Since we weren’t able to get to everyone’s questions during our chat on Explore, we answered them here. Watch the full chat here! https://youtu.be/MCtdzn13aSI.
How do eagles feet not freeze?
An eagle’s legs use counter-current heat exchange to control body temperature. Warm arterial blood flowing from an eagle’s core into its feet passes cool venous blood flowing the other way. Heat is exchanged, warming the blood flowing into its core and cooling the blood flowing into its feet. The cooler blood is still warm enough to prevent frostbite, but the lower temperature reduces the gradient between its insides and its outsides, preventing excessive heat loss through its feet. It has very few soft tissues in its long legs and feet, which are wrapped by thick, scaly skin that helps protect them from the cold. If its feet do get cold, it can always tuck them underneath its feathers. You can learn more about an eagle’s response to cold here: https://www.raptorresource.org/2019/01/17/flashback-blog-how-do-eagles-stay-warm-in-cold-weather/
Why do the parents still sit on their chicks after they are born?
A hatchling eaglet can’t thermoregulate, or control its temperature. The parents need to brood the eaglets to keep them warm. At ten to 15 days, dense ‘woolly’ thermal down begins to replace fuzzy white natal. Thermal down provides more insulation and helps nestling eaglets keep their body temperature relatively constant. However, thermal down isn’t as weatherproof as feathers, so they still might need brooding in extremely cold or wet weather.
Do any of the US eagles migrate or are they sedentary?
Some eagles migrate and some don’t, and eagles may undertake winter and summer migrations. Let’s start with the Decorah Eagles. As far as we know, every eagle on that territory has been a year-round resident. They have wonderful habitat and ample access to food during the winter, so they don’t leave. However, all of their young dispersed from the territory (even if D24 didn’t disperse very far) and at least two of them have made long migrations between their wintering grounds (Iowa) and summering grounds (Ontario). Adults tend to be more sedentary than sub-adults, although some (think eagles in central Canada or the US northern boreal forest) still migrate when winter begins to bite. You can follow the travels of all of the eagles we’ve tracked here: https://www.raptorresource.org/learning-tools/eagle-map/
What predators do they have to watch for?
Adult bald eagles don’t really have predators – i.e. something that specializes in hunting and eating them, or relies on them as a food source. While nothing specializes in taking nestling eaglets, raccoon will eat eggs and young (raccoon will eat everything they can get their paws on!) and owls will take young if they have a chance. We’re not sure that owls deliberately attack nests to take young, though – they might attack nests out of instinct or territoriality and take young secondarily.
Why do they nest without shade or shelter from elements? Why do they have white heads?
Eagles tend to nest toward the tops of trees, or on open cliff faces, because they need a lot of room to fly in and out, especially when carrying large sticks. Denser leaves and brush might provide more protection, but it would also be more difficult for them to access and build nests. Either way, there is no foliage when eagles begin nesting in Iowa, Minnesota, and Wisconsin, regardless of where they are nesting.
An eagle’s white head gets discussed quite a bit. In general, it is thought that adult eagles have white heads to help eagles distinguish sexually mature individuals at a distance. However, John pointed out that their white heads and tails would make them more cryptic during incubation, especially if they are nesting in snow-covered nests. This is an interesting idea and one worth exploring.
Do the nesting habits of eagles match any weather patterns?
There are two ways to think about that question: nesting chronology, or the time at which nesting occurs, and short-term response to immediate weather conditions.
Let’s start with nesting chronology. Nesting chronology is determined by daylight length and intensity. Daylight length appears to be a bigger factor among birds that nest in the temperate and frigid zones, while daylight intensity may play a bigger role among birds that nest in the subtropical and tropical zones.
Why? Birds need to be light weight to fly. During most of the year, their gonads are shrunken and inactive. But as daylight length increases following winter solstice, or daylight intensity increases following the end of the rainy season in tropical latitudes, a bird’s gonads ‘wake up’ and begin swelling. Sex hormones course through their bodies, which leads to nesting, sperm, eggs, and young.
So in general, a bird’s reproductive chronology isn’t influenced by weather. Our eagles will lay eggs in February or March regardless of temperature, windchill, or snowfall. However, weather can push laying a tiny bit earlier or later. We’ve observed that our eagles may shift egg-laying three or four days later in very cold, dry temps. This might be because females need to store a lot of water to produce an egg, and cold dry weather makes that harder.
How about the Florida eagles? The SW Florida eagles are well within the sub-tropical zone, so their reproductive activity is most likely influenced by light intensity instead of daylight length.
World Map created by KVD | (CC BY-SA 3.0)
Average monthly rainfall ramps up in early May and stays high through mid-September. What comes with rain? Clouds! What is diminished by clouds? Light intensity! Light intensity begins rapidly increasing in September, causing gonads to swell and eggs to be laid in November or very early December instead of February or March. However, the tropical breeding season is strongly correlated with weather, confusing the issue even further! But like their northern counterparts, the SWFL eagles will lay eggs regardless of immediate weather conditions like rainfall or bad storms.
John brought up a nesting eagle’s immediate response to weather conditions. Nesting eagles respond to external weather conditions by changing their wing configuration in inclement weather to protect the eggs from snow, sleet, and rain, adding fresh nesting material to keep eggs warm and dry, standing off the eggs to air and cool them, keeping them completely covered in very cold or wet weather, and so on.
We’ve written extensively about nesting chronology. Here are a few links for anyone who wants to learn more!
Just wondering why the eggs on the Decorah nest are different colours? I always thought their eggs were all white.
Egg color can change from near white by picking up staining or tannins from the nest material, prey items in the nest, and dirt, to name a few. The degree of color change is related to how long the eggs are in the nest. We have seen a significant difference in the color and darkness of the eggs at the Decorah Fish Hatchery nest this year. It was fairly easy to tell the difference between the oldest (more dark and dirty) to the newest egg.