In early August I spent a week in south Florida. No, I’m not crazy. I had some indoor painting to do there, and own a perfectly functioning air conditioner. It makes one wonder what people did in the South before AC. And more importantly, how do the birds handle this heat? Instead of watching the paint dry I ventured outside to do a little summertime birding and to solve this mystery.
The August humidity in Florida is oppressive; just get used to being damp while doing anything outside, including birding. We sweat in an attempt to cool our bodies through evaporation. Remember your high school thermodynamics; water going from liquid to gaseous phases requires energy and draws heat away from your skin. But birds don’t sweat; they do not have sweat glands. Despite this they still like to stay wet in the hot weather to take advantage of evaporative cooling.
Birds have also developed other behavioral and physiologic mechanisms to deal with extreme heat. They have a much higher metabolic rate than humans and a higher baseline temperature, as high as 108F degrees for some birds. Ninety degree days, therefore are not as critical for a bird as for us humans, however extreme temperatures can be a problem. Their behavioral adjustments to the heat strike me as just common sense, like things your mother would tell you. “Stay out of the hot mid-day sun, feed and play in the early morning or evening, bathe often, and drink a lot of water.”
I understand that soaring birds soar even higher on hot days, seeking cooler air. If all else fails, the birds can always consider an earlier fall migration or relocation to habitats at higher elevation. Indeed the ranges of many birds are expanding northward as the climate warms.
Avian physiologic adjustments to heat are interesting. Birds can increase their respiratory rate and breathe with an open bill, just like a panting dog. Think of the bird’s lungs as a heat exchanger, with heat passing from the hot blood to the relatively cooler air. Some birds take this thermoregulation to a higher level, by including a rapid vibration of the moist throat to enhance evaporation. This is called “gular fluttering” and can be seen with cormorants, night hawks, and doves.
Feathers, such vital insulating structures for cooler weather, work against the bird in the hot summer. Luckily birds also have some vascular featherless body parts, (legs, feet, bills, eye rings) that can also function as cooling heat exchangers. The huge bills of the tropical Toucans are very vascular and a good example of this cooling technique. When the temperature finally falls the blood flow to these parts decreases to maintain warmth.
Vultures, as you might expect, lead the way with the most disgusting cooling method. Urohydrosis is the sophisticated term for these birds urinating on their feet and legs to foster evaporative cooling. The drying white urate salts also better reflect the sun’s rays than the darker clean legs and feet. I’m told that multi-colored birds perch with their lighter and more reflective plumage toward the sun in hot weather, but have not observed this pattern myself.
Some of these cooling methods were evident while birding the Pelican Bay berm along the mangroves and further inland at the Corkscrew Swamp Sanctuary. The birds were hiding from me and the heat, especially the passerines where only a few were active in the deep shade of the cypress forest. There certainly was no shortage of water as the ditches and ponds were all full from the daily monsoons. The Florida waders were out in force, but I only saw one shorebird; a Willet frolicking alone in the Gulf surf.
Anhinga, a juvenile night heron, and a vulture were all seen holding their wings out, away from their bodies. This is for drying and evaporative cooling, but also to keep the insulating wing feathers away from the body.
Don’t forget the birder who must also adapt to the heat. Sunscreen, hats, water bottles, etc. are obvious, but I wasn’t prepared for the severe Florida humidity. The air temperature was similar to that of the Chesapeake region this time of year, but the humidity was brutal.
Dripping sweat clouding glasses and lenses was a constant battle, but on the plus side, there were no throngs crowding the birding hotspots. I made the 3 mile loop on the Corkscrew boardwalk and saw only two other birders. There were many more guides than patrons. The Pelican Bay berm and beach were almost empty with no joggers or bikers to dodge. If you prefer to bird alone, Florida in August beckons.
August is our yearly lull on the birding calendar for more reasons than just the heat. The excitement of breeding, nesting, and feeding hatchlings is subsiding. Birds are lying low, molting, and building up reserves for a possible long fall migration. For the full time residents of Florida, both avian and human, its just a time to relax, try to stay cool, and wait for the inevitable surge from the north, soon to begin.