This simulation models how migratory birds find their way across the Europe-to-Africa flyway using the Earth's magnetic field rather than a magnetic-polarity compass. It combines a simplified geomagnetic dipole model, in which the field inclination angle grows with latitude, with a stylised cryptochrome radical-pair response that peaks near the field intensity and angle the bird is tuned to. By adjusting the field, the inclination and wind, you can watch the modelled flight path drift away from the direct route and see navigation accuracy fall.
An inclination compass over a map of Europe and Africa. Field inclination is derived from a dipole formula, tan(I) = 2·tan(latitude), drawn as horizontal isoclines. A radical-pair rate function peaks at a reference intensity of 50 µT and 60° inclination, and a compass-error term grows as intensity and angle leave that optimum, bending the migration route.
Use the sliders to set B-field intensity (20-80 µT), inclination angle (0-90°), wind drift (-30 to 30°), and the breeding and wintering latitudes. Toggle the inclination isoclines and the internal compass dial on or off, and press Reset to restore defaults. The stat panel updates navigation accuracy, energy cost, compass error and radical-pair rate live.
The avian magnetic compass reads inclination, not polarity: birds sense the angle between the field lines and gravity, so they cannot tell magnetic north from south. Flip the field's polarity and the bird keeps heading the same way, a property confirmed in laboratory orientation experiments.
It is the ability of animals such as migratory birds, sea turtles and some insects to sense the Earth's magnetic field and use it for orientation and route-finding. Birds in particular rely on an inclination compass that reads the dip angle of the field lines rather than their north-south polarity, which helps them keep a heading over thousands of kilometres.
Cryptochrome is a light-sensitive protein in the bird's eye. When struck by blue light it forms a pair of radicals whose electron spins interconvert between singlet and triplet states, and the rate of that interconversion depends on the surrounding magnetic field's strength and direction. This simulation uses a simplified rate that peaks near 50 µT and 60° inclination to stand in for that quantum-sensing process.
The B-field slider sets intensity in microtesla and the inclination slider sets the dip angle in degrees, both of which feed the compass-error and radical-pair calculations. Wind drift adds a sideways push in degrees, while the breeding and wintering latitude sliders set the start and end of the route. The two checkboxes show or hide the inclination isoclines and the on-screen compass dial.
It is a teaching model, not a research-grade one. The inclination follows the correct dipole relationship, tan(I) = 2·tan(latitude), and the directional sensing and polarity-blindness reflect real findings. However, the radical-pair rate, compass error and energy cost use simplified illustrative formulas rather than measured biophysical constants.
Because inclination changes smoothly from 0° at the magnetic equator to 90° at the poles, the dip angle acts as a proxy for latitude. A bird tuned to a target inclination can fly until the field matches that value, giving it a built-in stop signal. The simulation shows this with horizontal isoclines that the migration route crosses on its way south.