Monday, 10 March 2014

Cheating the Winds

Cheating the winds

How to do Spring migration, bird-style...

Here in western Europe, the prevailing airstream in early spring is from north-east to south-west, except over the British Isles and NW France where incoming Atlantic systems can give local winds out of the west. In west Africa, the general pattern is much the same: high pressure over the Azores gives a more-or less constant anticyclonic (clockwise) rotation of the winds, keeping them flowing from north to south over coastal regions.

For birds returning north for their breeding season at this time of year, this presents a problem: head winds impede their progress and – if strong enough – could mean no progress at all. Fortunately, birds have a number of tactical ploys that they can use to lessen the adverse effects of this situation...

1) Wait for a window.

As part of the normal variations in weather, the prevailing winds can change. Periodic “equalization events” along the inter-tropical convergence zone can result in low pressure areas forming over the Sahara Desert and, coupled with this, a shift in position of the Azores High. For periods of several days, winds over the whole region swing round and give the birds more favourable conditions to gain ground.

2) Staging:

This is the tactic adopted by many larger migrants, including adult ospreys. On previous migrations, they have identified “refuelling stops” along their route, where food can be obtained and safe roosting sites found. Many individual birds use the same locations on every annual return journey, meaning that they can replenish flying energy and push forward on the next leg, even if winds are contrary.

3) Use the terrain:

Local geographical features can give favourable 
conditions, even when the prevailing winds are against. These include wind-shadow areas behind north-south hills and along coasts. “Wind funnels” can be formed at mountain passes (see diagram). A phenomenon called “adiabatic cooling” causes local air movements as a result of pressure differences down or across slopes. Birds seem to have an instinctive awareness of these processes and are often seen to exploit them.

4) Make an early start:

In maritime climate zones, the prevailing winds are often at their easiest just before and after dawn. A migrating bird can cover good distances at this time, perhaps resting instead during the later part of the day when the wind tends to be stronger.

5) Situational awareness:
"New Arrivals" by Martin Ridley

Many species of birds migrate in small flocks or extended family groups. It has been shown that skeins of geese keep an eye on the progress of other groups nearby, and will change altitude or direction if the birds ahead or to one side of their own group are getting a benefit.

6) Fly low, fly slow:

Unlike ourselves, birds have no concept of “average speeds” that have to be maintained during a flight. Their main flight parameter is energy usage, and this means that flying fast into a headwind makes no sense: it is not an energy-efficient way to travel. Keeping the speed down means that they get the same aerodynamic lift, and use the same amount of energy but cover less distance overall. This is not a problem in itself. It is better for a bird to arrive three days late at its destination than to be “on time” - but so knackered that it is unable to defend its breeding territory from others.

Wind speeds are generally lower close to the ground, so sticking to a low altitude in a headwind will make the best of a bad job.

Putting all these strategies together means that migrants can still get north and east towards their breeding sites – even when the weather charts say they ought to be struggling to do so.

1 comment:

  1. Due westerly gale here today (NW France) so not very encouraging for the migrating osprey loitering in mid-France waiting for a kinder wind.