Codes from Lambert & Fort, 2022 "No evidence that seasonal changes in large-scale environmental conditions drive migration in seabirds", Journal of Animal Ecology (https://doi.org/10.1111/1365-2656.13759). The source variables and files used for the analyses are not directly shareable online, but can be accessed upon request to the authors. The R codes are organized as follows: 00 and 01: define species presence by seasons on Marine Ecoregions, and map distributions 02: process environmental variables and compute the table to be used for PCA and niche determination 03: compute the PCA, the seasonal niches and quantify overlaps (for true distribution and simulated residencies, see main text for details) 04: explore and plot the results