Sunday, May 11, 2014

Big Bad Bald Eagle Mums

Brooding Bald Eagle (c) John Ashley
A female Bald Eagle broods her two chicks during a spring rain. One chick is peeking out from underneath.

(Mother's Day special edition)

In spite of what you may have heard, size really does matter, especially to female Bald Eagles. But in this case she has the advantage. Her extra heft has major impacts during nesting, when it's time to incubate delicate eggs and brood young eaglets - exactly what Montana's eagle moms are doing during Mother's Day. Two factors, weight discrepancy between the pair, and the weather during nesting season, can have a huge effect on how the eagle parents divvy up the family-rearing chores.

After the female lays 1-3 eggs, she and her mate both take turns incubating them. Montana's Bald Eagles generally incubate for 31 (mostly warm weather) to 35 days (mostly inclement weather). Chicks hatch between mid-March and mid-May. So by today, Mothers' Day, most of our successful Bald eagle nests should be cradling a baby or two that are really ugly, in a cute sort of way.

Bald Eagles incubate their eggs for about 95% of the daylight hours. If you average the hours, then the female spends about 10% more time incubating than the male, during daytime. She also takes the late shift and incubates their eggs through the night.

When their eggs finally hatch, both parents take turns brooding, or covering the chicks with their bodies to keep the little guys warm and dry. During the first 10 days after hatching, the parents will spend almost 80% of each day brooding their wobbly youngsters. As the voracious toddlers gain weight and grow feathers (downy feathers, body feathers, flight feathers), the time spent brooding falls to 3% at 51-60 days after hatching.

During that early, critical nestling stage, male Bald eagles handle about 30% of the brooding hours, while females work the other 70%. But before you judge these dads, this is actually a pretty good showing by Bald Eagles. By comparison, female brooding rates are 100% for Golden Eagles, 99% for Peregrine Falcons, and 95-100% for Osprey.

The steepest drop in brooding time happens 11-20 days after hatching, because this is age when chicks are finally fat enough to maintain their own internal body temperature, or "thermoregulate." The young eagles will fledge from their nest a little more than two months after hatching, but they'll still depend on their parents to feed them for another 2-3 weeks before wandering off to discover the world. In other words, typical teenagers.

Bald Eagles feeding two chicks (c) John Ashley
Can you tell which eagle parent is mom?
Here in Montana, female eagles can weigh up to 30% more than males. Our nesting females average 11.5 lbs, while adult males weigh in at just 9 lbs. And as you may recall from high school biology, a larger body is more efficient at maintaining a constant internal temperature, so female Bald Eagles have a substantial, built-in advantage in the heat-maintenance department.

This size disparity shows up as a gradient as you look from south to north, from warmer areas to colder ones. Female Bald Eagles nesting in balmy Arizona weigh 20% less, an average, than our Montana females (males only 8% less). And the females that migrate through Montana on their way to nests in cooler Canada weigh up to 13% more than our Montana gals. So an eagle pair's individual weights are fairly similar in the south, but they diverge more and more the farther north they nest.

Larger body size contributes to a second built-in advantage for females over males, at least while nesting - they show more skin. Most nesting birds either molt or pluck most of the small body feathers from their breast area in preparation for incubation. This patch of bare skin is called a "brood patch," and a nesting female Bald Eagle has a brood patch that's three times larger than her mate's.

Give dad credit for having a brood patch, but mom has much more bare skin to press against the eggs without insulating feathers getting in the way. This one-sided heating is the reason that Bald Eagles, like all birds, need to turn their eggs frequently during incubation.

To this point, we've been talking about averages. But averages fly out the window when bad weather rolls in. Because of their built-in physical advantages - more weight to produce heat, and larger brood patches - female Bald Eagles take over almost all of the incubation and brood duties during periods of precipitation (rain or snow), high winds or low temperatures. This is especially pronounced at nests that are more exposed to the sky (no overhead branches) and the elements.

During spells of wet, windy or cold weather, mom is simply the best-equipped parent to keep eggs and chicks alive. Dad pitches in by delivering food to her at the nest, but that's indirect care of the young, so he tends to get less credit for this labor. However, during good weather, the males might more than double the daylight hours that they spend tending to eggs and eaglets.

Basically, Bald Eagle parenting is a flex schedule, especially for mom. Her plus-size body means she's well-equipped to micro-manage the family nest environment, and she changes her behavior to fit the family's needs. Diminutive dad just pitches in wherever he can help out. And of course, that's why we have Mother's Day. We pause one day each year to recognize that, in most families, mom rules the roost.

We could end there, but I'll add a bonus note from the Bald Eagle world. Size also matters to the sub-adult Bald Eagles, but not body size. Young Bald Eagles weigh less than adult eagles of the same sex, but they actually have longer wing and tail feathers than the adults.

Instead of parenting, young birds are in flight school for their first four years. Less weight and longer feathers make it easier to fly at slower speeds. And slower flight speed is a great benefit for young eagles who are learning how to fly, chase, fish and hunt. The young guns who master all of these flight skills will earn a turn as the future Bald Eagle moms and dads.