For 7 days, between February 18 and 24, 2002, very strong winds (northwesterly to north-northwesterly) have blown over the Somontano in the province of Huesca, coinciding with the arrival of a very important number of migrating cranes in the area of the La Sotonera Reservoir. After a short stopover, there were many trials to fly north each day, but most of the cranes were forced to give up, due to the strong wind and the impossibility to cross the Pyrenees. Thus, they accumulated in the area, with particularly poor staying and flying conditions. Besides, food resources were diminished and were concentrated in small areas of the plateau north of the Sotonera depression.

Together with some members of our ornithologic society, the Ligue pour la Protection des Oiseaux - LPO Aquitaine (BirdLife France - délégation Aquitaine), who had travelled to Aragon for birdwatching, we were able to see the consecuences on large birds produced by the very-high-tension power line installed in the area.

The following people have also taken part in the observations: : Aline Bonzoumet - Marie-Françoise Canevet - Françoise Gérardin - Robert Guélin - Jacqueline Guiraud - Marie-Claude Julié - Huguette Laveissière - Daniel et Huguette Rodriguez.
 
 

Detail of dead birds recovered after crashing against the wires of high-tension power line REE 155. Period between February 22 and 26,2002.

RESULTs: 

53 cranes retrieved dead, of which 12 were juveniles (22,6 %), plus 1 Griffon vulture, along about 1350 m of power line and in a time lapse of just 6 days.

2 wounded immobile cranes observed for a few consecutive days on the reservoir shore died in the same place.

1 crane with a chopped and hanging left wing seen on the 26^th but not found afterwards.

3 recently wounded cranes (at least) were seen in the area with a limp leg or a chopped tarsus.

An important proportion (not counted) of the dead cranes had one or more wounds in legs and/or wings: fractures or sections of these limbs. No electrocution marks were noticed.

None of the birds were ringed.

(English version by DGa)

The Sotonera reservoir (Huesca) Aragón - Spain

Cranes and other bird species collisions against the high-tension power line REE 155, and consequences of the recent increase in the number of visitors to the area

The meaning of La Sotonera area for crane Grus grus migrations. 
The Sotonera (also called Tormos) reservoir, some 20km west-northwest of Huesca (Aragón) and some 160km north-northeast of Gallocanta, is a big artificial lake with high variations of its water level. In relation to birds, it shows a lot of waterfowl during winter, but its most important role is related with the whole population of cranes migrating through the western European route. During spring migration, after a stopover in Gallocanta lake, it is the last resting area before crossing the Pyrenean mountains, which are the most difficult step in the whole route from southwestern Spain to the breeding areas in northern Europe (see sketch map).

According to the observations made during the last years, probably a total of 80000 to 100.000 cranes at least (i.e. 60-74% of the whole western flyway population) will stopin this place, with simultaneous gatherings above 35.000 individuals (for instance on 19-20th February, 2002). The highest occupation period is between 15th February and 7th March. In autumn, the occupation is much less important, due to the much larger migratory front.

The area of Sotonera is included in the IBA Important Bird Areas inventory.

 

Phenology of the cranes in La Sotonera area. 
Cranes roost or use as resting areas during daytime some areas with scattered trees (Tamarix sp.) in the northern and northeastern flat edges of the reservoir. According to food availability (before departing to France) or during the afternoon (after migratory arrival) they disperse on the cultivated areas north of the reservoir (La Loma - El Cordero). Many of them will continue northwards to reach some corn stubbles (Mezquita - La Lavanera) (see map of space use).

Late in the afternoon the cranes come back to the borders of La Sotonera. If weather conditions are suitable, most of the cranes will continue their migratory journey on the next day, after mid-morning. A bad weather period can force them to stay several days. It is also possible that they depart on migration but have to return back due to bad weather and cloudy skies over the Pyrenees, after a hard effort of 3-4 hours flying, frequently with strong winds. When the cranes come from northeast (as they allways do when they find westerly or northwesterly winds), some flocks may try to stop in the surroundings of the small Valdabra reservoir, close to Huesca city. The number of birds gathered such way may be over 3.000 individuals. But this is not a quiet area, and they are frequently disturbed.

Finally, bad weather periods increase vulnerability of the already tired birds in the whole Sotonera area.


Elements specially dangerous for the cranes in the area. 
1- Electric powerlines. The high-tension powerline (ref. REE 155 2 SBN-VNV2) crossing the Sotonera basin to the north means a very high risk of collision for the cranes as well as for other big sized species:

- the poles are very high;
- the hanging cables between consecutive poles are very deep and irregular in high;
- the situation of the powerline along the northern slope of the basin, and then along the highest part is very difficult to see for the crane flocks as they fly up from the lower part of the Sotonera basin.

In spite of an additional effort to fly up when the cranes see the cables, they can hardly surpass the six big transport cables, but the higher ground cable, much thiner and almost invisible is responsible of most of the collisions (see pictures). When the cranes do not die immediatelly because of the collision, the most important wounds are in the legs and wings, which are extended in a last effort to avoid collision.

A very strong wind or a reduced visibility will increase very much the risk of collision.

Finally, there is a medium-tension powerline (ref. ERZ) in the vicinity, in the higher part of the basin, almost perpendicular to the high-tension powerline (see the pictures). It is also a potential danger of collision for the birds.


2- Human disturbances. The number of people comming “to see the cranes” is increasing from year to year. Nevertheless, without any previous information, and ignoring the ecological requirements of the species and the meaning of the area in the course of the migration, the “visitors” try to approach the birds. The cranes, very shy, fly at very long distances, and on weekends the disturbances continue even by night. The cranes are more and more instables and spend a lot of energy, not being allowed to rest and forage. In dry years, the low level of the water in the reservoir increase the possibilities to reach any resting area in the edges or the roosts, and the frequency of disturbances is also increased (February 1999, February 2002). There are also indirect consequences of disturbances: the more the cranes are forced to fly the more the probability of collision risks with powerlines.



Identifying the sections of the high-tension powerline most dangerous for the birds
MMany local flights along the daytime and most of departures on migration force the birds to cross the powerline (see the map of space use).

In the map of the powerline, we have numbered from 1 to 10 and marked in red colour the most dangerous sections for birds. On both sides, there are yellow marked sections represention unknown potential risks. In this year (February 2002), all the 53 dead birds were foud under sectors 1 to 5, which is a line about 1.350 m long. Sections n° 6 to 10 were only examined from a distance with binoculars.

In February 1999, we found by chance 2 dead cranes under section n° 6 and 3 Griffon vultures under the sections 1 to 4. In that year we observed the collision of a flock of cranes with the higher ground cable in section n° 4, with clear weather and almost no fog (see the firsrt two pictures).



Conclusion
The area of La Sotonera, is very important for the European crane population, and should be really protected. It seems necessary that this important step before crossing the Pyrenees would perform its important role.

The high-tension powerline REE 155, very dangerous for birds would need to be signalled on the cables by means of plastic spirals, particularly close to each other in the higher ground cable. In France, this kind of visual signalling (design A.M.B.E.) has shown to be effective and produced a drmatic decrease in the number of accidents in the equiped lines.

The medium-tension powerline sould be buried on the upper side of the basin.

On the other hand, some status of efective protection of the area coul be suported by a large informative campaign to the public, both on the site (information center, leaflets, informative posters at the entrance of the tracks...) and out of it (press, television...). During the staying period of the cranes, the access could be restricted to the public (apart from farming activities) in an area including the foraging grounds of the cranes and specially the resting areas and roosts (border of the lake from northeast to northwest). Some hidden observatories placed on high places easy to access could compensate the visitors, allowing them to enjoy the wonderfull spectacle of mass gatherings and migratory flights without disturbing the cranes.

The area of the Valdabra reservoir (Huesca) frequently disturbed by people would deserve special attention. Again this year (February 20th, 2002), we observed the regrettable spectacle of people sistematically fighting the cranes (more than 3.000 individuals) and other waterfowl from vehicles: cars and motor bikes.

Pierre PETIT          
Mayo de 2002