue stable isotope, and feather corticosterone analyses –– to explore seasonal, interannual, and individual variation in the foraging responses of Guanay cormorants and Peruvian boobies to differing ENSO conditions. I found that despite moderate sexual dimorphism in Peruvian boobies, there was little sex-based spatial, behavioral, or trophic niche segregation. However, there may be dietary niche partitioning, as males captured smaller and more variably-sized prey. Stable isotope analysis of blood and feather tissues revealed that while Guanay cormorants and Peruvian boobies differed in both their breeding and nonbreeding isotopic niche widths, both species displayed consistent nonbreeding niche width expansion with little evidence of seasonal individual consistency. However, cormorants were more responsive to weak El Niño conditions, with consistent niche width expansions during warm conditions. Lastly, while oceanographic condition and δ13C values were related to feather corticosterone levels, surprisingly, unfavorable conditions did not translate to higher feather corticosterone in either species. Overall, this combined use of methods across seasons and years provides multifaceted insight into the foraging ecologies of Guanay cormorants and Peruvian boobies in the NHCS.

Date

6-18-2025

Committee Chair

Bargu Ates, Sibel

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