Resumen de Stopover ecology of migrant songbirds in the ebro delta
Ana Sofia Guerreiro Duarte Rivaes da Silva
Birds perform incredible migration movements because they can fly, display efficient behaviour and show an extraordinary physiological and morphological adaptability. Birds use fat as fuel source to perform migration flights. Birds accumulate fat stores prior to migration and replenish them during stopovers along the migration route. Stopover sites may be high quality feeding grounds where they refuel quickly, or just resting spots where birds avoid flying in hazardous weather or stop after a long flight. Understanding the quality and availability of migration hot spots is vital for migratory bird conservation and knowing how these sites function for migrants provides information for conservation and management of suitable stopover areas. This thesis explores the stopover ecology of songbirds with different migratory behaviour during both migration seasons in the Ebro delta (NE Spain) using data from standardized bird-ringing campaigns. We investigated the factors influencing seasonal occurrence, body mass, stopover duration, fuel deposition and habitat preferences of six target species, analysed the site’s quality as a refuelling site and propose conservation measures for other coastal wetland type stopover areas.
Trans-Saharan and Mediterranean wintering birds showed different phenology patterns, and similar species avoid using the area at the same time. Our results reveal different migration strategies between long and short distance migrants due to time and energy constraints and age differences relative to phenology patterns are described for the first time in Chiffchaffs. Estimated real stopover durations were shorter than expected and the presence of transients was high. Mediterranean wintering species stayed longer in the site than trans-Saharan migrants. The study of fuel deposition rates (FDR) supported the hypothesis of the time minimization-based theories for long-distance migrants. During autumn, time pressed late trans-Saharan migrants may change from the usual hop strategy along the coast to perform longer jumps or eventually cross the Mediterranean. Chiffchaffs behaved like long-distance migrants regarding some stopover parameters as the result of the existence of different subspecies with different breeding origins using the area. Estimated FDR was low or inexistent for almost all species. All species were more active during the first morning hours. Long-distance migrants were active also during the day due to foraging to maintain fat deposits and leave that same day while Mediterranean wintering species showed activity during late afternoon with some birds probably already wintering in the area. Pied flycatchers lost weight during the daylight hours although late birds increased diurnal FDR supporting the theory of a direct flight to Africa. In general, diurnal FDR were low although nocturnal losses were not computed, and overall FDR may be low or negative. Heavy transients left the area near sunset while birds captured after dusk were probably staying in the area at least one more day.
Our results show that the Ebro delta is not the last stopover area before Mediterranean crossing in autumn but is the first stopover site after its crossing in spring for Willow Warblers and Pied Flycatchers, which perform loop migrations. The Ebro delta is a suitable one-day stopover area for transients and, even showing low refuelling rates, is essential for the overall success of songbird migration. The best stopover habitat would include the alternation of different habitats like reedbeds, bush patches and even trees. If reedbeds predominate, species not well adapted in terms of foraging habits, for instance, may relocate in other areas. Further studies on these movements should be carried out. It would be interesting to analyse more recent available data to explore possible changes in the migrant songbird community due to habitat variation observed in the site since the data here analysed was collected.
