From 2002 to 2022, yearly King Penguin Aptenodytes patagonicus chick mortality and mass data was collected in April, just before the winter fast. The same chicks were relocated after the winter to assess mortality. The data was collected on Kerguelen Island, located in the Southern Ocean. 

All data was collected at Ratmanoff’s colony in the Kerguelen archipelago (-49° 14′ 33″, 70° 33′ 40″), which hosts approximately 80 000-90 000 breeding pairs of king penguins (Barbraud et al. 2020). Chick mass and survival was assessed yearly from 2002-2022, apart from 2007 (i.e. 20 studied years). Randomly selected chicks were weighed and marked with fish tags (FloyTags, Seattle, USA) in April before the winter fast, except for 2009, where weighing occurred in May due to fieldwork constraints. The number of chicks marked annually varied from 28-55, with most years (14) having n = 40 chicks. The colony was again visited in spring (October-November) to relocate the marked chicks and assess winter mortality.