BA Pectoral muscles z
Figure 5.6 Comparison of the mass of different organs (lean dry mass (95% confidence interval)) of Garden Warblers Sylvia borin before (B, Ethiopia) and after (A, Egypt) crossing the Sahara Desert. From Biebach & Bauchinger (2003).
trans-Saharan migrants, namely the European Pied Flycatcher Ficedula hypoleuca, Willow Warbler Phylloscopus trochilus and Barn Swallow Hirundo rustica (Schwilch et al. 2002a).
Massive changes in body composition also occur in some shorebirds, which include some of the most impressive long-distance bird migrants, crossing some of the world's largest stretches of ocean or desert non-stop. Not only are they able rapidly to store and metabolise large amounts of fat, they also undergo many other physiological changes, affecting skeletal muscles and various internal organs (Piersma & Lindström 1997, Battley et al. 2000). Extreme changes were found in Bar-tailed Godwits Limosa lapponica baueri collected in Alaska as they hit a radiotower, just after take-off on a presumed trans-Pacific flight of at least 10 400 km to New Zealand. The majority of individuals in autumn are thought to make this flight non-stop, but some are seen to pause on various Pacific Islands (Chapter 6). The Alaskan birds had some of the highest fat contents recorded in birds, amounting to 55% of total body mass. They also had relatively large breast muscles and heart (= exercise organs), but very small gizzard, liver, kidneys and gut (= digestive organs). Upon departure in autumn, these long-distance migrants apparently dispensed with parts of their metabolic machinery that were not directly necessary during flight, presumably converting them to other tissue. They rebuilt them upon arrival at the migratory destination, in order that they could again feed at maximum efficiency (Piersma & Gill 1998). This temporarily reduced digestive function may have been more than compensated by savings on transport costs.
Attaining as much as 55% of fat in total body mass has other consequences. Not only must all of the bird's structure and systems be contained in 45% of the body mass at take-off, but consuming this huge proportion of fat requires heavy inroads into the remaining protein during flight. According to Pennycuick & Battley (2003), the journey to New Zealand (10400 km) would entail flying continuously for a week, but the fuel load was sufficient to fly much further, or alternatively to provide a reserve for adverse winds and navigation errors.
A second sample from the same population of godwits, obtained in spring before departure from New Zealand, would have run out of fat before reaching Alaska, but could have reached the Yellow Sea area, where these birds stage
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