<p>Although essential for survival, moult is energetically costly and has evolved to be temporally separated from other costly activities when resources may be limited. Wing-propelled pursuit divers in the family Alcidae (‘alcids’) with high wing loading may become flightless when wing area is reduced during primary feather moult, increasing their vulnerability to food scarcity and other threats. Different primary moult patterns require different durations to complete; a descendent pattern extends the moult period, while a synchronous pattern minimizes this period. As alcids typically moult while at sea, little is known about the flight feather moult patterns of most species. We investigated whether the primary flight feather moult pattern of Atlantic Puffin (<i>Fratercula arctica</i>) is synchronous or descendent using stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N). Using depredated puffin carcasses (<i>n</i> = 25) collected during breeding from James Island, Newfoundland, Canada from 2020 to 2022, differences in δ¹³C and δ¹⁵N between the first and second primary feather (P1–P2), which are&#xa0;likely grown simultaneously during both moult patterns, were compared with differences between P1–P5 and P1–P10. A lack of differences in δ¹³C and δ¹⁵N across all feather pairs suggested a synchronous moult pattern; however, higher P1–P10-differences in some birds indicated these individuals may perform a partial primary moult. These findings increase our understanding of moult patterns in Atlantic Puffins, and, along with future reaserch determining the timing and location of&#xa0;primary feather moult, will indicate vulnerable periods and places for this at-risk species.</p>