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Migrations from the Other Shore: The migratory behavior of birds

Migrations from the Other Shore: The migratory behavior of birds


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By Ornithological Collective Black Stork

The migratory behavior of birds opens up a wide field of study and research, in which many questions remain to be resolved.

The Strait of Gibraltar is a fundamental strategic point in the migration of the birds that inhabit the Upper Paleartic, since together with the Bosphorus Strait they become the points that are closest to the European and African continent.

Birds, especially large gliding species, need, in their seasonal journey, to change continents to one of these two places, since flying above the ocean involves a huge effort, because the sea absorbs all solar radiation and does not the so-called thermal currents are formed, currents of hot air that rise from the surface of the earth, thanks to the heat of the sun, and that rise slowly in a spiral; Thermal currents that are essential for the flight of large birds, which somehow rely on this rising hot air and that supports them in flight and avoids much of their effort.

Perfectly aware of these circumstances, expert ornithologists and fans are located in the vicinity of the Strait of Gibraltar, waiting for the birds to approach in their transcontinental movement. As a result of this increasingly popular influx of ornithologists, the data provided by these observations are increasingly varied and complete, especially in the post-nuptial migration period, that is, in the journey that the birds make, once the period is over. breeding, between July and October, according to each species, in a North-South direction, that is, from Europe to Africa.

The task of ornithologists in this period is, basically, to find a suitable observation point, and to locate and follow the birds until they get lost over the sea in the direction of Morocco. And this is the scope of the work with such conditions, to date, we have not found any work that provides other data: the birds are lost towards the South, but, empirically, we cannot know what happens next; what geographical points do birds look for in Morocco, what impact on individuals this litmus test caused by crossing over the sea causes, how long does it take to cross, what is the behavior of the birds when they reach the African shore.

To try to answer these and other questions, the Cigüeña Negra Ornithological Collective has been conducting field observations in African territory for two years, within the project called "Migration from the Other Shore".

"The Migration from the Other Shore", has an initial execution period of five years, between 1999 and 2004, and has as fundamental objectives: Find areas conducive to ornithological research in Moroccan territory, calculate time and speeds of bird crossing gliders, to study the influence of the weather conditions of the Strait of Gibraltar on the migration of birds, to determine the mortality due to fatigue of birds when crossing the Strait of Gibraltar.

For this, an important economic and human deployment is being carried out, since during the fifteen days in which the field work is carried out, work teams are placed simultaneously on both banks of the strait, and we can define a work methodology depending on the workplace. Thus, the volunteers who work on the Spanish shore, among other tasks, basically go to locate and count the flocks of birds, study their evolution and start of crossing, once the crossing has started, communicate data to the Moroccan team, and the volunteers in the Moroccan shore, they will locate the flocks of birds during the crossing, follow the birds and write down in detail: exact time and place of arrival, count, height, study the evolutions until their loss in the south. Thanks to the communication by radio transmitter and coordination between both shores, a very interesting traceability is obtained in the crossing dynamics of birds of prey and mainly storks.

The dates chosen for the field work in the years 1999 and 2000 was the first fortnight of August, a period in which the White Stork (Ciconia ciconia) and the Black Kite (Milvus migrans) are the most numerous and easy to observe species. , since they move in large groups, and their location is therefore more affordable.

Finally and to define the work context, highlight the enormous influence of meteorological conditions on the migration phenomenon, since the strong prevailing winds in the area determine the behavior of the birds and their crossing phenomenology.

Field work

The working day has a very defined approach, it is going to try to locate and follow the largest number of birds that cross the strait on that day. To do this, it is necessary to take into account the prevailing meteorological conditions, since the lateral wind makes it possible for the birds to reach the Moroccan coast on a coastline of 50 kilometers.

Thus, at dawn, in addition to having the weather forecast, a preliminary examination is carried out, since the area of ​​the Strait of Gibraltar is a very unstable place in its climatology, once evaluated, the position / s to be decided are decided. they will cover. In the event that the prevailing wind is west, more eastern positions will be covered, close to Punta Cyres and Ceuta; In the event that the wind, of any component, is null or scarce (less than 5 m / s), the coast south of Tarifa, around the Ksar El-Sghir valley, will be covered, and if the wind is from the east, the observers are placed in the western zone, near the city of Tangier.

The places chosen as observatories are points where, in addition to being located near the coastline, they have good visibility, both to the North, to the East and West, which allows to see at least 8 kilometers of coastline on both senses, it is also convenient that there is not an excessively high hill behind to be able to follow the birds on their itinerary to the south. So far, six different observatories have been used, along 45 kilometers of coastline.

The observatory will be busy throughout the day without interruption, usually between 7:00 a.m. and 4:30 p.m. The large gliders take advantage of the thermal currents that form in the Iberian Peninsula during the morning and noon to gain height and approach the strait, which means that the greatest influx of birds is between 11:00 a.m. and 2:00 p.m., however on August 9, 1999, a band of White Storks arrived on the African shore at 9:22 a.m. and on 10 / Aug / 99, another band of white storks arrived at the coast at 17:07 pm The earliest observation of storks arriving in Africa in the year 2000 was on 12 / Aug / 00 at 9:53 hs and the latest on August 10, 00 at 4:14 p.m. We have been able to verify how the Black Kites arrive in Africa from dawn until late afternoon, with the sun above the horizon line.

The handicap to overcome in the field work is the meteorological conditions, which are very unstable in the area, the almost permanent clouds and fogs prevent good visibility, and the winds can reach a speed of up to 30 m / s. The species best studied so far is the White Stork, due to its large number, showiness, and above all to the gregarious behavior that it manifests in its migratory phenomenon, being able to concentrate groups in the area of ​​the Strait of up to 9,000 individuals. In the year 1999, 20,483 storks could be observed arriving in Africa throughout 11 days of observations and in the year 2,000, 37,015 white storks in 13 observations. The largest number of individuals observed in the year 2,000, we think that it is mainly due a: a greater effort of observation (more hours and number of observers), as well as a better knowledge of the phenomenon thanks to the experience of the previous year, and greater knowledge of the study area. The notable difference in observations between one day and another is mainly due to atmospheric conditions; The crossing of this arm of the sea supposes an enormous physical effort for the birds so they take advantage of favorable conditions to cross, and when the weather worsens, they remain in the Iberian Peninsula, although some groups cross with strong winds, we have observed how the crossing decreases and it is even null with winds greater than 25 m / s, a phenomenon that occurred on 14 / Aug / 99, 7 / Aug / 00 and 8 / Aug / 00 when there was a strong eastern storm zone. Likewise, when the wind dies down, the individuals who were waiting start the crossing, causing veritable rushing of birds, as occurred on August 2, 00 (13,094 birds) and August 10, 00 (10,857 birds).

The White Storks, in case of favorable conditions, seem to have a very defined crossing route, looking for the departure and arrival valleys as close as possible longitudinally, thus, they depart from the Valley of the Sanctuary in Tarifa and choose the Oued Valley as their destination. -Al-kshar in Morocco, although with variable conditions they come out in a strip that goes from Bolonia in Tarifa, to Punta Carnero in Algeciras and arrives between Hayar El-Galuli in Punta Cyres to the Bay of Tanger. The departure and arrival points depend on the prevailing wind at that moment, because in the middle of the strait, the crosswind causes a "lateral drift", which causes the distance traveled above the water to exceed 22 kilometers on many occasions. With good visibility conditions, it has been possible to follow some groups throughout the tour, simultaneously from both banks, being able to trace their route almost exactly, while timing it, thanks to the coordination between the different groups of observers, thus we have learned that storks rowing over the sea between 12 and 20 minutes, a distance of between 16 and 25 kilometers. By continuously following some of these groups we have been able to calculate that storks cross the Strait of Gibraltar at an average speed of 96 km / h. A very high speed for some birds accustomed to cycling and gliding, which gives us an idea of ​​the enormous trial by fire that this journey represents for the species, many specimens arrive with obvious signs of fatigue, with their legs lowered and their beaks open. Although we have not been able to verify it with specific individuals, we believe that some specimen fails to pass this test and may perish at sea.

The height at which they cross is highly variable, normally during the morning and at noon they start from lower heights, although in the afternoon the starting height is much higher. During their maritime journey they lose altitude, some groups reaching the African coast a few meters from the ground, so the first action when they reach land again is to catch a thermal current to increase their height and thus overcome the hills and continue traveling south. In large groups it is easy to identify the strongest and most experienced individuals since they can arrive up to 4 minutes before other congeners that leave at the same time. At the same time, the lateral drift of the last individuals is much higher and the height lower. We have been able to observe groups of storks that when reaching Africa cover a distance of up to 4 kilometers on the coastline, with the group very stretched out, presenting a snake effect.

Ornithological Collective Black Stork Ornithological Station of TarifaCrt. N 340 Km 78,511380 Tarifa
e-mail: [email protected]: http://www.tarifainfo.com/cocn


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