Fig. 4. Raw isotope enrichment data from one individual, which was used to calculate its flight cost. Before flight (diamonds) the isotope enrichment data conforms to a linear regression, while after flight (squares) a polynomial regression provided the best fit. The regression equations were extrapolated to the time when flight began and ended and the gradient between these two values is the isotope elimination rate during the flight period. In this example k_c was 0.73 min^–1.