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Report on
Leshem, Yossi & Bahat, Ofer (1996): Flying with the birds (Hebrew), Yedioth Ahronoth & Chmed Books, Ltd.

(APUSList No. 1915) 


Chapter E  
The timing of migration, fat collecting and refueling. 

  Migratory birds pass sometimes above big areas of dry deserts or snowy mountains, where they cannot find food. They can be exposed to changeable harmful weather conditions, such as temperature under the freezing point, strong winds, rain, snow etc. In order to make their chances to succeed in the journey bigger, they have to arrange the timing of migration in the best possible way and to gather a big load of energy before the journey or during it.

The timing depends, above all, on the daylight time. But, birds use many other things to set the final timing of the departure. One of them is the weather. A rainy and stormy weather causes an early migration departure in the fall season. In other way, many birds that usually migrate to the south in the autumn stay sometimes for the winter in their nesting places when the weather is pleasant and allows them to find food.

Another factor that affects the timing is the birds' food.
Even the humans are being strongly affected by the quantity of consumed food.

The timing of migration depends on the geographical horizontal line where the bird is nesting.

Populations of a special kind, that nest in the southern areas, usually start to sit on the eggs before those from the northern areas, because of the more pleasant weather.

This is the reason why the southern birds finish the nesting earlier and migrate earlier as well.

A good example for that is the Apus apus - those who nest in Italy finish nesting at the end of July. Those who nest in north Germany finish the nesting at the beginning of August. The Common Swifts in Finland finish the nesting only at the beginning of September.

The during of migration affect the timing as well. Birds who have to migrate to distant areas have to finish the breeding before the departure. There are times that shortly after they hatched from the eggs, the chicks have to migrate with their parents.

Many kinds of birds eat huge amounts of food before going away in order to load their bodies with "fuel". Sometimes they gain 50% more to their weight before moving. The fat is located under the skin, in the two sides of the belly and on the chest. During the periods of rest the birds try to find more food to load themselves again. The weaker birds, those who lost more fat, have to rest for a longer period of time.

Researches about singing birds discovered that the amount of fat collected by a bird is adjusted to the length of the journey. Birds who have to migrate to remoter destinations have more fat in their body tissues.

Some water birds use a large amount of fat when they fly above the ocean, because of the need to burn large quantities of energy during the flight.

As bigger the bird is, as greater is the quantity of fat it gains. The fat accumulation affects the bird's migration. The researches on Kolibris in Northern America show that as much as 2 g (out of 4.5 g of the bird's weight) is fat. It allows it to fly 26 hours without resting. It flies 20 km/h so that it can fly 1000 km on its fat fuel until the Mexican Bay. Big birds like geese can fly 6000 km using their fat fuel. Storks and carnivore birds can pass thousands of kms without refueling, because of their way of flying ("surfing").

The choose of migration's route and the food in this route are very important to the ability of the birds to keep their fat fuels and to renew them. Many birds intend to stop in areas rich in food to rest and collect food to refuel their bodies. Many kinds of birds stop in Eilat on their way north because the area is a good place to find water, plants and food.

They can find it in the fields north of Eilat, where each species can find their own specific food.
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