One-fifth of all birds worldwide spend a large part of their lives travelling between breeding and wintering areas. Billions of birds pass between Europe and Africa during migration. In addition to the dangers associated with thousands of kilometres of flight, all that flying has a high energy cost. Migratory birds store fat reserves for this purpose, sometimes more than half of their total body weight. These reserves are replenished during the migration itself, at intermediate stops. One can estimate the quality of such a stopover by studying what food is available and how good it is. The higher the quality, the less energy a bird wastes by/in? searching for food.
At the stopovers around large ecological obstacles, such as the Sahara, extra reserves need to be stored. If the quality of these intermediate stops deteriorates, which is common under the influence of human interference or climate change, the consequences for the further migration period and the breeding season will be considerable.
To gain insight into the quality of such stopovers, Deboelpaep searched/dug? in both the spring and autumn migration seasons for invertebrates living in and on the sediment in the internationally protected wetlands in Western Greece. The Kalamas Delta, Ambracian Gulf and Lagoons of Messolonghi and Gialova are part of an important migratory route through which birds commute between Eurasia and Africa, just before or after crossing the Sahara. The invertebrates were identified, measured and weighed and their fat and protein content was determined. “By separating the different layers from each soil sample, we were able to distinguish which invertebrates were accessible to which bird species,” explains Deboelpaep. “Prey from the deepest layer can only be caught by waders probing into the sediment with longer beaks, such as the curlew (Numenius arquata), while short-beaked Kentish plovers (Charadrius alexandrinus) are restricted to prey on the surface. We were able to estimate which waders will find more food here.”
The study found larger differences in food availability and quality between the two migration seasons than between the four wetlands. There is more food present in spring than in autumn, both in the number of prey and in their total mass. Because the prey is relatively richer in fat and protein in spring, the total energy that waders can use to refuel appears to be seven times higher than in autumn. With more food available during spring, birds can reach their breeding areas faster and in better condition. “Based on our data, it seems plausible that especially during spring, Greek wetlands are attractive stopovers. Previous counts show that approximately twice as many waders pass through Greece in spring as in autumn,” says Deboelpaep. “By demonstrating that the wetlands differ little from one another, this study has also shown that the habitat classification of the European Natura 2000 legislation, which is applied in all European protected areas, can be used as a ‘template’ to estimate food quality and quantity on a larger scale (flyway scale).”
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Evelien Deboelpaep is a PhD student (FWO) under the supervision of Prof Nico Koedam and Prof Bram Vanschoenwinkel. Tina Coenegracht, Lore De Wolf, Alexandre Libert and Dr. Mathil Vandromme participated in this study in the field and/or in the lab.
Foto's (c) Evelien Deboelpaep/Hilde Meere