Technological advances in Global Location Sensor (GLS) dataloggers have enabled year-round tracking of seabird location and behavior, providing new insights into their ecology. My thesis used archived GLS datasets from Atlantic puffins (Fratercula arctica, n = 22) and razorbills (Alca torda, n = 28) breeding on the northeast Newfoundland coast (James Island) from 2020-2023. In Chapter 1, I developed a workflow to integrate light and immersion sensor data from these tags to categorize behaviours, including time spent at sea (wet), in burrows/crevices (dry, dark), and at the colony (dry, light). This study highlighted ways to use archived GLS datasets to investigate seabird behavior and phenology during under-studied periods with minimal disturbance. In Chapter 2, I used this workflow to investigate the phenology and time activity budgets of both species during the pre-laying period, when high energy demands for egg formation coincides with low prey availability and, thus, may represent a period of competition. I hypothesized that puffins and razorbills would partition their niches along multiple axes, including diet and breeding phenology, and that time activity budgets would differ. Razorbills arrived at the colony earlier than puffins but laid later, resulting in a longer pre-laying period. Although arrival was similar among years, both species adjusted lay dates (and hatch dates) among years. Females of both species spent more time at sea in most years, while males spent more time in burrows. Puffins spent more time at sea and in their burrows, but razorbills spent more time at the colony outside their rock crevices. Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in eggshell membranes (2022- 2023, n ~30 /species/year) indicated no isotopic niche overlap between the two species during pre-laying, with razorbills foraging at a higher trophic level (higher δ15N values) and exploiting a wider range of prey types (greater variation in δ13C values). These findings support the hypothesis that razorbills and puffins partition their niches during pre-laying, suggesting that differences in diet and activity timing may aid coexistence when high abundances of multiple species aggregate nearby breeding colonies. Niche adjustments among years, however, suggest phenotypic plasticity to varying conditions.