We investigated dietary preferences in Pigeon Guillemot (Cepphus columba) across different breeding stages in Puget Sound, WA. Observations of prey delivered to chicks were conducted during the breeding season (June–September) at 28 colonies on Whidbey Island over a 12-y period (2008–2019). We conducted stable isotope analysis on discarded eggshells collected below active Pigeon Guillemot burrows distributed across 9 colonies on Whidbey Island during the 2019 breeding season. We estimated the relative percent contribution of fish and invertebrates to the diet of pre-laying adult Pigeon Guillemot using the δ¹⁵N and δ¹³C in eggshell membrane tissue. Results of our mixing model showed that adults derive nearly 75% of their energy and nutrients from demersal fish species (rockfish, gunnel, and sculpin) during the pre-laying period, and prey-delivery observations showed that Pigeon Guillemot preferentially deliver gunnel (Pholidae) to chicks. These results demonstrate a consistent foraging pattern and dietary preference in this population over the course of 2 different stages during the breeding season.