Wader species wintering on the Banc d'Arguin increased their body mass by about 40% during the 4–6 weeks before their departure in spring. This estimate is based on 1) the empirical fact that most waders which had finished or suspended their body moult into summer plumage were heavy and thus ready to take off, and 2) the assumption that only the heaviest birds in the population left, which allowed the mass of disappearing waders to be estimated from counts and the frequency distribution of body masses in samples of captured birds. The mass gain on the Banc d'Arguin is just over 1% per day, when expressed as a proportion of winter mass. A review of studies on waders preparing for migration shows that 1) the total migratory reserve adds 20–80% to winter mass, 2) the rate of mass gain is 0.1–4% per day and 3. the period of mass increase lasts four weeks on average, but longer if waders prepare for spring migration on the wintering areas. We suggest that all wader species leaving the B anc d'Arguin at the end of April and the beginning of May are able to reach SE. and NW. Europe without refuelling. This seems only possible if current equations to predict flight range systematically under-estimate this range, even when the energetic benefits of favourable winds at high altitude are taken into account.
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