Depending on the distance travelled, a one-way migratory journey can last for several hours up to several months, including resting, fuelling and flight times. Some species that migrate long distances between northern and southern hemispheres spend around half of each year on migration.

The ratio of flight to stationary time during the migration periods of small passerines is typically around 1:7, matching theoretical predictions, and in larger species that travel by flapping flight the ratio is around 1:14 to 1:30, as large species have relatively higher flight costs than small ones, and take longer to accumulate the necessary body reserves. However, these ratios vary greatly between individuals in the same population, probably depending largely on their fuel deposition rates and on weather during the journey. Individuals from the same breeding area have been found to vary up to several-fold (in extreme cases up to 12-fold) in the time they take to complete roughly the same journey. Nevertheless, maximum observed migration speeds are fairly close to maximum theoretical estimates.

Although they need to spend less time feeding, small passerines do not always achieve higher migration speeds than larger species that also travel by flapping flight: typically 50-150 km per day versus 50-250 km per day. Among large birds, species that travel by soaring-gliding flight achieve faster average speeds (200400 km per day), mainly because they do not need to accumulate and maintain such large fuel reserves. Some pelagic seabirds travel even faster, with mean speeds up to 950 km per day recorded from albatrosses.

In birds that fly by flapping flight, as well as in those that migrate by soaring-gliding, migration speed generally increases with length of journey. Birds with twice as far to go take longer over the journey, but not twice as long.

In favourable winds, wild geese have been found to fly 5000 km over water within 60 hours, giving mean speeds of around 2000 km per day, or around 80 km per hour. Bar-tailed Godwits Limosa lapponica travelling from Alaska to New Zealand covered more than 10 400 km non-stop in seven days, around 1500 km per day, or 60 km per hour.

In some species that travel by flapping flight, fuelling times associated with large body size may make it impossible to breed, moult and migrate long distances within one year, and could thereby limit migration distances (and hence geographical ranges).

Common Starlings Sturnus vulgaris migrating at night

Was this article helpful?

0 0

Post a comment