Autumn migration of Savi's Warblers Locustella luscinioides in Portugal: differences in timing, fuel deposition rate and non-stop flight range between the age classes

Abstract

Capsule The autumn migration strategy of Portuguese Savi's Warblers differs between age classes in timing, fuel deposition rate and flight range. Aims To describe the autumn migration strategy of Savi's Warblers, and compare the age classes. Methods Data from 533 first captures and 149 retraps, held by the Portuguese Ringing Centre, were analysed, and potential non-stop flight ranges were estimated. Results Juveniles started migrating in early August whereas adults departed at the end of August and both vacated the breeding quarters in early October. In contrast to juveniles, adults showed a marked increase in mass and fat from the end of August and had a significantly greater fuel deposition rate. Accordingly, in adults, the average potential still-air flight range increased dramatically from 600 km in August to 1500 km in September, whereas in juveniles it remained approximately the same (about 150 km). The fattest adults had an average flight range of 2750 km, and so were able, theoretically, to reach the winter quarters without refuelling. Conclusions The autumn migration strategy of Portuguese Savi's Warblers differs between age-classes in timing, fuel deposition rate and flight range. Adult Savi's Warblers are time-selected migrants in autumn. The early departure of juveniles with smaller fuel stores is consistent with their lower fuel deposition rates, as expected under time-minimizing migration models, as is the fuel-overloading and by-passing of potential staging sites seen in adults. However, energy-selected migration cannot be excluded as a possible explanation for the juvenile migration pattern, as these birds left the breeding quarters with only enough fuel to reach the next staging site.