Hanover Bald Eagle Blog # 20

April 19, 2019

In partnership with Pennsylvania Game Commission   and Comcast Business .

Now that winter has drawn to a close, the temperature extremes faced by the Hanover family have shifted from cold concern to hindrance by heat. This week we have witnessed both parents and hatchlings engaged in what looks suspiciously like panting. In birds, we call this behavior “gular fluttering,” so named because the area of skin on a bird’s neck is called the gular and they will flutter this skin as a way of dissipating heat. Birds do not possess sweat glands, and therefore they engage in other physiological and behavioral methods to cool off.

Earlier in the season, we took a look at the cold hardiness of bald eagles, investigating their methods of heat retention. As we learned, bald eagles are incredibly efficient at trapping heat due to their surface area to volume ratio, as well as their incredibly insulative feathers, and behavioral adjustments such as nest placement, usage of roosts, and reduced activity levels.

During the summer when temperatures are higher, one would assume these same physiological and behavioral tools might make it difficult for bald eagles to handle the heat. However, once again, their adaptive nature comes to the rescue!

Birds and mammals are endotherms, meaning they produce their own body heat, in contrast to fish, amphibians, reptiles and insects, all of which are referred to as ectotherms, meaning they gain and lose heat through the environment (think of a lizard or snake sunning itself on a warm rock before nightfall, soaking up every bit of heat possible before sunset).

Most raptors have a core temperature of 40 °C (104-105°F). The degree of adjustment needed for raptors to maintain their optimal core temperature varies based on factors such as their size and activity level. Small raptors maintain a slightly higher temperature than larger raptors, due to their higher metabolism. Research shows that these “high- flight-speed” raptors, such as falcons, have a higher resting metabolic rate compared to “low-flight-speed” raptors such as red-tailed hawks and vultures. Adding to this difference is the amazing physiological reality that the hearts and flight muscles of smaller fast-paced raptors are nearly twice as large as bigger, slower-moving raptors in relation to overall body mass! This means that smaller raptors create and dissipate heat more rapidly than their larger cousins.

Several behavioral strategies are employed by these larger raptors to dissipate heat during sticky warm summer days, and at this point in the season breeding adults such as Liberty and Freedom are required to manage not only their own body temperature but that of their offspring as well. As a reminder, hatchling eagles cannot thermoregulate on their own and rely heavily on their parents to maintain an appropriate core temperature until about two weeks of age.

One method of cooling off is simply to alter posture. They will shift their body away from the sun to ensure that as small of an area as possible is receiving direct sunlight. Parent eagles will shade their hatchlings simply by providing a sun umbrella with their wings. This week we also observed Liberty and Freedom spreading their wings slightly as they sat in the nest, presumably to dissipate heat through their version of armpits, or “wingpits.”

Just as birds will fluff up their feathers to create an insulative pocket of air on cold days, they will do the opposite on hot days and essentially de-fluff to prevent that same pocket of air from forming at all. They will also keep their unfeathered body parts such as the beak, talons, and legs exposed on hot days to allow for heat dissipation and may also, like us, reduce their activity level on hot days.

As we watch the hatchlings develop, we will see Liberty and Freedom assisting them less and less in temperature regulation. At this very moment the feather tracks of the young are filling out and they are developing a second stage of downy feathers that will keep them insulated in the years to come. They are learning how to flutter their gular, and their internal physiology is getting closer to full function, preparing them for a life of thermoregulatory superpowers that characterize bald eagles and allow them to inhabit such a broad geographical range with blizzards, heat spells, and everything in between!


Check out this incredible story of three eagles raising three eaglets! On the Mississippi River, two adult males are assisting one adult female in raising young, and have been doing so since 2017!

Read more on NPR.


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