Organ structures and correlated metabolic features (e.g. metabolic rate) have often taken as fixed attributes of fully grown individual vertebrates. When measurements of these attributes became available they were often used as representative values for the species, disregarding the specific conditions during which the mesurement were made. Evidence is accumulating that the functional size of organs and aspects of the metabolic physiology of an individual may show great flexibility over timescales of weeks and even days depending on physiological status, environmental conditions and behavioural goals. This flexibility is a way for animals to cope successfully with a much wider range of conditions occurring during various life-cycle events than fixed metabolic machinery would allow. Such phenotypic flexibility is likely to be a common adaptive syndrome, typical of vertebrates living in variable environments.

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Rapid reversible changes in organ size as a component of adaptive behaviour

Important biological concepts typically have many definitions. Body condition is no exception; it is a multi-level phenomenon that can be defined either conceptually or operationally. Although at this time there is no clear consensus on the definition for body condition, conceptually, the term in many cases describes the degree to which an organism’s physiological state influences its performance (i.e., production, activity, or response to environmental conditions). Operational definitions for condition typically are based on some aspect of body composition (e.g., levels of nutrient stores or indirect indicators of such levels). Despite these complexities, numerous published studies have documented the effect of a bird’s nutritional status on some aspect of Darwinian fitness. The primary goals of this chapter are to describe the available methods of assessing condition, evaluate their accuracy and utility, illustrate how these methods have been applied, and clarify how they satisfy operational and conceptual definitions of body condition. The result will be to provide a primer for researchers interested in obtaining condition estimates, as well as for those wishing to improve upon existing methods.

Assessing Body Condition in Birds

The adaptive value of size changes in different organ and muscle groups was studied in red knots (Calidris canutus islandica) in relation to their migration. Birds were sampled on five occasions: at arrival in Iceland in May 1994, two times during subsequent refueling, at departure toward, and on return from, the high arctic breeding grounds. During their 24-d stopover in May, body mass increased from 144.3 to 214.5 g. Mass gains were lowest over the first week (0.85 g/d, only fat-free tissue deposited). Over the subsequent 10 d, average mass increased by 5.0 g/d (fat contributing 78%), and over the last week before takeoff, it increased by 2.0 g/d (fat contributing over 100% because of loss of lean components). There were no sex differences in body and fat mass gains. Over the first interval, lean masses of heart, stomach, and liver increased. During the middle 10 d, sizes of leg muscle, intestine, liver, and kidneys increased. Stomach mass decreased over the same interval. In the last interval before takeoff, the stomach atrophied further and the intestine, leg muscles, and liver became smaller too, but pectoral muscles and heart increased in size. Sizes of "exercise organs" such as pectoral muscle and heart were best correlated with body mass, whereas sizes of organs used during foraging (leg muscles) and nutrient extraction (intestine, liver) were best correlated with rate of mass gain. Kidneys changed little before takeoff, which suggests that they are needed as much during flight as during refueling.

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Rapid Changes in the Size of Different Functional Organ and Muscle Groups during Refueling in a Long‐Distance Migrating Shorebird

We documented fat loads and abdominal organ sizes of Bar-tailed Godwits (Limosa lapponica baueri) that died after colliding against a radar dome on the Alaska Peninsula, most likely just after takeoff on a trans-Pacific flight of 11,000 km, and of birds of the same subspecies just before northward departure from New Zealand. We compared these data with data on body composition of godwits of the smaller lapponica subspecies obtained during a northward stopover in The Netherlands. As a consequence of high amounts of subcutaneous and intraperitoneal fat, and very small fat-free mass, Bar-tailed Godwits from Alaska had relative fat loads that are among the highest ever recorded in birds (ca, 55% of fresh body mass). Compared with northbound godwits from New Zealand, the Alaskan birds had very small gizzards, livers, kidneys, and guts. This suggests that upon departure, long-distance migrants dispense with parts of their "metabolic machinery" that are not directly necessary during flight. and rebuild these organs upon arrival at the migratory destination.

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Guts Don't Fly: Small Digestive Organs in Obese Bar-Tailed Godwits

Avian long-distance migration involves the storage and expenditure of very large fuel loads. Birds may double in weight before take-off on flights of many 1000 km, and they may lose half their body mass over the subsequent few days that such trips take. Recent studies indicate that in addition to the storage and depletion of fat, the muscles and belly organs also undergo considerable changes in size in the course of such migrations. Such intraindividual and repeatedly reversed changes in stores and organ sizes represent a class of phenotypic plasticity called 'phenotypic flexibility'. Using preliminary comparative data for different populations of Bar-tailed Godwits Limosa lapponica, and several other shorebird species adding variation to the migration strategies sampled (Golden Plover Pluvialis apricaria, Ruff Philomachus pugnax, Red Knot Calidris canutus and Bristle-thighed Curlew Numenius tahitiensis), the thesis is developed that the size of the organs carried during take-off represent evolutionary compromises between their functions during the storage, flight and post-arrival phases of migration. In all cases fat-free tissue along with fat is deposited during fuel storage, but the proportions vary a great deal between similarly sized species. Just before departure on long-distance flights, exercise organs (pectoral muscle and heart) tend to show hypertrophy and nutritional organs (stomach, intestine and liver) tend to show atrophy. Reductions in nutritional organs appear most pronounced in (sub-) species that are about to overfly barren oceans with few or no opportunities for emergency landings. Migrant birds seem to show a great deal of adaptive flexibility, and the study of this flexibility may shed light on (presently unknown) physiological mechanisms as well as on correlated ecological constraints on bird migration.

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Phenotypic flexibility during migration: optimization of organ size contingent on the risks and rewards of fueling and flight?

We studied differences in body composition and basal metabolic rate (BMR, measured in postabsorptive birds under thermoneutral conditions at night) in two subspecies of red knots, Calidris canutus: one that spends the nonbreeding season under energetically costly climatic conditions at temperate latitudes (subspecies islandica in western Europe) and one that winters in the hot and humid tropics (subspecies canutus in West and South Africa). To examine whether the possible differences would be upheld under identical conditions, we kept both groups in captivity as well Body composition was quantified with respect to the fat and lean components of 10 "organs" (breast muscles, leg muscles, stomach, intestine, liver, kidneys, lungs, heart, and the skin, and skeleton and attached muscle). Captive birds had lighter lean tissues than wild birds, especially those of the stomach, intestine, kidneys, and liver (the nutritional organs). During the northern winter wild islandica knots had higher lean masses than canut...

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P. Wiersma

Papers

Variability in basal metabolic rate of a long-distance migrant shorebird (Red Knot, Calidris canutus) reflects shifts in organ sizes