Blog #466 Ospreys Never Sweat

May 29, 2024

Ursula holding wings out in a “mombrella” pose, cooling herself and shading her offspring.

If birds don’t sweat, then how do they stay cool? Looking up at the Salt Point nest, I see Ursula standing attentively with wings spread out like an umbrella, hence the Facebook moniker of “mombrella.” As soon as the sun is hot, Ursula, like any good, self-sacrificing Osprey mother, strikes her mombrella pose to shade her nestlings, sustaining this throughout the day for at least a month. When not asleep, the nestlingssquirm about the nest challenging Ursula to maintain their cover.

Ursula in Mombrella pose to protect herself and the nestlings from the heat and sun.

When not asleep, the nestlings squirm about the nest challenging Ursula to maintain their cover.Warm-blooded, Ospreys must produce their own body heat instead of depending on the environment for warmth. However, osplets are unable to do so before they are two weeks old and must depend on their mother’s body for comfort. Exposed to the elements, Ursula must brood her young, sheltering them with her body.

The highly successful Osprey lineage reaches back 23-33 million years to the Oligocene without much variation in their phenotype. Their unusually flexible thermoregulation results from countless seasonal migrations exposing the raptor to a vast range of conditions and habitats, from the steaming tropics to the frozen polar regions. Even in extreme fluctuating conditions, they stay at 101 °F provided they have ample fish to fuel their high metabolisms. But it takes more than physiological responses to keep Osprey systems running hot.

Sometime in their past, Ospreys modified adapted behaviors to support and augment their biochemical responses to temperature changes. Their autumn migrations south and spring migrations north are the quintessential behavioral adaptations to keep Ospreys at their desired body temperature. Unlike Ospreys of the past, modern Ospreys can maintain a stable metabolic temperature of 101º F.

Thermal imaging shows more heat loss from the legs than bill, courtesy of Svana Rogalla.

Ospreys lose body heat passively through unfeathered areas such as the highly porous bill, long legs, and large feet. Seen with thermal imaging, their leg veins and arteries are just below the skin, creating a passive, counter-current heat exchange. Thermal images also disclosed that even though the Osprey bill is porous, it playa a very minor role in cooling their bodies, contributing only 0.3% to the bird's total heat exchange. Similar studies of the long legs and the feet with the claws distended add 6–7%.

Adult panting and gular fluttering with mouth open, Flickr.

Curiously, thermal imaging of some Ospreys indicates that at 90°F, a cold “cutaneous” stripe on the head forms, which cools by water evaporation through the skin. Clearly, all these losses are not enough to prevent over-heating; other means of cooling must be in play.

Older nestling is able to pant, and hence thermoregulate on its own, courtesy of Dyfi.

An adult Osprey bathing offshore in Cayuga Lake.

When passive heat loss fails to cool the bird, behavioral adaptations acquired over the millennia bolster their thermoregulation. An Osprey’s first response to temperatures above the comfort range is to pant by opening its beak widely while breathing heavily. Panting forces air through the bird’s respiratory tract, removing excess body heat from surface blood vessels. Concurrent cooling from the evaporating moisture is also emitted by the damp lining of the respiratory tract. Heat loss is enhanced by vibrating their moist throat tissues—gular fluttering—by opening its mouth and expanding and contracting it in quick succession. Air passing over blood vessels lining the surface of the gular area and the moist lining of the respiratory tract dissipate excess heat through convection.

Male adults are not tied to their nest as their mates are, allowing them to shelter in the trees to keep cool or warm. Adult Ospreys can quickly cool off by wing spreading (holding the wings open), allowing air to pass over the less insulated ventral or underside. When both mother and offspring are hot, the mombrella stance serves to cool them both. When heat stressed, adults often wade into the water and drink or drag their legs and feet while flying over the water. Their wet feathers probably help cool the nestlings as well. Nestlings must get all the water they require from the fish they consume. Until they can fly, the nest-bound chicks do not have these options and must depend on panting in the shade of their mombrella.

Although minor heat losses from the body add up, it is the Ospreys behavioral adaptations to heat and cold that have enabled this bird to become a world traveler, able to cross scorching deserts and freezing oceans, and brood eggs in the snow. Migrating south in the fall and returning north in the early spring is, perhaps, depends critically on their quintessential ability thermoregulate.

Eyes to the sky!

Candace

Candace E. Cornell

Cayuga Lake Osprey Network

cec222@gmail.com

A black and white drawing of an eagle

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