Barrier crossing in small avian migrants: individual tracking reveals prolonged nocturnal flights into the day as a common migratory strategy.

Over decades it has been unclear how individual migratory songbirds cross large ecological barriers such as seas or deserts. By deploying light-level geolocators on four songbird species weighing only about 12 g, we found that these otherwise mainly nocturnal migrants seem to regularly extend their nocturnal flights into the day when crossing the Sahara Desert and the Mediterranean Sea. The proportion of the proposed diurnally flying birds gradually declined over the day with similar landing patterns in autumn and spring.

Fuelling in front of the barrier-are there age based behavioral differences in Garden Warblers Sylvia borin?

Garden Warblers Sylvia borin were studied during autumn stopover in Crete before crossing the barrier of the Mediterranean Sea and the Sahara Desert. Birds followed with transmitters show extensive stopover periods, which were longer in first-year birds, 16 days, compared with adult birds, 14 days. The distribution of body masses from birds trapped in fig trees were used to estimate the departure body mass and the results found indicate that both age categories on average depart with a fuel load close to 100% of lean body mass.

Information from the geomagnetic field triggers a reduced adrenocortical response in a migratory bird.

Long-distance migrants regularly pass ecological barriers, like the Sahara desert, where extensive fuel loads are necessary for a successful crossing. A central question is how inexperienced migrants know when to put on extensive fuel loads. Beside the endogenous rhythm, external cues have been suggested to be important.

Food intake and fuel deposition in a migratory bird is affected by multiple as well as single-step changes in the magnetic field.

Recent studies have shown that migratory thrush nightingales (Luscinia luscinia) experimentally treated with multiple changes of the magnetic field simulating a journey to their target stopover area in northern Egypt, increased fuel deposition as expected in preparation to cross the Sahara desert. To investigate the significance of food intake on the body mass changes observed, in the work described here we analysed food intake of the nightingales under study in those earlier experiments. Furthermore, to study whether a single change in the magnetic field directly to northern Egypt is sufficient to provide information for fuelling decisions, we performed a new experiment, exposing thrush nightingales trapped in Sweden, directly to a magnetic field of northern Egypt.

Fuelling decisions in migratory birds: geomagnetic cues override the seasonal effect.

Recent evaluations of both temporal and spatial precision in bird migration have called for external cues in addition to the inherited programme defining the migratory journey in terms of direction, distance and fuelling behaviour along the route. We used juvenile European robins (Erithacus rubecula) to study whether geomagnetic cues affect fuel deposition in a medium-distance migrant by simulating a migratory journey from southeast Sweden to the wintering area in southern Spain. In the late phase of the onset of autumn migration, robins exposed to the magnetic treatment attained a lower fuel load than control birds exposed to the ambient magnetic field of southeast Sweden.