SkarpChapter 2 of 3
Knowing Direction Versus Knowing Position
Learn the difference between a compass that supplies a heading and a map sense that indicates location. Follow simple displacement experiments showing how some birds use magnetic gradients, odors and familiar landmarks to correct their route.
1. Two Questions, Two Navigation Tools
Direction versus position
A compass answers "Which way should I go?" A map sense answers "Where am I relative to where I need to be?" These are different problems.
A bird can hold a correct migratory heading yet still miss its destination after an unexpected displacement. Correcting that error requires position information.
2. What a Bird Compass Can Use
A multi-cue compass
Birds may use the Sun, stars, polarized twilight, and Earth's magnetic field to choose and maintain a heading. Their systems can calibrate one another.
A magnetic inclination compass reads the tilt of magnetic field lines, distinguishing poleward from equatorward. A heading cue is not automatically a location cue.
3. Indigo Buntings: Reading the Sky
A rotating sky as a reference
Young indigo buntings learned from the apparent nightly rotation of stars. The stable-looking center of that rotation became their directional reference.
When the planetarium shifted the apparent rotation center, the birds shifted their preferred direction. That demonstrates a learned star compass, not a geographic map.
4. Thought Exercise: The Passenger Bird
Test the distinction
A bird is traveling south for winter. Strong winds carry it 1,000 kilometers west while it rests on a ship.
- If it has only a compass, what direction should it continue to fly?
- Why might that heading fail to bring it back to its usual migration corridor?
- What new information would it need to choose an eastward correction?
Answer: A compass preserves the original southward program. To correct westward, the bird must infer its current position relative to the corridor or destination. That inference is the job of a map sense.
5. A Magnetic Map Experiment
Virtual displacement
A coil system reproduced the magnetic field of a location near Moscow while reed warblers remained elsewhere. The experimental "move" happened in magnetic space.
Sham-operated birds with intact ophthalmic branches re-oriented as if displaced. Birds lacking input through the trigeminal ophthalmic branch did not. Here, magnetic cues contributed to a map.
6. Gulls: Position Information Can Smell Like a Place
Heading retained, correction lost
Olfactory-nerve-sectioned gulls still preferred a southerly direction after westward displacement, but they did not compensate toward their usual corridor.
The result supports olfactory information as a positional cue in this situation. It does not establish that smell is every bird's only navigation system.
7. Combine Cues Like a Navigator
Layered navigation
A compass maintains direction. A map detects position and supports correction. Familiar landmarks can add immediate local guidance where a bird has prior experience.
Ask two separate questions: "What tells the bird where to point?" and "What tells it where it is?" That prevents compass and map evidence from being confused.
8. Review the Core Terms
Flip each card and connect the term to the displacement experiments.
- Compass
- A system that provides a travel direction or heading, such as south during autumn migration.
- Map sense
- A system that provides information about current position relative to a destination or route.
- Inclination compass
- A magnetic compass that uses the tilt of Earth's magnetic field lines to distinguish poleward from equatorward.
- Virtual magnetic displacement
- An experiment that changes the magnetic field around a bird to mimic another geographic location without physically moving the bird.
- True navigation
- Correcting for an unexpected displacement by selecting a new bearing toward a goal or normal route.
Check Understanding
Choose the best interpretation of a displaced bird that keeps flying south instead of correcting eastward.
What does the bird's continued southward flight most strongly suggest?
- It may still have a compass heading but lacks, or cannot use, sufficient position information to compensate.
- It has proved that magnetic fields never influence bird navigation.
- It has precisely identified its location and chosen the shortest route home.
Show Answer
Answer: A) It may still have a compass heading but lacks, or cannot use, sufficient position information to compensate.
A steady southward course can show that a bird retains its migratory compass program. A corrective eastward turn would require information indicating that the bird is west of its intended corridor.
Key Terms
- compass
- A navigation system that supplies direction of travel.
- map sense
- A navigation system that supplies information about location relative to a destination.
- olfactory nerve
- A nerve carrying smell-related sensory information; experiments indicate it can be important for route correction in some displaced gulls.
- trigeminal nerve
- A cranial nerve with branches that have been investigated for their role in transmitting magnetic positional information in some birds.
- migration corridor
- The broad, population-specific region through which migrants normally travel.
- inclination compass
- A magnetic compass based on the angle at which Earth's magnetic field lines meet the surface.