By Dr Colin Walker BSc, BVSc, MRCVS, MACVSc (Avian health)
Pigeons do not have sweat glands and so cannot lose body heat through sweating. In birds, body heat is lost by evaporative cooling from the air sacs. When birds become hot, they start to pant. This increases air flow over the air sacs, in the process increasing evaporative loss, leading to cooling. Lost moisture is replaced by drinking. In this way, a normal body temperature and normal level of hydration are maintained.
What we do know is that at 25C, if deprived of water, pigeons become 5% dehydrated in only 24 hours. To put this in perspective, birds that are 5% dehydrated become quiet and fluffed up and will readily drink if water is provided. When 10% dehydrated, these symptoms become more exaggerated. When 15% dehydrated, there has been such a reduction in the circulating blood volume that various organs are inadequately perfused with blood to function normally and there is interference with the level of consciousness. Over 15% dehydrated, deaths start to occur.
We also know, importantly, that at 25C no dehydration will occur if birds are provided with water.
On races conducted on days over 25C, it can be assumed that unless birds drink along the way, they will arrive at their lofts suffering a degree of dehydration. Upon return, in health, a bird will drink readily, in the process replacing lost fluid reserves and a normal hydration status quickly reestablishes.
Problems arise in a number of situations.
High temperature – Temperatures over 25C require larger amounts of fluids to be evaporated from the air sacs to maintain normal body temperature, leading to a quicker onset of dehydration unless drinking occurs.
Either very high or very low humidity – Low humidity has a similar effect on the air sacs as high temperature. Low humidity tends to more readily dry the air sacs, leading to a more rapid fluid loss. Conversely, high humidity decreases evaporative cooling, leading to hyperthermia (high body temperature).
Head winds – The extra exertion involved in flying in to a head wind leads to an increased level of carbon dioxide in the blood stream, which increases respiratory rate. This increases fluid loss from the air sacs and also predisposes the birds to dehydration.
Races conducted in areas where surface water is not available to competing birds – Normal hydration can be maintained under quite extreme levels of temperature and humidity provided the birds have access to water. Lack of surface water means the birds have no way of replacing lost fluid reserves if they become dehydrated.
The combination of hot winds that are also dry and face the birds, with no access to water is a deadly one for pigeons. With exertion and heat, large amounts of fluid are lost from the system through evaporative cooling, leading to dehydration. As fluid reserves become depleted, evaporation from the air sacs can no longer occur. This means that the pigeon has lost its only method of cooling itself and so secondary hyperthermia starts to occur. As dehydration levels approach 15%, normal metabolic systems are disrupted and a decreased level of awareness occurs. In this situation, heavy losses can be expected.
It is worth noting that even on cool days the temperature within a race unit can quickly reach 25??q C because of the heat that pigeons generate (normal body temperature is 41.7??q C). This means that birds, if not offered water from the start of basketing until arrival at a race point the next morning, will already be suffering some degree of dehydration. The importance of insuring that birds have access to water whenever possible so they are fully hydrated at release cannot be overstated.