Every season has its challenges, but winter seems especially difficult. The hardships winter presents are the reason some animals hibernate or brumate. For those creatures, it's better to hunker down and sleep the winter away than risk not surviving the harsh conditions.
Of course, not all animals skip winter altogether. Many birds and mammals remain active, particularly on warm and sunny winter days. For some animals, winter is even a critical part of the year. Take bald eagles. For them, winter is the start of nesting season.
Eagles start preparing for nesting season even before winter begins. Between one and three months before laying eggs, a male and female bald eagle pair will begin working on their nests. In northern Illinois, this typically begins in November or December. Eagles reuse their nests from year to year but work on them each year to get them ready for the new clutch of eggs. Both the male and female work to construct or maintain the nest.
By late February, it's time to lay the eggs. The timing might seem odd considering how cold it is in February. This timeline does have its advantages, however. From the time eggs are laid until the time young eagles are ready to fledge the nest is about four months. That means eagles are learning how to fly and hunt sometime between May and June. During this time, their main food source — fish — is plentiful and easy to find.
Laying eggs in winter is also practical because it is easier for the adult eagles to keep the eggs warm in winter than it is to keep them cool in the summer. It also explains why eagles do not follow the same nesting timeline everywhere. Here in the Midwest, laying eggs in winter is common, but in the southern United States, breeding pairs of eagles begin the nesting process in the fall. This allows the eaglets to reach fledgling age before it gets too warm again in the spring.
Words to know
Brumate: A lethargic state similar to hibernation used by reptiles.
Clutch: A group of eggs fertilized at the same time and incubated together.
Hardship: Sever suffering.
Incubate: To sit on eggs to keep them warm and bring them to hatching.
Plentiful: Existing in great quantities.
The difference in when bald eagles nest based on where they live also creates differences in the size of eagles based on where they live. Bald eagles in the Midwest are larger than eagles in the southern United States because their larger size makes them more efficient at conserving heat. This helps them better incubate their eggs.
A breeding female will lay between one and three eggs, although she does not lay all the eggs at once. It generally takes between three and six days to lay a full clutch of eggs.
Once the eggs are laid, they are incubated for about 35 days before hatching. Both the male and female will sit on the eggs, although the female eagle is the primary incubator. The pair will trade places several times through the day, usually every one to four hours.
To incubate the eggs, both the male and female will create a brood patch on their breast by removing feathers. The bare skin of the brood patch is what incubates the eggs because it allows their body heat to warm them.
The eggs must be kept at a temperature of about 105 degrees Fahrenheit, but that's not the only purpose of the eagles sitting on the eggs. It also protects the eggs from predators and even extreme sunlight, which might make the eggs too hot. The eggs must also be turned regularly — about once every hour or two — so they are warmed evenly and they don't start to stick to the inside of the shell. Turning the eggs has to be done carefully so their sharp talons do not punch holes in the delicate shells.
Bald eagles aren't the only birds that lay their eggs in winter. Nesting in late winter or early spring is common for many owl species. Great horned owls typically lay their eggs in February or March. Eastern screech owls follow the same schedule, and barred owls typically lay their eggs in March.
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