Fuel management and stopover duration of Blackcaps Sylvia atricapilla stopping over in northern Spain during autumn migration period

Capsule Fuel load is correlated with fuel deposition rate; stopover duration is affected by arrival fuel load. Aims To determine the stopover duration, fuel management and flight ranges at departure of Blackcaps stopping over in northern Spain. Methods Systematic mist-netting and ringing allowed the use of mark-recapture Cormack-Jolly-Seber models for the estimation of stopover duration. Trapped birds were measured and weighed in order to estimate mass gain. FLIGHT software was used to estimate flight ranges. Results Stopover duration ranged from 3.6 to 13.6 days, and was negatively correlated with arrival body mass (assessed by body mass at the first capture event). On average, arrival body mass was 18.4 g, whilst body mass at departure was 19.8 g. No significant differences in arrival body mass and departure body mass were observed between age or sex classes. Mass deposition rate did not differ between age or sex classes (mean = 0.20 g/day). Birds recaptured one day after the first capture event lost mass, whilst recaptures from the second day onwards had a mean gain of mass; mass was observed to increase linearly with the stopover duration. Mass deposition rate was positively correlated with departure body mass. Finally, with a mean departure body mass of 19.8 g, a Blackcap stopping over in northern Spain should be able to fly up to 1100 km. Conclusions Stopover duration assessed by Cormack-Jolly-Seber models was longer than that observed in birds recaptured more than once ('minimum stopover duration'). Stopover was longer for birds arriving with less fuel. The positive relationship between departure body mass and mass deposition rate suggests a time-minimizing strategy. The lack of difference in fuel deposition rate between age and sex classes suggests a relatively abundant food supply at the study site, but other explanations might also account for the lack of age and sex differences, for example if competition for food was not determined by social hierarchies but by scramble competition. Departing fuel load would allow these birds to arrive at their wintering areas in southern Spain under still-air conditions, without needing to refuel.