Examples Of Changes In Body Composition

In addition to the storage and depletion of fat that occurs over the migration period, the muscles and internal organs can undergo considerable changes in size

(Piersma 1998). Such changes occur even in short-distance migrants making frequent stops but, as expected, they are much greater in birds making long uninterrupted flights (Table 5.5). They serve to adapt the bird for the journey, providing necessary fuel but also reducing unnecessary weight.

Body changes during migration have been studied in particular detail in the Garden Warbler Sylvia borin (Bairlein 1991a, 1991b, 1998, Biebach 1998, Biebach & Bauchinger 2003). This trans-Saharan migrant can increase its body mass from 18 g in summer or winter to a maximum of about 37 g shortly before setting out over the desert in autumn or spring (Bairlein 2003). This doubling in body mass is due largely to fat deposition, but also to increase in protein and water content. In autumn in northwest Africa, preparation takes 10-14 days, with mean rates of weight gain of 0.7-1.0 g per day, and maximum rates of 1.5 g per day (10% of lean body mass), depending on the food available.

By comparing Garden Warblers Sylvia borin caught in autumn in Turkey (just before a southward Mediterranean-Saharan flight) with others caught in spring in Sinai (just after a northward Saharan flight), Biebach (1998) concluded that about 70% of the loss in body mass during migration comprised fat, and the rest protein and associated water. The protein came partly from the breast and leg muscles which were reduced by 19%, but mostly from the digestive tract which was reduced by 39% (Biebach 1998). Overall, 2.2 g protein and 5.1 g fat were used on this trans-Saharan flight, in an approximate ratio of 1:2.3. Assuming a flight of 2200 km and a mean weight loss of 7.3 g, this gave a mean weight loss of 3.3 g per 1000 km flown, which was not very different from the 3.6 g per 1000 km calculated for this species at sites elsewhere by Bairlein (1991b). After the trans-Saharan flight, it took 1-2 days before gut function and metabolic intake returned to pre-flight level, and 2-3 days for the digestive tract to recover its size (Biebach 1998). This was consistent with the general finding that, after arrival at a stopover site, long-distance migrants often do not gain weight for 1-3 days, and may even lose weight in this period (mean weight loss in 11 passerine species = 4.4%, range 0-13%, Alerstam & Lindstrom 1990).

In another study of Garden Warblers Sylvia borin migrating north over the Sahara in spring, Biebach & Bauchinger (2003) estimated a generally lower rate of weight loss of 1.8 g per 1000 km, but obtained more detailed information of the loss from particular organs (Figure 5.6). The most pronounced weight reduction took place in the liver (57%) and gastrointestinal tract (50%), followed by the flight muscles (26%), leg muscles (14%) and heart (24%). It could not be determined whether some of these changes (such as gut reduction) occurred immediately before take-off, or whether they occurred during the flight itself, but the latter seemed more likely (from the general correlation between body mass and gut mass). By reducing the size of organs before or during a flight, Garden Warblers were estimated to save around one-fifth of the energy needed for the same flight made without organ reduction. Protein catabolism made up about 34% of the overall saving, reduced maintenance costs 22% and reduced flight costs 43%. Estimated savings were roughly the same whether birds flew continuously or intermittently (flying at night and resting by day). The reduction in energy costs has presumably been a major driving force in the evolution of organ flexibility in these birds, extending the maximum possible flight range. A similar ranking of organ reductions to the Garden Warbler occurred in three other

Table 5.5 Examples of phenotypic flexibility of the exercise and the nutritional organs of birds in relation to long-distance migration

Species

Ecological context Flexibility in exercise organs Flexibility in nutritional organs Source

Chaffinch Fringilla coelebs

Gray Catbird

Dumetella carolinensis

Garden Warbler Sylvia borin

Dusky Thrush Turdus naumanni

Pied Flycatcher Ficedula hypoleuca Willow Warbler Phylloscopus trochilus Barn Swallow Hirundo rustica Yellow Wagtail Motacilla flava

Stopover during southward migration

Stopover during southward migration

At autumn departure in Turkey and spring arrival in Egypt

Spring departure from Japan

Autumn migration during short flights compared with spring after a long flight

Spring departure from West Africa

No information

Increase by ca. 35% of pectoral muscle mass with increase in fat load

Decrease in pectoral, leg and heart muscle mass by 15-20% during migration

Pectoral muscle, lungs and heart increase in mass before migration

Decrease in pectoral, leg and heart muscle mass of 15-20% during migration

40-50% mass loss of liver and intestine with increasing fat loads

No information

Decrease in mass of digestive tract and liver by up to 50% during migration

No information

Decrease in liver and gastrointestinal tract of up to 50% before or during migration

Dolnik & Blyumental (1967)

Marsh (1984)

Hume & Biebach (1996), Biebach & Bauchinger (2003) Kuroda (1964)

Schwilch et al. (2002a)

Pectoral muscle increases in mass before migration

No information

Red Knot Calidris canutus canutus/islandica Red Knot Calidris canutus rogersi

Semipalmated Sandpiper Calidris pusilla Bar-tailed Godwit Limosa lapponica

Greater Snow

Goose Chen caerulescens atlanticus

Eared Grebe

Podiceps nigricollis

Northward migration along West Africa/ Europe seaboard

Before northward migration from New Zealand wintering area

Stopover before transoceanic southward flight At departure on trans-Pacific southward flight

Staging during northward migration

Before southward departure from moulting area

No information

Increase in heart and pectoral muscle mass with readiness to migrate

Increase in pectoral muscle and heart mass before departure

Probable increase in heart and pectoral muscle mass before departure No information

Increase in pectoral muscle and heart mass before departure

Decrease in stomach mass before northward departure

Decrease in stomach and intestine mass with readiness to migrate

No information

Very small gizzard, liver, kidneys and intestine at departure

Decrease in gizzard mass during spring stopover

Liver, stomach and intestine shrink by more than 50% before departure

Piersma et al. (1993)

Battley & Piersma (1997)

Driedzic et al. (1993)

Piersma & Gill (1998)

Gauthier et al. (1984)

Data from body composition analyses. Mainly from Piersma (1998).

In addition to the species listed, enlargement of breast muscles (as measured by fat-free dry weight) before migration has been found in a wide range of passerine migrants (see Wingfield et al. 1990) and in various shorebirds (Davidson & Evans 1988, Evans et al. 1992). In contrast, no obvious muscle growth in association with autumn migration was found in Wood Thrush Catharus mustelinus, Veery Catharus fuscescens and juvenile Willow Warbler Phylloscopus trochilus (Hicks 1967, Baggott 1975).

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