Body composition

The internal workings of birds are constantly changing in relation to different energetic demands. Migration has long been recognised as needing to be fuelled by large fat deposits, but recently it has become clear that changes are much more complex. 'Exercise organs' (such as the flight muscles, heart and lungs) increase in size before migration. Digestive organs may become enlarged as birds have to process more food to put on weight, but as birds do not feed during migratory flights, in some species the guts may be reduced in the week before departure. During flight itself, both fat and protein is broken down, resulting in birds being 'leaner' in very sense of the word upon arrival. These changes in lean tissue affect the overall 'working level' of the body, the metabolic rate, and basal metabolic rate varies as a consequence of the changes in body composition.

It's easy to see that studying body composition is often highly intrusive. While some methods allow estimation of coarse changes in lean and fat tissue in an animal's body, more detailed work typically involves dissection of specimens, which have been collected specifically for a study or have been accidental casualties or natural deaths. Body composition work on godwits and Red Knots in New Zealand and Alaska, for example, has been done on birds killed by a poacher, and confiscated, and on birds that flew into a tower and died. Accidental casualties can be very valuable for work like this.

What have body composition analyses of shorebirds revealed? In Red Knots, the gizzard mass directly determines how much food a bird can process, and on migratory stopovers the gizzard increases in mass, then decreases. This 'shrinking' of digestive organs before departure has been assumed to represent the typical shorebird, though knots may not necessarily be particularly representative in their gut size. Because knots have the hardest diet of any shorebird, they also have the largest and heaviest stomach size relative to body mass of any shorebird studied (see reference). Turnstones feeding on horseshoe crab eggs also show the same pattern, but Western Sandpipers migrating up the west coast of North America do not. It seems that these small sandpipers, which feed on tiny soft-bodied prey, do not enlarge their digestive systems until after they have started on migration, keeping their guts relatively large. More work needs to be done to determine how representative either of these situations are, and whether the apparent differences between knots and Western Sandpipers reflect their diets, relative gut masses, migratory flight strategies or some other factor. At the moment these 'models' are treated as if the only difference that is important is flight length.

The knot work has recently shown up an unexpected paradox in how birds adjust their gizzard sizes on migration. We normally assume that birds must increase their guts during fuelling and on stopover, as has been shown clearly in Iceland. For many knot populations, however, they can get away without enlarging their stomachs because they choose sites on migration that have much higher prey quality (more flesh for a given shell mass) than at their earlier fuelling sites. And not all knots build up their stomachs to the 'optimal' size for speedy fuelling. Knots in Northwest Australia ought to have a gizzard that weighs around 10 g if they are to fuel as quickly as possible. Instead, they keep their stomach much smaller (around 7.5 g) and take much longer to fuel. Such variation between populations provides exciting possibilities for biologists to exploit!