Mortal salt indigestion of red crossbills

Sometimes human activity has unpredictable consequences to certain species of our mountains. Who would have thought that salt spread on roads could kill red crossbills?

During winter time, workers from “Peñalara” Visitor Centre at the National Park who drive up to Cotos Pass, frequently observe many red crossbills dead o agonizing on the road. Apparently, this is caused by an unfortunate behaviour they have, eating the salt used to melt the ice on the roads. Many of them end up run over as they are not frightened of cars, or even return to the same spot although one bird of the same flock had been knocked down right beforehand. Occasionally, the citril finch joins the red crossbill at their salty feast.

Ornithologists had described this behaviour, in the forties, and though it is not clear yet why they do that, it has been suggested that they present a sodium deficiency in their granivorous diet, which they try to compensate through eating salt (sodium chloride) used to melt the ice on the roads.

Red Crossbill run over on the road. Author: Genoveva Tenthorey

We do not really know the real importance of this mortality rate compared to the whole population at Sierra de Guadarrama, but nevertheless, it entails a paradigmatic example of unpredictable or unknown consequences to some of our actions. On another blog post we warned about chronic salinization

of Navalmedio stream due to its proximity to the road and Navacerrada urban area. Our studies demonstrated that sodium’s increase was seventy times higher than similar brooks without any winter ice melters’ influence. However, the results indicated that aquatic invertebrates, which we supposed could be affected by this water contamination, did not present alarming symptoms. But we were looking at the wrong fauna group: it is birds who are suffering from the effect of salt on roads.

Moreover, wildlife is not the only one lured by salt. Every local from Sierra de Guadarrama knows that in winter they should be extremely careful while driving, as they often encounter cattle or horses licking salt from the road, unexpectedly. It would not be the first accident for that reason, unfortunately, and it will not be the last. Pine trees on the margins of the road are also affected by salt causing the death of many of them, year after year, or, in some cases, making them vulnerable to windstorms during which they can be knocked over across the road.

It is most likely that M-604 road users, between Rascafría and Los Cotos Pass, remember some green obituaries sticked to the trunk of dead pine trees on the margins of the road saying: “Pine tree killed by salt”, as a way to complain about that situation by the landowner of the pinewoods surrounding the road, Sociedad Anónima Belga de Los Pinares de El Paular.

Salt spread on the road at Sierra de Guadarrama. Author: Genoveva Tenthorey

From Peñalara Natural Park Authority first and later on from Sierra de Guadarrama National Park’s, several proposals to mitigate the problem were offered whilst a new way of dealing with ice on roads was applied. Among which felling pine trees inside a 15-20 metres belt – on both margins of the road- was considered. These belts had to fulfill two conditions sine qua non. Firstly, the existence of an undercanopy consisting of broadleaved species, arboreal and shrubby, capable of replacing fallen pine trees; secondly, the pine canopy would cast a shadow over the road for most of the day. That proposal aimed to allow more insolation on the road, as broadleaved species would lose their leaves in winter, as well as to reduce accidents due to falling trees. It was also a fire risk prevention belt.

Another proposal was waterproofing road gutters and directing runoff water towards desalting catch basins, which entailed substantial investment and whose functioning depended, also, on the current method of administering salt: fan-shaped application with an excessive radius reaching slopes and forest’s hillsides.

It is well known that in other parts of the world, alternatives to sodium chloride have been proposed, although, for the moment, it is difficult to find a satisfactory solution. In the meantime, at the National Park, the solution to this problem might be a better coordination with people responsible for winter viability, the reduction of salt use to the minimum, testing other alternatives in certain spots with a higher level of salinization and carrying on with the assessment of the impact caused by it.

Red crossbills might just be the tip of the iceberg.