Blog # 279 What’s it like to be an Adult Osprey Part 1: Mombrella

In May most Osprey mothers are sitting tight to the nest incubating their clutches and hardly visible to observers. By June the opposite is true. Mothers often stand on their nests in plain sight looking like she’s about to fly with their wings held partly out and relaxed downward. This “mobrella” pose shades their nestlings like an umbrella from the heat and UV rays. Ophelia will hold a mombrella pose for an entire sunny day, 9am 5pm+. For the last few weeks, we’ve had a stretch of dry weather in the 80-90°F, requiring Ophelia to stand on constant mombrella duty over her panting young. 

When the young are small, they fit neatly under their mother’s wings, but the larger the nestlings, the harder they are to keep shaded. Four-week-old nestlings are now too large to fit underneath Ophelia’s wings, but they huddle as much as they can in her shadow to stay out of the burning sun. Instead of sleeping all day as they did a few weeks ago, the nest mates now spend more time restlessly moving about the nest,  rearranging nest materials and playing house, preening, and stretching their awkward wings and legs. Much of their awake time is spent panting or gular fluttering to keep cool and looking over the side of the nest, and, yes, defecating. View footage of the nestling from the Salt Point Osprey Nest Cam.

Ophelia stands with wings drooping in her Mombrella pose, creating a shadow to cool her rather large offspring in their 4th week, Lucky (L) and Hope (R). Salt Point Osprey Nest Cam screenshot.

Standing like this circulates air across Ophelia’s body and all over her wings,  

cooling her off while her body shades her offspring, still small enough (3 weeks old) in this image to fit under their mother’s wings. Salt Point Osprey Nest Cam screenshot. 

How can Ophelia, with her dark chocolate back and wings, tolerate the scorching sun all day? Since dark colors absorb heat, one might think her deep brown feathers cause overheating. It turns out it’s just the opposite—dark feathers help with cooling. The heat absorbed by dark feathers is concentrated on the feather surface where the slightest breeze can blow it away. In fact, breezes circulates air across Ophelia’s body and all over her wings, diffusing 95% of the heat away from the surface of her dark feathers by convection, making intense sunlight tolerable. The plumage color, as well as the microstructure and micro-optical properties of feathers, combine with environmental properties, such as wind temperature and speed, to determine the small amount of sunlight Ophelia absorbs. 

Flying also cools off the Osprey as wind passes all over his body,  

courtesy of Karel and Cindy Sedlacek. 

By contrast, the Osprey’s light-colored feathers on its underside trap heat closer to the  skin, where it’s harder for cooling breezes to reach. In a breeze, the skin of an Osprey  stays much cooler than the skin of a white-feathered gull, tern, or egret. Only dark birds  like osprey, eagles, turkey vultures, and cormorants are active on the lake in the hottest  sun. 

Flying and bathing are other means the Osprey have to instantly cool off. Occasionally  during the day, Ophelia may leave the nestlings alone for one or two minutes at a time  to take a quick cooling flight, dip in the lake, or just to drag her feet in the water. She  takes a calculated risk in doing so as this exposes the young to possible predation. However, Orpheus is usually nearby when Ophelia takes a break, keeping guard from a  shady cottonwood. 

I learned the principles of heat transfer fifty years ago when living with a Tuareg tribe in  the Sahara Desert and experiencing seeming endless days of beating sun with  temperatures up to 115 °F. These people were known as “les Hommes Bleu” for the bluish color of their skin imparted by the dark indigo-dyed robes and turbans they wore.  Over these dark robes, thick woolen capes were added to keep cool in the day and  warm at night. Steady desert breezes blowing across and within these flowing garments  take the heat away by convection, the same way the microclimate on an Osprey’s dark  feathers keeps it cool.

L-R: Dark-robed Tuareg man leading camels in Spanish Sahara,  

turbaned les hommes bleu in typical blue-black garments.  

The sunglasses were a present. 

Birds have no mechanism to perspire, requiring an alternate means of thermoregulating  in hot weather. Male Ospreys, like Orpheus, seek the cooler microclimates of the trees while Ophelia and other mothers must stay in the open with her nestlings in the nest.  Ospreys pant to cool off, but not in the mammalian panting style involving the tongue.  It’s a panting unique to birds and reptiles called gular fluttering, which loses less water  than mammalian panting. 

When the Osprey opens its mouth to “flutter” its neck muscles it loses body heat in the  process. Panting avian style increases air movement over the bird’s air sacs amplifying  moisture evaporation and the loss of body heat. To do this, Ospreys must stay  hydrated: they get their fluids from the fresh fish they consume. Their body  temperatures are maintained around 104 °F. At higher temperatures, proteins carrying  critical information to vital organs denature, killing the bird. 

Ophelia and a panting Lucky. 

Like the Tuareg, Ospreys can tolerate the heat of the direct sun owing to a suite of  physiological and behavioral adaptations which enhance their convection abilities—dark  plumage, insulating feathers, gular fluttering, flying, resting during the hottest hours, and  bathing. 

Eyes to the sky!  

Candace  

Candace E. Cornell  

Friends of Salt Point  

Lansing, NY  

cec222@gmail.com

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Ospreys of Salt Point 

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