Some fanciers say that pigeons do not sweat, but have you ever held a pigeon on a hot day and felt dampness? Pigeons do not have sweat glands like humans, none the less cooling does take place at the skin's surface through a process called cutaneous evaporation.
Cutaneous means "relating to the skin" and cutaneous evaporation is a process by which our pigeons cool themselves by evaporating water vapor off of their skin. Like I said, pigeons do not have sweat glands that produce drops of sweat, rather, they have capillary sized channels which move moisture to the skin's surface where water vapor is released.
Not only are our birds able to cool themselves by way of cutaneous evaporation, but they are able to rapidly change the rate of evaporation at the skin's surface in response to changes in air temperature and humidity.
The primary method by which our birds cool themselves is by respiratory evaporation which takes place in the air sacs and airways. The respiratory system is a "heat pipe", with cooler dryer air being inhaled and warmer moister air being exhaled. Under ideal situations, a pigeon would be breathing in air that contains 1% - 2% moisture and exhales air that contains 5% - 6% moisture.
If a pigeon has to race in weather where the moisture content of the air is reaching the saturation point, then it will be less able to use evaporative cooling at the surfaces of the air sacs and airways (respiratory evaporation) in order to cool itself. This is because, the air being breathed in, has little capacity to hold additional moisture and therefore, the air sacs are unable to utilize respiratory evaporation in order to cool the blood, which in turn, cools the body.
During conditions where respiratory evaporation is inhibited by high moisture content in the air, the rate of cutaneous evaporation must increase at the skin's surface (just below the feathers and along the exposed skin of the legs and feet) in order to cool the bird in flight. Studies have shown that cutaneous evaporation can increase by as much as 78% to compensate for a bird's inability to fully utilize respiratory evaporative cooling.
Convection is a third method by which a pigeon may regulate its body heat. Convection's effectiveness is directly related to the difference in the air temperature and the bird's body temperature. The lower the air temperature the faster cooling can take place via convection. This cooling takes place as the cooler air passes over the feathers and skin, drawing off heat, and also as this cooler air passes through the air sacs and airways.
A pigeon in flight on a windless day moves air across itself at the rate of about 1312 ypm. At the same time, the bird is breathing approximately 480 - 600 times a minute, one breath for each wing stroke. This creates a lot of air movement which depending on the difference in temperature between the air and the racing pigeon, draws heat away from the bird and cools the skin and air sacs.
So then, how can we use weather statistics; like temperature, humidity and dewpoint tell us something about the effort our birds will need to exert in flight? Obviously, the hotter the air temperature, the less convection cooling will take place. The higher the relative humidity, the less evaporative cooling will take place. The higher the air temperature and the higher the relative humidity, the less effective evaporative and convection cooling become.
Dewpoint tell us at what air temperature, relative humidity would reach 100% (zero possible respiratory evaporative cooling). When the dewpoint is high, this means that the air temperature is high and the humidity is high and this means that both convection cooling and evaporative cooling are less effective.
If dewpoint temperature is low, say 44 degrees, this means that as the temperature approaches 44 degrees, the air would become saturated with the maximum moisture it could hold and evaporative cooling would approach zero. However, at 44 degrees, convection cooling would be very efficient as a pigeon's body temperature in flight is between 104 and 109 degrees, so a great amount of convection cooling could take place.
If dewpoint temperature is high, say 80 degrees, this means that as the temperature approaches 80 degrees, the air would become saturated with the maximum moisture it could hold, and evaporative cooling (both respiratory and cutaneous) would approach zero. At the same time, if the air temperature did reach 80 degrees, convection cooling would be much less efficient than say at 44 degrees.
As far as dewpoint readings go, the higher the dewpoint temperature, the harder the race will be for our birds. It will be hard for them to utilize evaporative cooling and it will be hard for them to utilize convection cooling. You know what overheating does to an engine, it runs much less efficiently and it may fail from heat related fatigue.
I should point out that dewpoint temperature is always less than current air temperature. That is, since warmer air has the capacity to hold more water vapor than cooler air, the current air temperature is alway greater than the dewpoint temperature. As the current air temperature reaches the "dewpoint temperature" it can no longer hold the current volume of water vapor and must shed water vapor, usually in the form of rain.
On the other hand, air which is both dry and hot, allows for maximum evaporative cooling (because the air is dry) but allows very little convection cooling (because the air is hot). This condition accelerates dehydration which then further limits the possibility of evaporative cooling and if continued leads to a shuts down of the internal organs which must use water to flush out toxic waste product, generated by the pigeon flight muscles.
At this point, you might be wondering how a pigeon can have evaporative cooling from under a covering of feathers? One might think that those feathers covering the skin would inhibit the birds from releasing the water vapor (and stored heat) associated with cutaneous evaporation. This is not the case, for several reasons; A pigeon in flight, flaps its wings about 8 - 10 times a second (or more if needed) . As it draws those outstretched wings to their full extension and then pulls them down, there is great feather displacement along the body, wing butts, and all along the wing (covering feathers, secondary feathers, and primary flight feathers). The same mechanisms are occurring along the tail feathers and the supporting covering feathers.
Feathers function somewhat like a car radiator. What I mean is that the sturcture of a feather and the structure of a radiator are alike. Feathers have a quill or hollow shaft filled with air, that functions much like the copper tubing of a car radiator and feathers have vanes extending out from the shafts that greatly increases the surface area available for cooling, much like the fins along the copper tubing of a car radiator increase cooling capacity. In this way, heat is pulled from the pigeon's body and released along the whole surface of the feather structure.
Consider the wings, they expose a great amount of surface area as air passes over and under the wings while in flight, the bone structure is hollow and feather covering s make up 90% of the wing's mass. Everyone has seen a "blood quills" and this just confirms that that major feathers of the wings are closely linked to the blood supply and are able to carry heat out of the blood supply, cooling the blood before it returns from the wing back to the breast and heart area. The birds are even able to increase the distance between the feather tips to allow greater air flow (or lesser) depending on heating needs.
This is another reason why the gap between the 8th, 9th and 10th flights can be important. The greater the gap a pigeon is able to produce, the greater the wing profile and more air passes over and under the wing feathers. The more air passing over and under the wing feathers, the greater the capacity for heat exchange. On long races this gap can be important in coooling the bird while on shorter races there is less need for efficient heat exchange along the wings so wing tip fanning or extention would be less important to short distance birds.
Consider how proportionately large the primary feathers are they are located on the outer arm (wing) of the bird and this arm is basically hollow bones, skin and blood vessels. While in flight, an extended wing has air currents moving over the forward wing edge and then aerodynamically following the feather edges to the tips of the feathers. This moving of air across the vanes of the feather produce great capacity for cooling.