Bird migration Study Guide

📖 Core Concepts Migration: Twice‑yearly, seasonal movement between breeding and wintering grounds, usually north‑south. Photoperiod: Change in day length that triggers hormonal changes and migratory restlessness (Zugunruhe). Navigation cues: Sun compass, celestial stars, Earth’s magnetic field, visual landmarks, and olfactory signals. Flyways: Broad front that narrows into preferred routes; often follow coastlines, rivers, mountain ridges, or updraft corridors. Partial migration: Within a species, some populations migrate while others stay resident. Leap‑frog vs. chain migration: Leap‑frog – high‑latitude birds migrate farther south than low‑latitude birds; chain – populations move in a graded north‑south sequence. Flocking & V‑formation: Reduces individual energy use by 12–20 % and lowers predation risk. Nocturnal migration: Night‑time flight with contact calls; reduces predation and overheating, allows daytime stop‑over feeding. Physiological preparation: Fat deposition (≈55 % body mass for longest nonstop flights), hormonal shifts, molting timing. Protandry: Males of polygynous species arrive at breeding sites earlier than females. 📌 Must Remember Primary cue: Photoperiod → hormone cascade → Zugunruhe. Record‑holders: Arctic tern (96 000 km/yr), bar‑tailed godwit (11 000 km nonstop). Long‑distance migrants: 1 800 of 10 000 bird species. Energy saving: V‑formation = 12–20 % less energy than solo flight. Altitude range: Most migrations 150–600 m; bar‑headed geese reach >6 540 m. Navigation mechanisms: Sun compass (time‑adjusted), magnetic radical‑pair & magnetite, learned landmarks. Threats: Power lines, wind farms, habitat loss at stopovers, climate‑induced phenological mismatches. 🔄 Key Processes Photoperiod detection → hypothalamic hormone release → Zugunruhe (migratory restlessness). Fat accumulation: Increase body fat to 30‑55 % of mass for long flights. Departure: Long‑distance: trigger by day‑length, innate program. Short‑distance: cue by local weather (e.g., wind, temperature). Navigation (during flight): Use Sun compass (adjust for time of day). Switch to stellar cues at night. Magnetoreception provides latitude (field strength) and directional info. Incorporate visual landmarks & odor cues when available. Stopover use: Refuel (fat deposition), rest, and possibly learn route (social learning). Return migration: Often different route (genetically programmed vs. learned). 🔍 Key Comparisons Long‑distance vs. short‑distance migrants Cue: Photoperiod vs. immediate weather. Navigation: Strong genetic program vs. flexible, cue‑driven. Leap‑frog vs. chain migration Leap‑frog: Higher latitudes travel farther south, overtaking lower‑latitude birds. Chain: All populations shift north‑south in the same order. Nocturnal vs. diurnal migration Nocturnal: Lower predation, avoids heat, uses night sky stars. Diurnal soaring: Relies on thermals, avoids large water bodies. ⚠️ Common Misunderstandings “All birds migrate north in spring.” Only true for the majority in the Northern Hemisphere; Southern Hemisphere patterns are reversed and less extensive. “Migration is purely learned.” Many species have a strong genetic component; learning refines but does not create the basic route. “Stopovers are optional.” For long‑distance migrants, stopovers are essential for refueling; skipping them leads to mortality. “All nocturnal migrants are passerines.” Many waterbirds and raptors also migrate at night under favorable conditions. 🧠 Mental Models / Intuition Clock‑Compass Model: Imagine the Sun as a moving clock hand; birds adjust their heading based on the time of day, keeping a constant “hour‑hand” direction. Fuel‑Tank Analogy: Fat stores are a bird’s gasoline; the longer the nonstop leg, the larger the “tank” needed (≈½ body weight for the longest flights). Barrier‑Detour Trade‑off: Visualize a river (barrier) and a bridge (detour). Adding up to 20 % extra distance can be worth it to avoid dangerous zones (e.g., large water bodies for soaring birds). 🚩 Exceptions & Edge Cases Altitudinal migrants: Move vertically rather than latitudinally; cues are more temperature‑driven than photoperiod. Partial migrants: Populations split between migratory and resident strategies, often driven by resource stability. Sexual protandry: Not universal; occurs mainly in polygynous, sexually dimorphic species. Magnetoreception mechanisms: Both radical‑pair (light‑dependent) and magnetite‑based (field‑strength) systems can operate simultaneously. 📍 When to Use Which Predict migration timing → use photoperiod models for long‑distance species; use local weather forecasts for short‑distance/altitudinal migrants. Identify navigation cue dominance → night‑time observations → prioritize stellar cues; daytime soaring → prioritize Sun compass and thermals. Choose monitoring method → Radar: large‑scale, night‑time flight intensity. Acoustic recording: species‑specific nocturnal flight calls. Satellite telemetry: precise route & stopover use for tracked individuals. Conservation action → focus on protecting stopover habitats for long‑distance migrants; mitigate collision risk (power lines, wind farms) along major flyways. 👀 Patterns to Recognize V‑formation flocking → energy‑saving behavior in large waterfowl and some raptors. Seasonal direction flip: northward in spring, southward in autumn (Northern Hemisphere). Stopover “hotspots”: narrow geographic bottlenecks (e.g., Strait of Gibraltar, Bosphorus) where many species concentrate. Night‑time flight calls → indicate nocturnal migration and can be used to estimate flock size. 🗂️ Exam Traps “Photoperiod is the only cue for all migrants.” – Short‑distance migrants rely heavily on immediate weather. “All long‑distance migrants fly nonstop.” – Most require a chain of stopovers; only a few (e.g., bar‑tailed godwit) achieve extreme nonstop legs. “Magnetic navigation works the same for every bird.” – Different species may prioritize magnetic intensity (latitude) vs. inclination (direction). “Partial migration means some individuals never migrate.” – In many cases, the same species may shift between migratory and resident strategies across years depending on conditions. “Flocking always reduces predation risk.” – While generally true, large congregations at bottlenecks can attract specialist predators (e.g., Eleonora’s falcon).