Basin Notes - December 2001

It's All in the Diet

Jim is going to do a marathon. He has been in training, exercising and practising, starting at a few miles and slowly building up to many miles. He's also paying attention to his diet and has doubled his weight!

If this was your next door neighbour he would not get off the stating line, let alone complete 26 miles 385yards. But if he was a bird, feasting on the swarming flies on the arctic tundra then he would be in prime shape to complete the trip to Montrose Basin.

Birds have a different metabolism that allows them to build up fat to an amount that would kill our human marathon runner, Jim. Clearly embarking on a trans-arctic flight without stopping demands drastic preparations, but is there not any alternative?

For many ducks, geese and waders that winter in the UK, the flight from Canada, Greenland, Iceland or Russia is mainly over the sea, so the opportunities for refuelling stops are few. Even over land, suitable feeding and roosting sites are being built on by man, making Montrose Basin a precious remnant of the once more numerous estuarine sites that were available in the past.

However, it should be obvious that doubling its body weight will make the act of flying much harder for the bird and it would use up more energy and hence reduce its fat store more quickly. Catch 22? No. Recent research has found that the bird can increase the efficiency of its giant flight muscles to cope with the increased load. This change in the structure of its muscles does mean, however, that it is not as agile a flyer as it is during its summer slimness. So it is slightly less able to avoid avian predators but once on the migration flight it can cruise along using its stored fat load very efficiently.

The experiments showed that waders would use up about 80g of their body weight in covering a 4,000 km leg on migration. These birds had increased their weight from 110g to 190g prior to their flight, so they need to feed rapidly to put on weight for the next leg of the journey.

Those migrants like the pink-footed geese, which have been providing glorious displays at dawn and dusk as they flight in and out of the Basin, have another trick up their sleeves. Those beautifully choreographed vee-shaped flight patterns are actually a method of reducing the energy expended in flying. The lead bird has to fly normally, but the bird following is helped along by the vortex formed in the air by the wings of the bird in front. To get maximum assistance, the second bird has to be behind and to one side of the leader, which gives rise to the characteristic vee formation.

The bird at the very end of the vee is at a slight disadvantage as it hard to keep station so, while it does not need to fly as hard as the lead bird it is worse off than the birds ahead of it in the vee. The gently changing formations that we see is the result of the lead bird wanting a rest and another bird taking its place at the head of the vee. So overall, the birds in the vee use less energy than those who fly alone.

Birds have been migrating for thousands of years so it hardly surprising that they are well adapted to the rigours of the journey. We are only just beginning to understand the nature of these adaptations, but we can still enjoy the spectacle that our winter visitors provide.