How Does a Bird Climb Without Flapping? Find the Rising Air and Circle

The ground heats unevenly, because a ploughed field, a car park or a rocky slope absorbs far more sunlight than grass, woodland or water. Air in contact with a warm patch is heated, becomes less dense than the air around it and eventually breaks away as a rising bubble or a continuous column. That column can extend a considerable height, frequently to the level where the rising air cools enough for its moisture to condense, which is why fair weather cumulus clouds mark the tops of the strongest ones and are the most useful sign available from below.

Using a column requires a specific and counterintuitive manoeuvre:

• •The bird circles tightly inside the rising air rather than flying through
• •It sinks relative to the air continuously, since it is still gliding
• •The air rises faster than it sinks, so it climbs overall
• •Circling tightly keeps it inside the strongest part of the column
• •It then leaves and glides towards the next one, losing height
• •The cycle repeats, giving long distance travel with almost no flapping

Measurements on soaring birds carrying recorders show the saving is enormous. Heart rate and metabolic measurements indicate that soaring flight costs a large bird only a little above resting, since holding the wings out requires far less work than beating them, and a vulture can remain airborne for hours at a cost that flapping flight would make impossible. That is why soaring birds are large and heavy with long broad wings, a combination that makes flapping expensive and gliding efficient. The trade is that the technique works only in daylight, only over land, and only when the sun has warmed the ground, which constrains their whole way of life.

Warm columns are not the only lift available and experienced birds and pilots use all of them. Wind striking a ridge is forced upward, producing a band of rising air along the slope that persists as long as the wind blows and works at any time of day, which is why gulls hang motionless over a cliff edge and why glider pilots fly ridges in winter. Mountain waves form downwind of a range when stable air is disturbed, producing smooth rising bands that can reach the stratosphere. Air converging at the boundary between two air masses lifts along a line that can run for tens of kilometres, which birds follow rather than circling in.

Dependence on rising air shapes migration routes decisively and produces the great bottlenecks where birds concentrate. Columns do not form over water, because the sea heats far more slowly and evenly than land, so soaring migrants cannot cross wide sea and instead funnel to the narrowest crossings. Enormous numbers pass over Gibraltar, the Bosphorus, the Strait of Messina and the Israeli rift valley, with counts of hundreds of thousands at a single site in a season. Those bottlenecks make the birds easy to count and easy to shoot, which is why several of them are both major research sites and major conservation problems. Departure also waits until the morning has warmed.

Unevenly heated ground releases columns of warm air, and a bird circling inside one sinks through the air while the air carries it up faster, so it climbs. Soaring costs a large bird little above resting, which is why such birds are heavy with long broad wings. Columns do not form over sea, so soaring migrants funnel into narrow crossings in enormous numbers.