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Migration
Overview
  • Flight performance
  • Migrating White Storks don't want to loose too much weight
  • Migration
  • This really happened
  • Ringing Storks
  • Tagging Storks with satellite transmitters

Flight performance

Storks are relative heavy birds with large, broad wings. Flapping flight, which is continuously moving the wings up and down is for such a species a very energy consuming activity. To safe energy the storks will try to cover long distances by soaring and gliding with very few wing movements.

During flight the storks will use upward air streams. These places of rising air are found where the wind blows against hills or mountains and where the air is warmed up and rises at places heated by the sun. When the higher air layers are colder the warm air will rise and thermals are formed. The ground surface is irregular with different types of soils and vegetation. Thanks to this irregular surface is the warm air rising in thermal bubbles and not in a continuous layer.


Migrating White Storks don't want to loose too much weight

Why storks use thermals can be explained when we look at the energy expenditure and the importance for the survival chances of storks to save energy. Food is scarce, especially along the migration route. That is one of the reasons why every individual stork should be very cautious with its energy expenditure. Storks store an important amount of their energy reserves in fat, that is mainly stored between the intestines. It is important for a stork to leave in time to its winter quarters and to start its migration at least at the end of the summer. The weather conditions become rapidly worse later in the season.

Even when a stork is standing still its uses energy, this basal metabolism is around 7,92 kcal/hour which means that the bird will loose 0,83 g of fat per hour. On one day a stork will use 190 kcal, which means it will loose 20 g fat per day.

Standing still doesn’t bring the stork to Africa. When a stork flaps with its wings it is using 242 kcal/hour. When a stork is soaring the stork is using only 12 kcal/hour ! This is 20 times less !!!

A stork on migration will fly about 10 hours a day and covers in Europe about 200 km per day and in Africa 300 km per day. When we take a stork of 3,4 kg we can calculate what amount of energy this bird will use during its flight.

 

Soaring flight

 

 

Active (10 hours)

Rest (14 hours)

Fat use in one day

12,6 g

11,6 g

Total one day

24,2 g fat used

 

 

 

Flapping flight

 

 

Active (10 hours)

Rest (14 hours)

Fat use in one day

253,6 g

11,6 g

Total one day

265,2 g fat used

 

 

 

 


Migration

In winter have storks difficulties to survive in northern Europe because their main food source –earthworms, insects, …- is not easy to find. The storks leave the breeding area and migrate southwards in the direction of Africa.

Some storks stay in southern Spain, other migrate all the way to South Africa, a trip of 12.000 km ! The storks on their way to Africa will try to avoid water crossing because of the lack of thermals above the water. For the same reason they will avoid large forested areas. ‘Flemish’ storks belong to the western population and cross the Mediterranean Sea at the Strait of Gibraltar, eastern birds will cross the Bosphorus in Turkey.

In the morning are the storks waiting for the sun to heat up the ground and thermals are formed. About 4 hours after sunrise are the thermals strong enough and the first birds will take off. When conditions are good than they only have to flap their wings a few times to gain height and then the warm air will do the rest.

About half an hour after take off, have the birds reached a good height and from then on they gain height and speed only by using the rising air in the thermals. They will soar to a height of about 700 m and then leave the thermal and glide in the migration direction. This way they can cover large distances with a minimum of flapping and glide to speeds of up to 70 km/h. While gliding the birds loose height and descend to about 450 m, they will again look for thermals and soar. The birds will flap their wings when they get behind and when they are afraid to loose contact with the flock or when they have to cross areas with few thermals.

Thermals are invisible, because it is only hot air. When a stork enters a thermal it is suddenly pushed up by the rising air. When the storks leave the thermal at its top, they will disperse while they are gliding. When a bird finds a new strong thermal, all birds will join in and start soaring. There is a relation between the size of the flock and the speed they can find thermals and eventually the large flocks might fly faster.

When the weather conditions are good for soaring, will the storks fly 7, 8 hours to 10 hours a day. This way they can cover a distance of up to 500 km a day. The weather conditions in Europe are most of the time not ideal and storks cover only a mean distance of about 220 km. It is much warmer in Africa and there they fly a mean distance of 290 km a day.

The White Stork is a strong flocking species, especially on migration, and individuals will react on the behaviour of other storks. This means that in a certain area all birds with the same migratory urge will take off or land at the same time. It is still uncertain why the storks always react to the behaviour of the flock but it is in most cases certainly no disadvantage for storks to fly in a flock. During autumn migration are storks migrating fast and are not feeding very often, which means there is no competition for food. They do need water and will land at ponds or lakes to drink.

Far away from the nest site is the composition of stork flocks pure coincidence. Young storks leave their nest site 1 to 2 weeks before their parents. And although the adults often start together they easily loose each other in southern Europe when the flocks are very large. Storks look at each other during the flight to find the thermals, but they don’t keep contact with calls, as geese or cranes do. During winter is food scarce and storks have to cover large distances to find enough to eat. When they have insufficient energy reserves they are so hungry that they will eat even butterflies or look for food remains on garbage dumps.

The spring migration is spread over a longer period. The birds are covering a shorter distance because the weather conditions are worse. They will return in January – April to their breeding grounds.


This really happened

 

The British missionary W.M. Thomson describes how in 1846 a weakened stork was captured in Israel. He was very surprised when he noticed that the bird was carrying a silver neck-lace and a note. In this letter the countess from the place Gotzen in Germany wrote that this stork has been breeding on the castle for many years and that in a heavy storm the nest of the breeding bird was blown away. The wounded stork was well cared for by the countess and released with the neck-lace. Unfortunately was the stork to weakened to be saved once again and it died in Israel.


Ringing Storks

In the 20th century, researchers have started to put metal rings at the stork legs. This ring is like a passport of the stork. On this ring you can find the address of the ringing centre (in Belgium is this : KBIN, Vautierstraat 29, 1040 Brussels) and a unique number which you also find on a passport.

In the research centre are all details of the stork kept in files, such as the day and place where it is ringed, age, number of young in the nest and any additional data. When somebody in Europe or Africa will find this ring and asks for information, he will be informed of all details known of this bird.

Because the numbers and letters on the old metal rings of only 1 cm high were very hard to read, 3 cm rings are in use since 1997.


Tagging Storks with satellite transmitters

The storks are equipped with 35 g solar powered satellite transmitters. The transmitter is placed on the back of the stork with nylon and teflon  ropes.
The transmitters are PTT-100s constructed by Microwave Telemetry (http://www.microwavetelemetry.com) and North Star www.northstarst.com

The biggest advantage of using satellite transmitters is that data on the behaviour and the migration route can be collected for individual birds. From a ringed individual you will only get information about the time and place where the bird is ringed and where it is recaptured or found. Because storks are very conspicuous birds there is a recovery rate of 20 to 25 %, which is high for a bird. From a tagged stork can we collect hundreds to thousands of locations and follow it from day to day for an important part of its life. From a German stork we collected thanks to the satellite in 10 months about 1.400 locations (data Vogelwarte Radolfzell). This is of course a enormous amount of information.

Every 50 seconds is the transmitter sending a signal that can be picked up by a satellite. These are NOAA-satellites that follow a polar orbit and can receive and store all information. When the satellites are in contact with the ground station they will transmit all the data. The data are then analysed in the ground station in Toulouse, France. The localisation of the stork is measured using the Doppler-effect, by comparing the received frequencies several times with the standard frequency.

Every day the transmitter can give up to 8 locations.

The locations and information of the sensors in the transmitter are send to us as follows :

14559 Date : 30.07.99 19:30:38 LC : 1  IQ : 50
Lat1 : 51.003N Lon1 : 4.483 E  Lat2 : 45.733 N Lon2 : 23.114 W
Nb mes : 006 Nb mes >-120 dB : 000  Best level : -126 dB
Pass duration : 379s  NOPC : 3
Calcul freq : 401 653408.1 Hz  Altitude : 80 m
00 06 61 01

First line : number of the transmitter, date, hour (GMT), accuracy of the location  (LC), quality of the receipt (IQ)
Second line : two locations that are calculated using Doppler effect, in this example the first location is the right one (Mechelen, Planckendael)
Third line : number of received signals, number of signals > –120dB and the best reception in dB.
Fourth line : contact period between transmitter and satellite, number of tests (how longer the contact is, the better the locations can be).
Fifth line : calculated frequency of the transmitter, altitude.
Sixth line : sensor data : temperature (coded), battery level, counter, activity counter (0 to 255)

We use the locations provided by the satellite to produce the maps with the migratory movements of the storks. With these coordinats and a GPS (Global Positioning System) is it possible to go to a certain place and observe the bird.