Sunday 5 February 2012


Vast rafts of ducks on Sierra Brava Reservoir (Cáceres) on the morning of 14 January 2012.
No, it's not a swarm of mosquitoes or a negative of the Milky Way but several thousand ducks
resting up on part of the reservoir (click to see it larger). By Javier Prieta.

The celebration of World Wetlands Day is as good a time as any to look at one of Extremadura's most outstanding waterfowl sites. Sierra Brava Reservoir lies in the southeast of the province of Cáceres in the catchment area of the River Guadiana, the whole reservoir falling within the municipality of Zorita. It forms part of the irrigation scheme called Central Zone of Extremadura, comprising several reservoirs and a vast area of cropfields, mainly maize, rice and orchards. Built in 1994, Sierra Brava did not become half full until 1998. In January 2002 it was "discovered" by the birding world. In 1995 the traditional wintering wildfowl count fell into disuse in Extremadura so there is no information on the wintering wildfowl of its early years. In January 2002, the recently created GOCE group (Grupo Ornitológico Cacereño, today SEO-Cáceres) decided to re-establish the wildfowl count in Cáceres. Their findings were a huge surprise. No one imagined such a massive concentration of ducks on one reservoir, reckoned to be about 61,000. And Sierra Brava is not alone; other smaller and larger reservoirs of the Central Zone also have huge duck flocks.

Ten years later SEO-Cáceres repeated the wintering wildfowl count in January 2012, aiming to take in the whole of Extremadura. Obviously this time Sierra Brava wasn't to come as such a big surprise. On Saturday 14 six members of SEO-Cáceres set out to census the wetlands of the Central Zone, concentrating on Sierra Brava. Anyone who knows the reservoir will realise that it is impossible to count the birds in situ (see above photo). The chosen method, partially tried out in earlier years, was a mixture of photographic and direct counting. Weather forecasts predicted mist for that day and it even rained on the way there but the morning turned out to be splendid, sunny and windless. This enabled us to make a detailed telescoping scan of the famous rafts of ducks from a nearby high point. For four hours the least numerous species were counted (Greylag Goose, Shelduck, Wigeon and Gadwall), the shores and inlet waters were checked out while also sketching out the proportions of the most abundant species. The fieldwork thus came up with no final result.

The second part of the study came back at home, counting one by one the photographed birds and also indicating species. The working material amounted to 199 photographs and several pages crammed with notes. The photos with fewer ducks were counted on the computer screen, while the more complex photos were printed out in colour. The number of birds per photo ranged from three to 2093 (mean of 252), adding up to 50,187, which turned out to be 86% of the total (58,000). Ascertaining how many corresponded to each species was not as difficult as might be thought since the duck species do tend to flock together on the reservoir. Thus, thanks to 72 field observation points, counting 9703 individual birds (c.20% of the total), one of the great rafts was seen to be formed 99.9% by Shovelers. The figures obtained for this species are reckoned to be accurate. The other big raft was formed almost exclusively by Teal at one end and Pintail at the other, merging in the centre. In most of the photos it proved possible to identify the species, so estimates were made only in doubtful cases in certain parts of the reservoir. Now it remained only to tally these findings with the field counts of scarcer species and zones not visible in the photos (the Mallards occupy above all the outer zones and shores).

Detail of Sierra Brava Reservoir (Cáceres) on 14-01-2012. A total of 1019 ducks has been counted in this image, the immense majority being Teal (Anas crecca) with some Pintail (Anas acuta) in the foreground. By Eva Palacios.

As in other posts of this blog, the idea was to check the previous information published on Sierra Brava and the Central Zone. There is little to go on but we do have the official January counts of Extremadura passed on by SEO/BirdLife to the Environment Ministry. These figures are summed up in the tables below. Unfortunately there is no indication of the methods used, the authors of the count or the dates (some counts seem to have been conducted in February), so we decided to eschew any thorough check. The trend graph shows wild swings; this came as a surprise to us since the many visits paid to Sierra Brava over the last 10 years showed no such thing. It is also surprising to find that some counts give figures of zero Teal or only 140 Shoveler, figures that hardly tally with the real situation. The total Sierra Brava figures, ranging from 11,000 to 102,000 Anatidae, coots and grebes, have been quite controversial for various reasons. At least on two occasions there were simultaneous counts: in 2002 on the same day, throwing up estimates of 45,000 in one case and 73,000 in the other (66% difference; we decided to split this difference); in 2003 the two estimates were similar (about 100,000 birds) but with a very different species breakdown. Despite this, Sierra Brava's importance is unquestionable, accounting as it does for over half of Extremadura's wintering Anatidae on most occasions. If we consider the Central Zone as a whole the area ranks third in Spain for wintering wildfowl, coming behind only Doñana and Ebro Delta.

We don't know if photographic counts were employed on earlier occasions; judging from our 2012 experience, however, it is the method we recommend. Obviously there will be some error but we believe it to be simple and objective. The main limitation might be bad weather with bad light or especially, wind, so the right days do need to be carefully chosen. Coverage also needs to be improved on shores and some of the inlet waters to detect above all Mallards, which might well have been underestimated by this current count.

In case anyone is wondering, we also tried a computer count, using Photoshop, as well as the photographic count. The method works and it proved possible to determine the number of points per photo, but the figures came out 20-30% below the direct count since 2 or more adjacent birds might merge into a single point.

The 2012 count in the Central Zone of Extremadura forms part of the Extremadura Wintering Wildfowl Count brokered by SEO-Cáceres. The most meritworthy work in this endeavour was carried out by Eva Palacios, who patiently took the photos and even more painstakingly counted up the ducks on each one. Javier Prieta completed the fieldwork and came up with the final figures for Sierra Brava. Sergio Mayordomo, Marcelino Cardalliaguet, César Clemente and Javier Mahíllo censused the rest of the Central Zone. Independently, Miguel Ángel Romo and Javier Pérez Gordillo covered the ponds of Manzanares (Campo Lugar, Cáceres). Lastly, our thanks go to Raúl Guzmán for coming up with the alternate computer method and to Blas Molina (SEO/BirdLife) for the figures from the counts of earlier years.