Stopover of bluethroats Luscina svecica cyanecula in northern Iberia during the autumn migration period
Published: Volume 57(1), June 2010. Pages 69-85.
Fuel management and stopover duration are parameters of great interest for the understanding of bird migration strategies. Our aim was to study the stopover of bluethroats Luscinia svecica cyanecula in northern Iberia. Data were collected at Badina de Escudera lagoon (reedbeds), Villafranca (42º 16'N 01º 42'W), Ebro Valley. Systematic mist-netting was performed during the autumn migration period between 2003 and 2006. The timing of passage did not vary between age and sex classes, with the bulk of migrants being captured in September. The stopover duration was very long (nearly 30 days) compared to that reported from other stopover sites. Main causes explaining these results could be: (i) an atypical September with unfavourable meteorological conditions, and (ii) the interference of apparently wintering specimens. Apart from a small fraction of birds with very large fuel loads, a mean fuel load of nearly 15 % over lean mass suggests that, if a fraction of migrants stopping-over in Badina overwinter in tropical Africa, they must refuel in areas further south to gain sufficient fuel before crossing the Sahara. Conversely, we cannot rule out that these less loaded birds overwintered within the circum-Mediterranean region. Adults were more fuel loaded than juveniles, but this was not due to a higher fuel deposition rate (mean 0.08 g/day). Bluethroats recaptured from 6 to 10 days after the first capture showed a higher mass deposition rate than those recaptured after 10 days or more. This suggests that birds with long stopovers were not true fuelling migrants, but more probably wintering specimens with balanced energy budgets. A long initial lost of mass (5 days) agrees with both the high competition and the interference of wintering specimens hypotheses. In conclusion, Badina de Escudera, in northern Iberia, is an area used by a true, stopping-over population, as well as by apparently wintering specimens.