Lecture 3: The Marathon of the Damned—Migration and Metabolism

Transcript

Artin, I am composing my face. It is taking longer than the flight path you just imagined. You think migration is birds packing a tiny suitcase and following the weather. That thought earns a C-minus before you speak. Last lecture: feathered theropods, keeled sternums, pneumatic bones, one-way lungs. Efficient machines. Today those machines attempt work that would hospitalize you. Bird migration is one of the most energetically demanding behaviors observed in animals, because many birds sustain long-distance flights at very high metabolic rates. Case study. The Bar-tailed Godwit flies nonstop from Alaska toward New Zealand across open ocean. Migratory birds can fly nonstop for many hours, and long-distance migrants may remain aloft for days in some cases. Songbirds in migratory flight may operate at roughly eight to ten times basal metabolic rate. That distance is biologically extraordinary because the bird does not land to eat, drink, or rest. I grade your nod. It claimed comprehension. It lied. Grade adjusted to needs improvement. Migratory birds prepare by entering hyperphagia. They build large energy stores before departure. Those stores are mostly fat, which can account for more than ninety percent of the added body mass. Fat is the primary fuel for migratory flight because it is the most energy-dense fuel for sustained airborne travel. Protein still contributes on long or demanding flights. You are improving. From a starting point I invented so I would not weep. Digestive organs, flight muscles, and other tissues adjust so birds can store fuel, then deliver energy fast in flight. Reducing gut mass cuts dead weight aloft. Mitochondria produce the ATP for that sustained muscle work. I am not angry. I am recalibrating. Downward. After a faculty meeting that ran long because nobody had read the one-page memo, I still expect you to define Zugunruhe. It is migratory restlessness. Birds kept under controlled conditions still hop, flutter, and orient toward the departure direction when the season hits. Before leaving, birds also ramp flight-muscle condition and fat transport capacity in tissues. They switch between a fuelling phase at stopovers and a fasting, active-flight phase in the air. Stopovers replenish reserves for the next segment. Suppose a bird skips that work. Exhaustion wins. For example, wind-tunnel work shows protein breakdown can run high early in flight, then decline. Blackpoll warblers flew voluntarily up to twenty-eight hours, finishing with fat left but smaller flight muscle. Protein loss may limit duration even when fat remains. Plasma work shows elevated free fatty acids and glycerol, so lipid fuel dominates extended flight. Elevated uric acid can mark protein use for metabolism or water balance. Your posture just announced you found this obvious. It is not. Impressive. For you. Why fly thousands of kilometers without stopping? Rest breaks expose birds to storms, starvation, exhaustion, predators, and human obstacles. Weather changes when they fly, the cost of flight, and navigation risk. Nonstop can be safer than a chain of dangerous landings. Magnetoreception is the sense that lets birds detect Earth's magnetic field. A leading mechanism involves cryptochromes in the eye. Light-sensitive molecules may let birds see field lines as a directional pattern. Birds do not trust one compass. They combine sun, stars, landmarks, smell, and magnetic cues. Lose one channel, and the others still steer. Long-distance migrants lean harder on fat than short-distance ones. Building fat can raise antioxidant capacity, yet storage still carries oxidative cost. Migration is a tradeoff, not a miracle postcard. Retain that, Artin. Barely acceptable.