| Abstract |
Billions of passerines are migrating over the Sahara desert and the Mediterranean Sea every year on their way to-and-from wintering areas in tropical Africa. Here, the garden warbler Sylvia borin, one of the most common migrants, are used to study the stopover ecology of small birds before and after crossing ecological barriers in the Eastern Mediterranean.
The median passage of the species at south East Europe takes place on the 16th of September according to data from Antikythira, a small Greek island between Crete and Peloponnesus. The first birds arrived at the study site are individuals with large wing length and small fuel loads while the last individuals arrived at the study site have small wing length and larger fuel loads. These patterns are most probably due to differential migration between the age classes.
To look into the preparation of the species just before the desert and sea crossing in autumn, inexperienced in previous migration garden warblers were studied on Crete. Body mass data collected from Crete and a site on Lesvos indicate that most birds do not fuel for the desert crossing further north. For the first time, detailed information about stopover duration close to the Sahara desert was studied by using light weight radio-transmitters. Results from Crete show that most first-year garden warblers arrive with relatively small fuel loads in relation to lean body mass (P<30%), stay for 13- 20 days and depart with an average fuel load of about 100%. Radio-tagged birds performed small scale movements initially and took advantage of fig fruits. Birds trapped at fig trees were heavier than birds trapped with tape lures, showing that tape lures can bias the sample of migrants trapped. The precise fuelling pattern found indicates that first-year migrants must also include external spatial cues to make the preparation for crossing the desert in the right area.
The findings regarding the departure fuel load from Crete were used to estimate the body mass loss when crossing the ecological barrier taking into consideration the season specific length of the barrier. In autumn, with an average fuel load of about 100 % of lean body mass, a large proportion of the birds were not able to cross the desert in still air, but northerly winds prevail during September and with wind assistance only one out of 14 failed.
Details on the strategies and routes followed by migrants in central and western Mediterranean during spring have been revealed thanks to a large network of ringing stations established in this region. Whether the same patterns occur in eastern Mediterranean is unknown. Garden warblers were studied on three small Greek islands, Antikythira, Gavdos and Strofadia during spring, just after the desert and sea crossing. The median date of passage of the species from east south Europe during spring migration is the 5th of May according to detailed data from Antikythira.
Comparing the data from the three islands to the findings from central and western Mediterranean locations, it seems that garden warblers migrates a broad front along the hole rage of the Mediterranean and that no re-fuelling takes place close to north African coast in the eastern flyway. On the other hand, when data from literature were analyzed together with those of this study, it seemed that the correlation between mean time of capture and body mass and latitude that has been shown for the central and western Mediterranean is absent if the whole Mediterranean region is examined. The physical condition of birds landing on Antikythira is close to the average for the central and western Mediterranean area, while a large proportion of birds trapped on Gavdos but especially on Strofadia were extremely light, indicating exhaustion.
The body mass data after the desert crossing in spring from Antikythira was used to calculate the departure body mass from the south of the desert. In spring, the departure body mass of the average bird arriving to Antikythira was estimated to be 34.6 g, which corresponds to a fuel load of 116 %. In both autumn and spring the desert crossing included four or five nights of flying. The results indicate that passerine birds need to attain substantial fuel loads in advance and that this is more critical in spring, when the distance to cross is larger and wind conditions are not as favourable as in autumn.
During long-distance flights, birds catabolize not only fat but also protein, which results in structural or functional loss as protein is stored in organs. In order to investigate breast muscle size in relation to body mass in garden warblers before and after crossing the barriers in autumn and spring migration, respectively, breast muscle size was measured by moulding the shape of the muscles of alive birds. Samples were collected on Antikythira during spring after the barrier crossing and on Crete during autumn, where garden warblers prepare for the barrier crossing on their southward migration. Breast muscle size on Antikythira was significantly smaller than the equivalent size measured on Crete. In spring breast muscle size was correlated with body mass while in autumn no such correlation was evident. The positive correlation between breast muscle and body mass can be regarded as adaptive, since decreasing body mass requires less mechanical power. During extended flights, when fat reserves are insufficient, the use of protein can also be a lifeline to reach the destination. The lack of any association during autumn implies that the increase of breast muscle, as a preparation for the barrier crossing, does not take place en route from breeding sites to the Mediterranean area.
Lastly, to investigate whether the exhausted from the long and intensive flight over the Sahara desert and the Mediterranean Sea garden warblers arriving depleted to the first stopover are able to refuel quickly a caging experiment were conducted on Antikythira. Twenty-nine newly arrived Garden Warblers were caged and given free access to food and water for five days. Birds were separated into two groups, according to their size-specific fuel load. No difference was found in the amount of food eaten between the two groups. Depleted birds, however, managed to achieve a significantly higher body mass gain than non-depleted birds during the experimental period. Our results indicate that depleted Garden Warblers arriving at suitable stopover sites after the barrier crossing can start refuelling directly, if given access to food, and that they can even transform consumed food into body mass gain to a larger extent than birds arriving with some fuel reserves remaining.
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