| Summary: | During the breeding season, seabirds may respond to shifts in prey regimes, which can lead to changes in diet, foraging behavior and breeding success. In coastal Newfoundland, seabirds rely on capelin (Mallotus villosus), a forage fish that migrates into coastal areas during the summer to spawn, thereby increasing prey availability. However, with increasing variability in ocean climate, capelin biomass as well as the timing of spawning and inshore arrival has become highly variable. In Chapter 1, I measured the association between capelin biomass and three niche metrics for chick-rearing razorbills (Alca torda): diet, foraging effort and body condition. For three years, I estimated dietary proportions using blood stable isotope ratios (δ15N, δ13C), foraging and dive metrics from GPS and TDR loggers and assessed colony-based measures of body condition with body mass and wing length. During higher capelin biomass years (2017, 2019), foraging effort of razorbills decreased with a higher proportion of capelin in the diet after they arrived inshore to spawn, but not in the year with lower capelin biomass (2020), which was also associated with lower adult and chick body condition. In Chapter 2, I explored how sympatrically breeding razorbills, Atlantic puffins (Fratercula arctica) and common murres (Uria aalge) respond to inter-annual (2017, 2018, 2019) variation in capelin biomass using blood stable isotope ratios (δ15N, δ13C) to estimate isotopic niche, as a proxy of dietary niche. At the species-level, niche breadth generally contracted and trophic position shifted toward capelin during the year of higher capelin biomass (2018) relative to years of lower biomass (2017, 2019), indicating a higher reliance on capelin and lower diversity of prey in the diet when more capelin were available. At the assemblage-level, trophic diversity was lower and niche overlap was higher during high capelin availability years (2018) relative to low capelin availability years (2017), suggesting shifts in species interactions with ...
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