The fieldwork took place at the black-legged kittiwake colony called "Krykkjefjellet" in Kongsfjorden, Svalbard during the chick-rearing part of the breeding season in 2013 and 2014. 
In the column for the clutch/brood: "P" means chick and "W" means egg. E.g. "2P1W" means the nest contained 
two chicks and 1 egg.
In the data file, all entries are for individual birds; i.e. none of the birds in 2014 were sampled in 2013.
The thyroid hormones and the PFASs were analysed in plasma. 
For the PFASs, "NA" values are given when the concentration was below the limit of detection (0.05 ng/mL for all).
For TT4, "NA" values are given when the variation between the duplicate samples exceeded 15%. 
Please refer to the associated article (Ask et al. 2021, https://doi.org/10.1002/etc.4978) for more information.