Fig. 4. Typical example of the raw flight data and the methods used to calculate k_c over the flight period for (A) sunbirds and (B) starlings. A linear regression was fitted before flight and forward-extrapolated to the adjusted time when flight began (accounting for time spent on perches). A polynomial regression was fitted to the enrichment after flight and back-extrapolated to the adjusted time when the flight ended (as described by Hambly et al., 2002). The resulting gradient between these two points was the isotope elimination rate (k_c, min^-1) over the flight period.