Stopover ecology of migrant songbirds at the Ebro delta = Ecologia de parada migratòria de passeriformes al Delta de l'Ebre

Abstract

Migration associated with movements between breeding and wintering areas allow animals to maximize fitness in response to seasonal changes in resources (Dingle 2014). Some of the most incredible migrations in the animal world are performed by birds which are possibly one of the best-studied migratory groups. Prior to migration, birds accumulate fat stores that may account for up to 50% of their body mass (Nielsen and Riis 2013) and once these fat stores are depleted after a flight bout they are replenished at stopover sites along the migration route. Migration is a dangerous life stage, which means that migrants have higher probability of perish while migrating than non-migrants. The mortality rates during the migratory period may be at least 15 times higher compared to that in the stationary periods of the annual life cycle (Sillett and Holmes 2002). Moreover, imagine small passerine birds, many of them inexperienced birds, born only some months before the beginning of migration, sometimes weighting just 10g or less, flying by night thousands of kilometres to encounter new kinds of habitats (Mettke-Hofmann and Greenberg 2005), competition for food resources (Moore and Yong 1991), predators (Lindström 1989), and inclement weather (Newton 2007), all while needing to maintain adequate fat reserves to perform their long-distance flight successfully. In fact, migrant bird populations suffer nowadays a sustained decline as shown in analyses of bird population trends across Europe (Sanderson et al., 2006; Vickery et al. 2014), North America (Ballard et al. 2003) and East Asia (MacKinnon et al. 2012). This decline is strongly associated with the accumulative impact of certain human activities such as habitat loss (Aharon-Rotman 2016), hunting (Raine 2016, Clausen et al. 2017) and climate change (Jiao et al. 2016) on the areas used by birds in their migratory routes (Crick 2004), especially on stopover sites (Bairlein 2016).

Understanding the quality and availability of highly used stopover sites is vital for migratory bird conservation (Mehlman et al. 2005) and to know where migratory birds stop during their migrations and how the sites function for migrants will provide information for conservation and management of suitable stopover areas and therefore, help in the development of full-life-cycle specific conservation plans. The Iberian Peninsula geographically connects Africa and Western Europe and migrants use it in transit between breeding and wintering grounds during both migration periods (Newton 2008). It becomes an ideal scenario for the study of migratory strategies (Bruderer and Liechti 1999) especially if we consider also Mediterranean wintering birds. This thesis focusses on the significance of the Ebro delta, as the second most important wetland in the Western Mediterranean, for the migration of songbirds and draw an overall picture of the way migrant passerines use the area as a stopover site. Since the Ebro delta is a highly humanized area where several important economic activities take place (e.g. rice production, hunting, fishing and tourism), as well as a very fragile territory due to severe problems of coastal regression and subsidence, the information here gathered aims to help in future conservation plans that should take into account not only breeding species but also wintering and migrating ones, what will give even more importance to this coastal wetland, which we must protect at all costs. Therefore, this thesis work overs the behaviour of both short/medium and long-distance migrants, i.e. Mediterranean wintering and trans-Saharan species, during their stopover at a coastal lagoon in the Ebro delta during both migration seasons (spring and autumn). The tool used to investigate the issues exposed above was a cheap one and available to everyone who asks for it: the ring-recovery data. Bird ringing is long used to study migration processes, and although nowadays more trendy and modern methods are available (e.g. satellite telemetry, geolocation loggers), not all of them are affordable to everyone who wants to use them. If we realize that the EURING Data Bank (EDB) is a unique set of mark–reencounter data on European birds with more than 10 million encounter records and that is at the disposal of the scientific and conservation communities (du Feu et al. 2016) and that ringing is still required as the key technique for facilitating our understanding of migration (Bairlein and Schaub 2009), we can use its potential to evaluate the importance of stopover areas and contribute to their conservation and also of the species that depend on these areas to survive. Of course, the new methods are very useful and sometimes a lot more precise and the combination of these methods with ringing data will improve considerably our knowledge about the migration phenomenon in the future. But while we look for financing to tag our birds with geolocators, let’s start working out the more basic questions.