Module 7: Flight Physiology & Fat

Pre-migratory hyperphagia is a phenotypic switch: day-length, photoperiodic history, and hormonal cues upregulate feeding rate 2–4× the maintenance baseline, suppress reproductive activity, and redirect the resulting anabolism almost entirely into lipid storage. In the blackpoll warbler (Nisbet 1975) body mass rises from a lean \(\sim 12\,\)g to \(\sim 24\,\)g over roughly three weeks at Atlantic-coastal staging sites. On the night of departure the bird is ~50% fat by mass. In the bar-tailed godwit (Limosa lapponica baueri), Gill et al. (2009) satellite-tracked individuals departing Alaska at 550 g and arriving in New Zealand at 270 g after a nonstop 11 000 km flight—fat loss of approximately 51% of departure mass.

Lipid as the migrant’s fuel

Fat is the supreme storage fuel for three reasons. Energy density: \(9.4\,\)kcal/g (~39 kJ/g), more than twice the dry weight of glycogen ( \(\sim 4\,\)kcal/g). Water dependence: glycogen is stored with 3–4 g water per gram of polymer; fat is anhydrous. Metabolic water yield: complete oxidation of 1 g of fat produces ~1.07 g of metabolic water, whereas protein yields only 0.4 g/g and carbohydrate 0.6 g/g. Long-distance passerines and shorebirds therefore exploit fat to maximum advantage: the oxidation product is both energy and water, and the storage form is lightweight.

Neuroendocrine control: orexin, leptin, corticosterone

The hypothalamic orexin/hypocretinsystem—identified in mammals as a driver of wakefulness and feeding—is upregulated in pre-migratory songbirds and coordinates the hyperphagic state with nocturnal restlessness (Zugunruhe). Corticosterone rises modestly, promoting lipid mobilisation. Leptin-like signalling is poorly characterised in birds but appears to provide a satiety feedback that is relaxed during hyperphagia. Together these signals produce a stereotyped behavioural package: high feeding rate during day and dusk, then restless nocturnal activity once fuel stores exceed threshold.

Colin Pennycuick’s 1989 book Bird Flight Performance synthesised avian aerodynamics into a closed-form power-vs-speed model with three ingredients: induced power (to generate lift against weight), parasite power (body drag), and profile power (wing drag). Induced power falls with airspeed (easier to generate lift at higher speed), parasite power rises as \(v^3\), and profile power is roughly flat. The sum is a U-shaped curve with a minimum at the maximum-range speed V_mr—the speed that minimises energy consumption per unit distance travelled, which is slightly greater than the minimum-power speed V_mp.

Under this model, the cost per metre travelled, \(c(v) = P(v) / v\), is minimised at \(V_\mathrm{mr}\). When the bird carries extra fat mass the total weight \(W = (m_\mathrm{lean} + m_f) g\) rises, the induced power rises as \(W^2\), and the wing-loading ( \(W/S_w\)) grows. The U-shaped cost curve shifts upwards and rightwards: V_mrincreases, and the minimum cost per km increases too. The result is a trade-off: more fat means more total energy onboard, but every kilometre flown costs more.

Maximum range and fat load

For a bird that burns fat at a specific efficiency, the total flight range is \(R = \eta_\mathrm{mus}\, e_\mathrm{fat}\, m_f / (P(v)/v) \)evaluated at \(V_\mathrm{mr}\). Because cost per km increases with fat load, the range-versus-fat curve initially rises rapidly and then curves over, approaching an asymptote near the point where the wing-loading penalty offsets the added fuel. In practice flight speeds are typically selected near V_mr, and the fat load is not maximised but rather set to the energy needed to clear the next barrier (Atlantic crossing, desert, mountain range).

A casual account might suggest that fat is burned exclusively and that protein is spared. The reality is subtler. Jenni & Jenni-Eiermann (1998) showed, by analysis of plasma beta-hydroxybutyrate, free fatty acids, uric acid, and triglycerides over the course of long flights, that protein is oxidised as a secondary fuel—approximately 10–15% of total energy in many passerines, rising as fat stores approach depletion. The proteins catabolised come disproportionately from the digestive tract, kidney, and liver, whereas flight-muscle protein is largely spared. In some long-distance migrants the flight-muscle mass even increases during hyperphagia by up to 10–15% in anticipation of the sustained workload.

Heart and flight-muscle enlargement

Echo-cardiographic and dissection studies of long-distance migrants have documented 15–30% enlargement of the pectoralis major and 10–20% enlargement of the cardiac mass in the weeks before migration. The changes are reversible: upon arrival at the non-breeding ground, flight-muscle mass regresses back to baseline within 10–14 days. Battley et al. (2000) directly measured the bar-tailed godwit arrival condition at New Zealand and confirmed both the pre-departure hypertrophy and the postflight lean-mass rebuilding phase.

Theunis Piersma (Ibis, 1998; Avian Invasions, 2011) introduced the flexible-phenotype framework as a unifying concept for migrant physiology: most organs in long-distance migrants are capable of rapid, reversible, large-amplitude size changes that match the physiological demand at each phase of the annual cycle. Examples:

• Digestive tract and liver: hypertrophy during hyperphagia (up to +80% mass); atrophy during nonstop flight (−25–40%) because the gut is dormant.
• Flight muscle (pectoralis): +15–30% during pre-migration; slight atrophy during prolonged flight; regrowth after arrival.
• Heart: +10–20% hypertrophy before migration to sustain aerobic demand.
• Kidney: atrophy during flight (reduced excretion demand) and regrowth at stopover.
• Reproductive organs: fully atrophied during migration, rebuilt at the breeding ground.

The underlying mechanism is classical hormone-driven protein turnover (growth hormone, IGF-1, testosterone, corticosterone) acting on organ-specific stem-cell pools. Piersma’s framework reframes the migrant body as a set of context-sensitive modules whose function can be maximised for the current phase at the cost of reversible mass investment.

Zugunruhe—German for ‘migration urge’—is the stereotyped nocturnal activity shown by many songbirds in the weeks surrounding migration. Caged birds that would otherwise rest quietly through the night begin to flutter, fly against the orientation cage walls, and orient toward the appropriate compass bearing for the season (south in autumn, north in spring). The intensity and duration of Zugunruhe correlates with the natural migration distance and direction of the population.

Peter Berthold (Experientia, 1990; Nature, 1992) showed that in the blackcap (Sylvia atricapilla), Zugunruhe is strongly heritable ( \(h^2 \approx 0.5\)) and that artificial selection can shift the migration programme in a few generations. Crosses between sedentary Cape-Verde-origin and migratory central-European blackcaps produce F₁hybrids with intermediate Zugunruhe amplitudes. Berthold’s work demonstrated the genetic basis of the migratory phenotype, including direction, distance, and the timing and duration of activity bursts.

During flight the migrating bird does not burn a single fuel. The initial 5–15 minutes of flight are powered primarily by muscle glycogen—rapid glycolysis supplies ATP at high rate but exhausts quickly. Sustained endurance flight then switches almost entirely to fat oxidation, which supplies ATP at lower rate but from a much larger reservoir. The transition is mediated by increases in circulating free fatty acids, upregulation of mitochondrial beta-oxidation, and elevated carnitine palmitoyltransferase activity.

Endurance-athlete parallel

The analogy with human endurance athletes is illuminating. A marathoner burns glycogen for the first 20 minutes and progressively fat for the remainder, with the ‘wall’ at km 30 corresponding to glycogen depletion. Migratory passerines show essentially the same hierarchy but scaled to tens of hours of flight and with a much higher fraction of total energy supplied by fat. The mitochondrial density of avian pectoralis (35–45% of cell volume in long-distance migrants) is far greater than in human muscle (~5% in sedentary subjects, ~15–20% in elite endurance athletes).

Migratory and food-caching bird species have disproportionately large hippocampi relative to their body size (Sherry et al., 1989; Krebs, 1990). The hippocampus mediates spatial memory, and the correlation with memory-intensive ecological demand is tight: black-capped chickadees, Clark’s nutcrackers, and marsh tits all cache thousands of seeds per year and locate them months later, and all have markedly enlarged hippocampi. Some migrants (Eurasian reed warbler, garden warbler) show seasonal neurogenesis in the hippocampus that tracks the migratory calendar.

This is significant for migration biology because it establishes that the neural hardware for long-distance navigation is at least partly plastic and environmentally responsive, not purely genetically hard-wired. The relevant circuits include hippocampal CA1, cortical-like pallial regions homologous to mammalian entorhinal cortex, and the cluster N region identified (Mouritsen et al., 2005) as essential for night-time magnetic orientation (see Module 2).

The fuel mix during migration is tightly regulated. Insulin falls during the fasting phase of long flights, promoting lipolysis; glucagon rises; catecholamines (noradrenaline) stimulate hormone-sensitive lipase in adipose tissue. The resulting free fatty acids are released into circulation, bound to serum albumin, and delivered to flight muscle where they enter the mitochondrial matrix via carnitine palmitoyltransferase-1 (CPT-1). Beta-oxidation produces acetyl-CoA, which enters the Krebs cycle for full oxidation.

Corticosterone shows a biphasic pattern during migration: it rises before departure (preparing lipolytic machinery and suppressing appetite) and rises again at the end of long flights (mobilising remaining reserves). Jenni-Eiermann & Jenni (J. Exp. Biol., 2001) measured plasma corticosterone in captured migrants and showed patterns consistent with a two-stage hormonal orchestration of fuel use.

A bird flying nonstop for 70–200 hours must address the problem of sleep. Rattenborg et al. (Nature Communications, 2016) recorded electroencephalograms from great frigatebirds during transoceanic flights and showed that the birds engage in unihemispheric slow-wave sleep—one cerebral hemisphere sleeping while the other remains awake, with the open eye monitoring the environment. The total amount of sleep obtained during flight is much less than during roosting (only 0.7 h per day versus 12 h on land), consistent with a dramatic flight-induced reduction in sleep requirement.

For smaller migrants like passerines, in-flight sleep is less well studied but likely involves similar partial-hemisphere strategies or brief ~10-second microsleeps. The implication for the study of sleep biology is profound: the mammalian model of 8 consecutive hours of rest is not universal, and long-range migratory birds demonstrate that sleep can be fractioned, unilateral, and short-duration without apparent long-term harm.

The wings of long-distance migrants are long, narrow, and pointed—high aspect ratio, moderate camber, low wing-loading. This geometry minimises induced drag at cruise speeds. In contrast, resident species that do not migrate have shorter, rounder wings optimised for manoeuvrability at the cost of efficiency. Comparative wing-shape analysis (Lockwood 1998) shows that within a single family (e.g., Turdidae), the migratory subspecies carry longer wing-tip feathers by 5–10% relative to their sedentary conspecifics.

Aspect ratio and wing loading

Aspect ratio \(\Lambda = b^2/S_w\) and wing loading \(w = mg/S_w\) jointly determine cruise efficiency. Long-distance migrants typically have \(\Lambda \in [7, 10]\) whereas short-distance or non-migrants have \(\Lambda \in [5, 7]\). Higher aspect ratio lowers induced power at the cost of moment of inertia, making sharp turns slower. This trade-off between manoeuvrability and efficiency is a classical ecomorphological theme.

The super-generation monarch presents a striking insect analogue to the passerine story. In late August the newly-eclosed adult enters reproductive diapause (juvenile hormone suppressed), stores 125–175 mg of lipid in the abdomen (~25–30% of body mass), and commences a 4 000 km migration at airspeeds of 3–5 m/s. Unlike passerines the monarch exploits extensive thermal soaring: Gibo & Pallett (Can. J. Zool., 1979) documented thermal-circling behaviour indistinguishable from that of migrating hawks, and energy expenditure during such soaring is minimal. On a typical migration day the super-gen monarch covers 50–100 km; total fat burned over 60 days is ~50 mg, leaving reserves to support five months of winter cluster dormancy.

Flight is one of the most thermally challenging activities in biology. Chemical power of 1–3 W is being dissipated as heat in a body mass of 10–30 g, giving a mass-specific heat production rate 30–100× resting metabolism. Evaporative and convective cooling must match this rate or body temperature rises rapidly. Birds regulate by flying at cooler altitudes (1–2 km AGL on warm days), flying at night when ambient temperatures are lower (nocturnal migration), and using panting when necessary.

Extreme altitudes are used by some migrants. Bar-headed geese (Anser indicus) cross the Himalayas at 5–7 km, sustained flight at 30 kPa ambient pressure. Their haemoglobin has amino-acid substitutions giving increased O₂ affinity, and their capillary density and mitochondrial density are the highest known in any bird. Whooper swans migrating over Greenland at 8 km have been recorded; common cranes and Eurasian oystercatchers cross the Alps at 3–4 km. The cost of altitude is compensated by lower air density (reduced parasite drag), tailwinds in the upper troposphere, and thermoregulation in cooler air.

Avian pectoralis muscle is among the most mitochondria-rich tissue in the animal kingdom. In long-distance migrants (bar-tailed godwit, blackpoll warbler, common swift), electron microscopy shows mitochondrial volume densities of 35–45% of the muscle fibre volume. For comparison, sedentary human quadriceps has ~5% and elite marathoners ~20%. The aerobic scope—the ratio of maximum sustainable to basal metabolic rate—can exceed 20× in migrant birds, well beyond the ~5× ceiling in humans and most mammals.

This extreme aerobic capacity imposes design constraints on other systems. Capillary density is correspondingly high, with 2–3 capillaries per muscle fibre compared with 1 in mammalian muscle. Cardiac output at peak migratory effort reaches 300–400 ml/min/kg body mass in some shorebirds. These are physiological parameters that push biological materials to the limits of what is possible with oxygen-powered metabolism.

Most migrants do not fly nonstop. The typical strategy is a series of flight legs interspersed with stopovers of 1–10 days at sites rich in food. At each stopover the bird rapidly replenishes fat stores at rates of 2–5% body mass per day. Higher quality stopover sites support higher fuel-deposition rates and therefore shorter stopover durations; poor sites extend the stopover and delay arrival.

Schaub et al. (J. Avian Biology, 2004) modelled stopover strategy as an optimisation problem: the bird chooses when to depart based on current fat stores, remaining distance, and expected future habitat quality. The model predicts threshold-based departure decisions, in which the bird leaves as soon as fat stores exceed a minimum required for the next leg. Real birds approximate this strategy with noise and with weather-dependent adjustments: unfavourable winds postpone departure even when fat reserves are adequate.

A migrating bird is an ambulatory dehydration experiment. Respiratory water loss via evaporation at the ventilatory surface is the dominant water expense; under typical flight conditions (\(\dot V\mathrm{O}_2 \approx 300\) ml/min, ambient T ~ 10°C) a 25 g warbler can lose 0.2–0.4 g/h of water. Over a 70-hour nonstop transatlantic flight, cumulative water loss totals 15–30 g, which would be lethal if not offset. Three mechanisms prevent catastrophic dehydration.

• Metabolic water from fat oxidation yields 1.07 g per g fat burned, accounting for most of the loss.
• Lean-mass catabolism releases bound intracellular water when protein is broken down; Jenni 1998 estimated 0.4 g/g lean mass released.
• Nasal salt glands of marine shorebirds secrete concentrated saline to reduce urinary water losses.

Even with these mechanisms, some shorebirds arrive in a mildly dehydrated state and recover rapidly by drinking at the stopover site. Species that cannot reach fresh water immediately upon arrival face acute mortality. The water balance is therefore another hard constraint on migration distance, alongside fat reserves and aerodynamic limits.

Migration is expensive. Across taxa the long-distance migrant solves the energetic problem by exploiting lipid as a hydrogen-rich, water-anhydrous, density-packed fuel, and by matching organ morphology to the phase of the annual cycle. The Pennycuick power curve sets cruise speed; the U-shaped fat-carrying trade-off sets optimal fuel load; the Piersma flexible-phenotype framework explains how heart, muscle, and digestive tract are reorganised in days. Zugunruheprovides a behavioural entry into the genetic architecture of the migratory programme. The chapters that follow turn from physiology and mechanism to conservation: how are these finely-tuned migratory adaptations holding up against a rapidly warming planet?