Strategic size changes of internal organs and muscle tissue in the Bar‐tailed Godwit during fat storage on a spring stopover site

Abstract

Summary In several migrant bird species it has now been established that refuelling is typified not only by fat deposition, but also by changes in lean body protein. The body composition of a long‐distance migratory shorebird, the Bar‐tailed Godwit Limosa lapponica taymyrensis, was examined on a stopover site in spring to evaluate changes in lean dry mass of organ and muscle tissue over the period of refuelling. It was hypothesized that tissues which support long‐distance flight would hypertrophy and reach peak size towards the end of refuelling, i.e. immediately before the 4500‐km bout of flight to Arctic breeding grounds. In contrast, it was hypothesized that tissues involved in digestion would initially hypertrophy, but then atrophy during the final stages of stopover, so that wing loading during long‐distance flight would be minimized. Flight muscles of godwits steadily increased in lean dry mass during refuelling and reached peak size in birds preparing to depart the stopover site – possibly in an effort to maximize power output. In contrast, the lean dry mass of stomach, liver, kidneys and intestines of godwits increased only during the early stages of refuelling, indicating a quick initial growth of organs involved in the support of refuelling activities. Additionally, in birds readying for departure, the intestines no longer differed in size from those of recent arrivals, suggesting a reduction in size of digestive tissues prior to flight. Such changes in body composition appear consistent with those found in another shorebird and in a grebe preparing for a bout of migratory flight, suggesting a common pattern in allocation of lean body mass in at least some migratory species during fuelling.