ARE RESERVOIRS GOOD OR BAD FOR BIRDS?

Sierra Brava Reservoir, Zorita, Cáceres (Ángel Sánchez)

Every month this blog publishes a list of the most notable observations of the previous month. Even the most cursory glance at one of these monthly lists shows a striking fact: the sheer number of these observations that are made in reservoirs. The percentage was particularly high last October, at about 80% of the total. As ecologists, however, we always come out strongly against any new reservoir project. Why the paradox? If reservoirs are so bad why do we spend so much time birdwatching in them? This begs a question we cannot afford to shirk.

The answer is at once very simple and very difficult, simple in principle and difficult as it actually pans out in practice. Basically, reservoirs might be good wintering and passage sites but they are very poor breeding grounds. The huge sheet of water seen from the air pulls down many migrating birds for a pit-stop, especially wildfowl and other aquatic birds like ducks, geese and waders. On winter nights these reservoirs can also be excellent roosting sites, especially for such an iconic Extremadura bird as the Crane, and we all flock to reservoirs like Rosarito and Sierra Brava to watch gobsmacked the huge incoming flocks of these magnificent birds. As breeding sites, however, reservoirs are pretty hopeless. The everchanging banks give lakeside vegetation no chance to take hold, making it very difficult or impossible for any phragmites reedbeds to build up, a sine qua non for typical wetland species to breed there. There are exceptions of course. Arrocampo, due to its particular purpose of cooling the Almaráz nuclear power plant, has to hold its water levels fairly steady. This has allowed at least a substantial reedmace bed to grow there, if not a phragmites reedbed. This has now become home to breeding species like Purple Heron and Purple Swamphen, scarce elsewhere in the region. In Rosarito Reservoir, straddling the Cáceres-Toledo border, there is now a large and stable Cormorant colony. Little Terns sometimes try to breed there, without much success. In general, however, reservoirs provide few birds with breeding territory but they wipe out huge swathes of it for others. The amount of breeding grounds that are lost by such a sudden and aggressive act as flooding a whole river valley is incalculable and irreversible.

Hence the dichotomy: reservoirs can be great wintering sites (by day and night) and migration stopover points, but at what cost to the birds that previously bred in the area? The huge rafts of ducks floating on the Sierra Brava reservoir are a staggering sight. But most of them don't even feed there, simply resting up there before winging out to feed on the surrounding rice fields. The Cranes that nowadays roost in our reservoirs have almost certainly been roosting in the general area for centuries; it's just that nowadays they are concentrated in an area that, well managed, offers them greater security. (Badly managed, reservoirs might even turn out to be harmful for them. Take the case of Rosarito. Even though it is a SPA site, fishermen are allowed to drive their cars right to the water's edge and quads and motorbikes scramble on the sand when water levels are low, precisely when the Cranes are arriving tired from their autumn odyssey and are in greatest need of rest). The rare waders that turn up on the muddy edges of our reservoirs, however interesting they might be (and we all might twitch for them as a once-in-a-lifetime event) are largely irrelevant. They could almost certainly have rested up equally well in thousands of other sites. But the Great and Little Bustards that have been driven out for good?

Neither is it all about birds. Damning rivers has dire effects for fish-life; hence Extremadura's rather poor ichtyofauna. Fish are in fact the most threatened group of vertebrates in the region. Our otters are probably also hard put to deal with all the shifting water levels and changes in the region's water courses. The biggest known heronry in Extremadura was drowned for ever beneath the waters of the Sierra Brava reservoir, now a paradoxical birdwatching Mecca.

The damage is particularly severe when a reservoir project involves a site of great importance for a threatened species, like the Monteagudo project in the River Tiétar (Ávila), which would destroy breeding territory of Imperial Eagle. If, god forbid, it should go ahead, I guess that in 10 or 20 years, like the fallible mortals we are, always looking for the quick fix, we would all wend our way there to tick off the latest rarity that turns up on spring or autumn passage. But I'm equally sure that all of us, put on the spot, would gladly swap this juicy but one-off sighting for the Imperial Eagle or even the buzzard or scores of scrub warblers that bred there in the past and have now had to look for other outlets in an ever-dwindling range of possibilities.

Dave Langlois. Villanueva de la Vera

Sierra Brava Reservoir, Zorita, Cáceres (Ángel Sánchez)

Every month this blog publishes a list of the most notable observations of the previous month. Even the most cursory glance at one of these monthly lists shows a striking fact: the sheer number of these observations that are made in reservoirs. The percentage was particularly high last October, at about 80% of the total. As ecologists, however, we always come out strongly against any new reservoir project. Why the paradox? If reservoirs are so bad why do we spend so much time birdwatching in them? This begs a question we cannot afford to shirk.

The answer is at once very simple and very difficult, simple in principle and difficult as it actually pans out in practice. Basically, reservoirs might be good wintering and passage sites but they are very poor breeding grounds. The huge sheet of water seen from the air pulls down many migrating birds for a pit-stop, especially wildfowl and other aquatic birds like ducks, geese and waders. On winter nights these reservoirs can also be excellent roosting sites, especially for such an iconic Extremadura bird as the Crane, and we all flock to reservoirs like Rosarito and Sierra Brava to watch gobsmacked the huge incoming flocks of these magnificent birds. As breeding sites, however, reservoirs are pretty hopeless. The everchanging banks give lakeside vegetation no chance to take hold, making it very difficult or impossible for any phragmites reedbeds to build up, a sine qua non for typical wetland species to breed there. There are exceptions of course. Arrocampo, due to its particular purpose of cooling the Almaráz nuclear power plant, has to hold its water levels fairly steady. This has allowed at least a substantial reedmace bed to grow there, if not a phragmites reedbed. This has now become home to breeding species like Purple Heron and Purple Swamphen, scarce elsewhere in the region. In Rosarito Reservoir, straddling the Cáceres-Toledo border, there is now a large and stable Cormorant colony. Little Terns sometimes try to breed there, without much success. In general, however, reservoirs provide few birds with breeding territory but they wipe out huge swathes of it for others. The amount of breeding grounds that are lost by such a sudden and aggressive act as flooding a whole river valley is incalculable and irreversible.

Hence the dichotomy: reservoirs can be great wintering sites (by day and night) and migration stopover points, but at what cost to the birds that previously bred in the area? The huge rafts of ducks floating on the Sierra Brava reservoir are a staggering sight. But most of them don't even feed there, simply resting up there before winging out to feed on the surrounding rice fields. The Cranes that nowadays roost in our reservoirs have almost certainly been roosting in the general area for centuries; it's just that nowadays they are concentrated in an area that, well managed, offers them greater security. (Badly managed, reservoirs might even turn out to be harmful for them. Take the case of Rosarito. Even though it is a SPA site, fishermen are allowed to drive their cars right to the water's edge and quads and motorbikes scramble on the sand when water levels are low, precisely when the Cranes are arriving tired from their autumn odyssey and are in greatest need of rest). The rare waders that turn up on the muddy edges of our reservoirs, however interesting they might be (and we all might twitch for them as a once-in-a-lifetime event) are largely irrelevant. They could almost certainly have rested up equally well in thousands of other sites. But the Great and Little Bustards that have been driven out for good?

Neither is it all about birds. Damning rivers has dire effects for fish-life; hence Extremadura's rather poor ichtyofauna. Fish are in fact the most threatened group of vertebrates in the region. Our otters are probably also hard put to deal with all the shifting water levels and changes in the region's water courses. The biggest known heronry in Extremadura was drowned for ever beneath the waters of the Sierra Brava reservoir, now a paradoxical birdwatching Mecca.

The damage is particularly severe when a reservoir project involves a site of great importance for a threatened species, like the Monteagudo project in the River Tiétar (Ávila), which would destroy breeding territory of Imperial Eagle. If, god forbid, it should go ahead, I guess that in 10 or 20 years, like the fallible mortals we are, always looking for the quick fix, we would all wend our way there to tick off the latest rarity that turns up on spring or autumn passage. But I'm equally sure that all of us, put on the spot, would gladly swap this juicy but one-off sighting for the Imperial Eagle or even the buzzard or scores of scrub warblers that bred there in the past and have now had to look for other outlets in an ever-dwindling range of possibilities.

Dave Langlois. Villanueva de la Vera