Latitudinal gradients in air density create "invisible topography" at sea level affecting animal flight costs

Data and code supporting the findings of the study:  For details about the attributes in the data files please see the "README.txt" file. Code included in the study: The R script:"Shepard et al_Changing_air_density_affects_flight_costs_in_low_flying birds.R" is used to run the statistical tests and models presented in the study. The R script: "Shepard et al_density matters at sea level_script_DensityAltitudeEstimation.R" , is used to estimate Density altitude based on the air density over the ocean downloaded from the ERA5 monthly means global re-analysis. For details please see the supplementary information of the study. The R script: "EstimatePowerBasedOnAirDensityOverTheOcean.R" , is used to estimate flight power based on the air density over the ocean downloaded from the ERA5 monthly means global re-analysis. The data for this script is provided in the files: ERA5_AirDensityOverTheOcean_WithEstimatedPower_DJF_Months.csv, ERA5_AirDensityOverTheOcean_WithEstimatedPower_JJA_Months.csv and ERA5_AirDensityOverTheOcean_WithEstimatedPower_For_JJA_AndDJF_Months.csv and the shapefile in the zip file: RedTailedTropicBirds_Distribution.zip For details please see the supplementary information of the study.