Abstract
In spring, long-distance migrants are considered to adopt a time-minimizing strategy to promote early arrival at breeding sites. The phenology of spring migration was examined and compared between two insular stopover sites in Greece and Sweden for Icterine Warbler, Wood Warbler, Spotted Flycatcher and Collared Flycatcher. All of them migrate due north which means that some proportion of birds that pass through Greece are heading to Scandinavia. The Collared Flycatcher had the earliest and the Icterine Warbler the latest arrival time. The differences in median dates between Greece and Sweden were 3–4 weeks and the passages in Sweden were generally more condensed in time. The average overall speed estimates were very similar and varied between 129 and 137 km/d. In most of the species higher speed estimates were associated with years when birds arrived late in Greece. After crossing continental Europe birds arrive at the Swedish study site with significantly higher body masses compared to when they arrive in Greece and this might indicate a preparation for arriving at breeding grounds with some overload.
Highlights
According to optimal migration theory, a migratory bird might have to choose between minimizing the overall time on migration, energy expenditure or predation risk (Alerstam & Lindström 1990)
The median arrival dates of Icterine Warbler, Wood Warbler, Spotted Flycatcher and Collared Flycatcher varied between 19 April and 8 May at Antikythira and between 8 and 27 May at Gotland
The median arrival dates differed significantly between the species at Antikythira (Kruskal-Wallis, H=25.35 df=3, p=0.0001) with the median arrival dates of the Icterine Warbler and the Spotted Flycatcher being different from the median arrival date of the Wood Warbler and the Collared Flycatcher (Mann-Whitney pairwise, Bonferonni corrected; Icterine Warbler vs. Wood Warbler: p=0.005, Icterine Warbler vs. Collared Flycatcher: p=0.05, Spotted Flycatcher vs. Wood Warbler : p=0.007, Spotted Flycatcher vs. Collared Flycatcher: p=0.005 )
Summary
According to optimal migration theory, a migratory bird might have to choose between minimizing the overall time on migration, energy expenditure or predation risk (Alerstam & Lindström 1990). The importance of reaching the breeding grounds early might give rise to a time-minimizing strategy rather than energy-minimization (Kokko 1999, Newton 2008). As migrants need to optimally modulate their travel costs in relation to time, energy and safety while crossing those diverse sectors, migration speed and body mass might vary between different geographic sectors along the migration route (Bojarinova et al 2008, Yohannes et al 2009a, b). It is well established that several bird species have advanced their spring migration with the current climate changes (e.g. Hüppop & Hüppop 2003, Stervander et al 2005, Thorup et al 2007, Hedlund et al 2014), which makes it important to follow and analyse the phenology of migratory birds
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