1   BIRD MIGRATION

The spring migration takes place between mid-FEB and the end of MAY, the most active part being in APR and MAY. This migration is more dispersed and less spectacular than the autumn one. The principal direction over Belgium is NE.

The autumn migration lasts for more than three months. The first flight movements can already be seen from the end of JUL, while the last movements of any significance are completed by the end of NOV. They have a typical stream of movement with periods of relative calm interspersed with periods of very intense migration. Between mid-AUG and mid-NOV migratory birds are particularly active. The movement takes place in a mostly SW direction.

When on actual migration, most birds make long flights at often high levels in contrast to their flying at relative low altitudes during their stay in the concentration areas.

Although a great part of the bird population that is dangerous to aircraft, start migrating from concentration areas, according to radar observations migration often appears to take place over a broad front, covering nearly the entire Belgian territory.

As a result of radar observations it is known that mass migration takes place when

• tail winds are not exceeding 5 M/SEC;
• the barometric pressure is above 1020 HPA;
• the temperature is at least 2 degrees below the average in autumn or above the average in spring;
• clouds are covering less than 4 octas.

This can be completely in contrast to what will be observed visually.

Heavy night migration may occur early in autumn (from mid-AUG onwards) and later in spring (till the end of MAY). During the winter months sudden snow and frost may stimulate very large numbers of water- and open land birds (geese, ducks, waders, starlings, thrushes and larks) to move to the south.

The bird strike risk resulting from these winter flights (during a period of sudden thaw, in opposite direction) is particulary high in the northern part of the country.

2   CONCENTRATIONS

As elsewhere in the world, headlands, inland waters and shallow estuaries attract flocks of birds for breeding, roosting and feeding at various times of the year. Within 10 NM or so of such locations concentrations of birds flying mostly below 1 000 FT may be encountered.

In order to lessen the risk of bird strikes, pilots of low flying aircraft should, whenever possible, avoid flying at less than 1 000 FT above surface level over areas where birds are likely to concentrate. Where it is necessary to fly lower than this, pilots should bear in mind that the risk of bird strike increases with speed (it is a fact that birds rarely hit an object moving slower than 80 KT).

Apart from endangering aircraft by flying close to bird colonies, the breeding of the birds may be upset and the practice should be avoided on conservation grounds. It should also be appreciated that, especially in the case of sea bird colonies, concentrations of birds may be soaring on lee waves downwind of the areas where they breed.

3   AREAS WITH SENSITIVE FAUNA

4   MILITARY BIRD MIGRATION OBSERVATION SYSTEM

Military training and flight operations usually take place at low altitudes, where a lot of birds are present, especially near coasts and during migration periods. To prevent bird strikes, the Aviation Safety Directorate (ASD) of Belgian Defence has put in place a warning system in cooperation with national and/or international agencies.