Long-distance seasonal migration is energetically demanding and time consuming. For example, some passerine birds are on migration during six months of the year, travel more than 10 000 km and make use of many fuelling sites. Migration requires many different decisions, and an individual bird will face a variety of situations. This thesis investigates how long-distance migratory passerine birds use time and energy under different circumstances.
In a comparison of Sylvia warblers, it was shown that more northern populations within species spend shorter time at breeding grounds and have a higher speed during autumn migration. Higher overall speed was also shown in species undertaking longer migrations. This indicates that selection has favoured a behaviour that will economise time during long-distance migration. Results from the last part of the spring migration in the Ortolan Bunting Emberiza hortulana indicate that early arrival is important in males while females might take other considerations into account. Willow Warbler Phylloscopus trochilus males arriving at breeding sites following spring migration carried an overload of fuel, which might be important during the transition between migration and breeding. Since acquisition of this reserve probably delayed their arrival, it also indicates that an individual's future activities have to be considered at some stages of migration.
Whitethroats Sylvia communis with access to food ad libitum at the start of autumn migration increased their fuel deposition rate and departed with a larger fuel load. At the feeder, no correlation between fuel deposition rate and departure fuel load was found, which is theoretically expected for birds minimising time on migration. Juvenile birds in post-juvenile moult attained a much higher stable body mass at the feeder than under natural conditions. Before departure they abruptly increased their gross food intake by about 70% resulting in a period of body mass gain. Whitethroat behaviour at the feeder agreed with predictions in a finite-distance model for time-minimisation during migration, in which birds take the total migratory distance into account. However, both the compensatory increase in migration speed in late birds that was found in several Sylvia warblers, and the higher departure fuel loads found in late Whitethroats at the feeder indicate that early birds also take other considerations than time-minimisation into account.
The effect of increased fuel load on the take-off ability under a simulated predator attack in Blackcaps Sylvia atricapilla, was shown to be much less than previously shown for other species. The large fuel loads that often exist, and the intense foraging normally combined with fuelling, will probably still place them at increased risk of predation during migration. In a simulated stopover situation, Blackcaps with an imminent risk of predation initially increase their food intake and fuelling rate compared with a control group. The pattern of night activity indicated that they also choose to leave earlier and with a lower fuel load than birds in the control group. This clearly shows that stopover behaviour can be adjusted to perceived predation risk.
Evolution is expected to favour a behaviour where survival is maximised during migration. However, it is not always possible to maximise behaviour and performance in all respect simultaneously. It is suggested that the optimal migration strategy may be to minimise overall mortality risk, achieved through a balance between consequences of arrival time, starvation risk and exposure to predation during migration
Stockholm: Department of Zoology, Stockholm University , 1997. , 34 p.