Estrategias de migración del género Acrocephalus en la Península Ibérica

Resumen

The migration strategies of Acrocephalus warblers, the Reed Warbler A. scirpaceus and the Sedge Warbler A. schoenobaenus, during the autumn migration in the Iberian Peninsula were studied at two levels: on one hand, the spatial organization of migration within Iberia; on the other hand, the behaviour at stopover areas. For the study of the spatial organization of migration in Iberia, we used ringing data of birds captured at a net of sampling sites, covering the main migration routes within Iberia. Reed Warblers passing through western Iberia (WI) had shorter wings than those in central (CI), eastern (EI) and southwestern (SW) Iberia, suggesting that birds in WI migrated shorter distances, a fact also supported by recovery data. Although Reed Warblers showed some population overlap when passing through Iberia, we found that birds passing through EI and CI came from areas further to the east (continental Europe) than those passing through WI (mainly British Isles), thus supporting parallel migration of different populations within Iberia. Reed Warblers tended to converge in southwestern Iberia, suggesting an effect of nearby geographical barriers. Reed Warblers in Iberia had the necessary fuel needed to arrive in northern Africa but not to tropical Africa. However, body mass patterns varied depending on the geographical region (EI, CI, WI). Date did not affect body mass in CI and WI, but it did in EI, where heavier birds tended to pass later. Thus, the factors shaping body mass of Reed Warblers in Iberia before the sea crossing to Africa seemed to be more complex than just the distance to this geographical barrier, with underlying stopover quality-associated factors possibly playing a relevant role. For the study of stopover behaviour, we used ringing and radiotracking data obtained at a relevant Iberian stopover site, the Jaizubia marshlands in Guipuzcoa. Firstly, we modelled landing probabilities using reverse-time capture-recapture models for the Sedge Warbler. As expected, rain showed a positive effect on the landing probabilities of Sedge Warblers at Jaizubia, indicating that birds avoid flying during rainfall and prefer to interrupt their migration, but wind conditions did not influence landing probabilities. Secondly, we modelled departure probabilities using Cormack-Jolly-Seber models. Sedge Warblers were more likely to depart with high tailwind values and late in the season and, contrary to expectations, with decreasing Sedge Warbler abundance. Selecting tailwind values allows birds to cover longer distances with the same energy amount, thus saving energy and time. The results also show how birds passing later in the season were more likely to depart from Jaizubia, suggesting that time pressure increases as the season progresses, which force migrants to increase their migration velocity with date. The negative relationship between bird abundance and departure probabilities may be explained by two hypotheses: a high sedge warbler’s abundance is an indicator of high food (i.e. aphids) availability and/or, a high sedge warbler’s abundance reduces individual predation risk, allowing birds to reduce anti-predator awareness and to maximize their foraging and fuelling rates. Additionally, the proximity of stopover sites to a geographic barrier (the sea band between Iberia and Africa) modified the importance of the factors that migrants take into account when deciding whether to stay or leave stopover sites. In northern Iberia sites, departure probabilities of Reed Warblers were independent of fuel load, whereas in southern Iberia birds were more likely to depart with higher body masses. Also, departure probabilities from the different stopover sites were not related with the fuel deposition rate experienced by birds in those sites. Reed Warblers departed irrespective of wind conditions in both regions, contrary to what was found for Sedge Warblers at Jaizubia. The wind-selectivity (or the lack of it) in the initiation of migratory flights may be related to the type of migration strategy adopted by the different species (length of flight bouts and amount of fat reserves accumulated). Finally, we studied the spatial behaviour and habitat selection of Reed Warblers at Jaizubia with radiotracking data, considering three different groups: local adult birds which were still at their breeding site in Jaizubia, migrating first-year birds (originating from beyond Iberian peninsula), and migrating adult birds. Overall, Reed Warbler established non-exclusive home ranges, smaller than the whole study area. Migrating first-year birds had larger home ranges than both local and migrating adults and to move more widely within Jaizubia. They also showed lower fat deposition rates than adults, although differences were not significant. The proportion of habitats in home ranges (reed-beds and tidal flats being the most abundant habitats) was similar amongst groups. The spatial distribution and habitat use of organisms have been theorised to follow an ideal-free or ideal-despotic distribution. However, according to our results, other complex underlying mechanisms apart from habitats availability and density of birds may play an important role in shaping the spatial behaviour of birds at stopover sites (e.g. site familiarity).