Abstract
BackgroundPerformance of migrating birds can be affected by a number of intrinsic and extrinsic factors like morphology, meteorological conditions and migration strategies. We compared travel speeds of four raptor species during their crossing of the Sahara desert. Focusing the analyses on this region allows us to compare different species under equivalent conditions in order to disentangle which factors affect migratory performance.Methodology/Principal FindingWe tracked raptors using GPS satellite transmitters from Sweden, Spain and Italy, and evaluated their migratory performance at both an hourly and a daily scale. Hourly data (flight speed and altitude for intervals of two hours) were analyzed in relation to time of day, species and season, and daily data (distance between roosting sites) in relation to species, season, day length and tailwind support.Conclusions/SignificanceDespite a clear variation in morphology, interspecific differences were generally very small, and did only arise in spring, with long-distance migrants (>5000 km: osprey and Western marsh-harrier) being faster than species that migrate shorter distances (Egyptian vulture and short-toed eagle). Our results suggest that the most important factor explaining hourly variation in flight speed is time of day, while at a daily scale, tailwind support is the most important factor explaining variation in daily distance, raising new questions about the consequences of possible future changes in worldwide wind patterns.
Highlights
The migratory performance of birds can be shaped by intrinsic factors like species morphology, experience and migration strategy [1,2,3] and by extrinsic factors like landscape properties and meteorological variables [4,5,6,7]
The migratory performance was evaluated for the raptors’ crossing of the Sahara desert along the flight routes shown in Figure 1. (a) Hourly Scale Concerning the circadian pattern of migration, speeds were generally slower early in the morning and late in the afternoon, being much higher around noon
In this study we compared the behaviour and migratory performance of different bird species migrating across the same regions and during the same periods
Summary
The migratory performance of birds can be shaped by intrinsic factors like species morphology, experience and migration strategy [1,2,3] and by extrinsic factors like landscape properties and meteorological variables [4,5,6,7]. Diurnal migrants flying over land can migrate by flapping or soaring flight. As energy consumption during flapping flight increases steeply with body mass, soaring migration is more advantageous for larger bird species [8,9]. Performance of migrating birds can be affected by a number of intrinsic and extrinsic factors like morphology, meteorological conditions and migration strategies. Focusing the analyses on this region allows us to compare different species under equivalent conditions in order to disentangle which factors affect migratory performance
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