← Responses of plants and animals to their external environment

6 Navigation over long distances

Navigation is finding the way to a specific distant destination — a step beyond simple orientation, since it requires the animal to know where it is relative to where it is going, not just react to one nearby stimulus.

Navigation cues, and when each is used

• Sun compass: the position of the sun, corrected for time of day using an internal clock — usable only by day, and only when the sky is not overcast.
• Stellar (star) navigation: the position of stars, especially a fixed point such as the celestial pole that other stars appear to rotate around — usable at night, but still requires an internal clock to allow for the sky's apparent rotation.
• Earth's magnetic field: detected directly, giving a compass sense that works day or night and in any weather — the fallback cue when visual cues are unavailable.
• Landmarks and scent: coastlines, mountain ranges, or chemical cues from a home stream, used at closer range once the animal is near its destination.
• Real animals typically combine several of these methods, which reduces the risk of ending up badly off course if any one cue is temporarily unavailable (cloud cover, an eclipse, unfamiliar terrain).

Worked example Choosing a navigation method for a night migrant The indigo bunting migrates only at night. Explain why it cannot use a sun compass, and name a method it could use instead. A sun compass requires the sun to be visible — impossible at night, so this cue is ruled out immediately. Stellar navigation is available at night: the bunting can use the position of the stars, especially a fixed point such as the celestial pole, to hold a heading. Because the whole sky appears to rotate around that fixed point as the night goes on, the bunting also needs an internal biological clock to correct for that apparent movement — the environmental cue alone is not enough. ✓ Stellar navigation, using a fixed reference point in the night sky, corrected by an internal clock for the sky's apparent rotation. ✕

Orientation is not the same as navigation

A taxis or a kinesis moves an animal relative to ONE nearby, currently-detectable stimulus (light, moisture, touch) — it needs no sense of where "home" or the destination actually is. Navigation requires knowing your position relative to a distant, currently-undetectable goal, which is why it needs a genuine compass sense (sun, stars, magnetic field) plus, usually, an internal clock to correct it. Calling long-distance migration "a taxis" misses this distinction and will not earn the mark.

Practise just this