Stress Physiology of Grey-Faced Petrels : interannual measures of feather corticosterone as a conservation tool

Abstract

Seabirds are ecologically relevant top predators and ecosystem engineers, but also the most threatened group of birds globally. Seabirds face many threats, but among the most critical is overall ocean health. Monitoring ocean health is difficult and expensive, but breeding seabirds have been touted as a potential low-cost bioassay of ocean health. However, the effectiveness of that relationship remains unclear and warrants further validation. Understanding how shifts in ocean conditions influence foraging opportunities for seabirds may assist the management of populations and inform on the dynamics of ocean conditions. Changes in food supply and potential nutritional challenges are often reflected in seabird physiology. Specifically, stress hormones often show a strong link to food supply, suggesting they are useful tool for linking ocean health, foraging conditions, and seabird population demographics. In birds, corticosterone (hereafter CORT) is the predominant stress hormone, and responses to stressors are reflected in changing CORT levels. Because CORT levels are reflected in developing feathers, measures of feather CORT (fCORT) may provide useful estimates of environmental stressors. However, the pattern of variation in stress hormone levels varies among species and their environmental context, thus requires validation from intra-species studies to draw reliable conclusions. Grey-faced Petrels (Pterodroma gouldi) are long lived, have high site-fidelity and are widespread, which provides good scope for robust validation experiments. Here, I determine whether variation in fCORT can be used as a proxy of ocean conditions and as a monitoring tool for population breeding success. I used reproductive data and feather samples collected over four breeding seasons (2017, 2019, 2020, 2021). I measured fCORT levels from chicks at Ihumoana Island. To investigate whether fCORT reflects changes in relevant ocean conditions, I analysed the relationships between fCORT and remote sensing data (sea surface temperature - SST/chlorophyll-a - Chl-a), measurements of feather quality, stable isotopes, and breeding success (i.e., chick’s body condition, fledging success for laid eggs). I also examine measures of parent quality (fCORT and quality, mass, age) to determine which factors predict breeding success. I found that chick stress levels fluctuated among seasons, but variation was not tightly coupled to ocean conditions. More stressful ocean conditions (High SST/low Chl-a) predicted lighter chicks, but also a decline in fCORT. Increased fCORT and lower feather quality (i.e., brightness) predicted reduced breeding success. Parental mass predicted chick body condition, but not chick fCORT. Further, foraging distance, as suggested by stable isotopes, varied annually, and increasing foraging trip distance from the colony was correlated with worse feather quality and showed year specific relationships with fCORT. This study reveals that taken together, fCORT and feather quality measures offer a promising tool for seabird conservation. Future studies should aim to confirm the underlying mechanisms of fCORT variation and relationship to study variables by exploring foraging behaviour and dietary patterns in search for the best proxy of ocean conditions affecting Grey-faced petrel populations.