The spring migration is stimulated by temperature and moisture, with almost all activity at night. The busiest time is just after it has got dark. Toads will migrate if the ground is dry, but greater numbers are observed if the grass is damp through either dew or rain. The largest numbers of toads are found on mild evenings when it is pouring with rain. Due to the extremes of the British climate toad numbers vary greatly from one night to the next.
A review of the facts and figures from previous years clearly shows the importance of air temperature at dusk in relation to the number of toads on the move. The graph below shows the maximum number of toads collected on a single night, but please note that when the dusk temperature is below 6.5C there are regular evenings when no toads are found.
Of course many other factors such as dampness affect numbers. Often, most numbers are found at dusk or shortly after, particularly in the early stages of the migration.
In our early years we encountered a small number of moribund or sick-looking toads at the Madingley toad crossing. In all cases these toads had red areas of skin, and in the more pronounced cases small pieces of grit adhering to their undersides. Believing these toads to be diseased we isolated them in moist containers. In almost all cases the toads died within a few hours.
During the Spring 1996 migration there appeared to be a heavier than usual residue of salts on the roads. Due to heavy trafficking this was concentrated in approximately 60cm wide bands along either side of the road. At the same time we started to find increasing numbers of ‘sticky red toads’. It was soon apparent that all of these toads had been in contact with the road surface itself and therefore exposed to the salt.
Symptoms varied depending upon the degree of exposure to salt with the least affected toads having a slight pink discoloration to their undersides. In contrast, the worst cases often had substantial areas of markedly red skin, sometimes covering much of their bodies. In addition a sticky skin secretion was noticeable, to which pieces of grit up to a few millimetres in length were adhered. Heavily contaminated individuals clearly looked sick and lethargic and in some cases it was difficult to tell whether they were dead or alive.
In one instance we observed a toad walk off the grass verge and onto the road surface, where judging by its behaviour, it immediately encountered salt. Its gait became more rapid and it started turning back towards the verge, demonstrating an almost instantaneous avoidance reaction. On another occasion a toad had managed to cross a less heavily salted section of road, but by the time it had reached the far side its movement was more hurried and it appeared to be stumbling over its front legs and hitting its chin on the ground. Other individuals were found completely moribund and very close to death in the middle of the road. Judging by the frequency with which we re-patrolled each section of road, animals could be this severely affected in less than 10-15 minutes. In many cases toads were gaining access to roads where kerbing was in situ and presumably they were unable to retrace their steps once encountering the salt. Our observations suggest that on more heavily salted sections of roads, toads have very little chance of crossing successfully. They either avoid crossing entirely or by attempting to cross become heavily contaminated.
The problem seems worse when the roads remain continuously dry after a salt application. After rain it appears to improve somewhat, despite there still being salt on the road. This may be because the salt particles have a greater tendency to adhere to the road surface. Sweeping of the roads by a road sweeping vehicle dramatically improves conditions, and we have in the past out of desperation attempted to manually sweep some of the worst affected areas, but this proved impractical and very time consuming.
Our observations have raised doubts over the effectiveness of amphibian tunnels during periods of heavy salting. It is inevitable that some salt must find its way into the tunnels though vents during the salt application itself and this is compounded as it is becomes scattered by vehicle tyres or washed in by rainwater run-off. As on the road surface, relatively low levels of residue inside a tunnel might harm migrating amphibians, but high levels would at best prevent them from using the tunnel, and at worst would kill them. In the light of this we now believe it to be good practice to clean tunnels as thoroughly as possible both at the start of the migration season, and also after each mid-migration salting operation.
Thankfully in Madingley, in areas where temporary fencing is in situ, we are able to prevent the majority of toads reaching the road surface, thus reducing contact with the salt. Having identified the problems, it is now our practice when the roads are heavily salted to treat rescued toads in the following way. All amphibians picked off the road surfaces are immediately (or within minutes ), washed in lake water to remove any external salt contamination. We then keep them in containers of lake water to help counteract any dehydration that may have taken place, and hopefully also to soothe their irritated skins. This definitely seems to help and increases survival rates. After being held in water for a time, individuals showing only mild pinkness on the underside are released in the lake on the same evening as they are captured. It is hoped that all of these make a full recovery. More heavily salted individuals are retained for a number of days in shallow trays of water (so that the nostrils of immobile individuals remain above water level). This proves only partially successful, some slowly recover but others die.
We are uncertain what the prognosis for even moderately contaminated animals might be without this rapid access to water. We manage to keep the total number of salt casualties quite low, but the picture would be very different if we did not have such an intensive rescue operation.