Inability to regain normal body mass despite extensive refuelling in great reed warblers following the trans‐Sahara crossing during spring migration

Abstract

Migratory birds wintering in Africa face the challenge of passing the Sahara desert with few opportunities to forage. During spring migration birds thus arrive in the Mediterranean area with very low energy reserves after crossing the desert. Since early arrival to the breeding grounds often is of importance to maximize reproductive success, finding stopover sites with good refuelling possibilities after the Saharan passage is of utmost importance. Here we report on extensive fuelling in the great reed warbler Acrocephalus arundinaceus on the south coast of Crete in spring, the first land that they encounter after crossing the Sahara desert and the Mediterranean Sea in this area. Birds were studied at a river mouth and due to an exceptional high recapture rate (45 and 51% in two successive years), we were able to get information about stopover behaviour in 56 individual great reed warblers during two spring seasons. The large proportion of trapped great reed warbler compared to other species and the large number of recaptures suggest that great reed warblers actively choose this area for stopover. They stayed on average 3–4 d, increased on average about 3.5 g in body mass and the average rate of body mass increase was 4.8% of lean body mass d–1. Wing length affected the rate of increase and indicated that females have a slower increase than males. The results found show that great reed warblers at this site regularly deposit larger fuel loads than needed for one continued flight stage. The low body mass found in great reed warblers (also in birds with high fat scores) is a strong indication that birds staging at Anapodaris still had not been able to rebuild their structural tissue after the strenuous Sahara crossing, suggesting that rebuilding structural tissue may take longer time than previously thought.

Individual and sex‐related patterns of prolonged flights during both day and night by great reed warblers crossing the Mediterranean Sea and Sahara Desert

“…Prior to crossing the Mediterranean Sea and the Sahara Desert, migrant birds must store extensive fuel reserves, mainly fat, as they might not be able to restore the energy resources by feeding at stopovers during the barrier crossing (Fry et al 1970, Ottosson et al 2005, Fransson et al 2006, 2008, 2017). Furthermore, while crossing the Sahara Desert there is a risk of dehydration during flight, especially during autumn (Carmi et al 1992, Klaassen 2004, Schmaljohann et al 2007a, 2009).…”

Highway(s) overhead: Strong differences in wetland connectivity and protected status challenge waterbird migration along the four Palearctic‐Afrotropical flyways

“…Birds travelling to and from sub‐Saharan Africa must cross the Sahara Desert, an area that spans up to 2000 km from north to south, where thermoregulation and replenishing energy reserves are particularly challenging (Fransson et al, 2017; Lok et al, 2015; Schmaljohann et al, 2007). The adjoining crossing of the Mediterranean Sea is another stretch of up to 700 km open water without stopovers.…”

Stopover Ecology of the European Turtle Dove (Streptopelia turtur), a Threatened Migratory Bird Species, after the Crossing of an Extended Ecological Barrier

“…Migratory birds spend the vast majority of the duration of the migratory journey at stopover sites (e.g., [14][15][16]). Stopover sites are used heavily for replenishing the fuel load (e.g., [17]), especially during spring after the crossing of the Sahara and the Mediterranean in the central and western parts of the eastern flyway, where a significant portion of birds have been reported to be energy-depleted [18][19][20][21][22]. Nevertheless, the importance of stopover sites, such as islands, just after the barrier crossing is not only limited to refuelling purposes [23][24][25].…”