We’ve talked about how long it takes bald eagle eggs to hatch after they are laid (an average of 38 days from first egg to first hatch in Decorah), how long hatch takes once pip starts (it can take upwards of 24 hours), and how long it takes Mom to lay each egg (she usually lays the second egg about two to three days after the first, and the third egg roughly four days after the second egg). But how long does it take Mom, DNF, or Ma FSV to make and lay an egg?
Let’s start with copulation. We think that female bald eagles begin laying eggs five to ten days after productive mating begins. In 2018, Mom laid her first egg at 7:28PM CT on February 21st, or about eleven days after copulation went from casual to frequent…and very determined on Mom’s part. Between February 10th and February 18th, Mom and Dad copulated nine times that we saw and had an additional 2 failed attempts. Mom took the lead five times – beak biting Dad, footing him, loudly vocalizing her intentions, and mounting him while wagging her tail (https://youtu.be/I4xlu9IEItY). You didn’t need to be a male bald eagle to know that Mom meant business!
So we know that bald eagles ensure fertile eggs by copulating regularly, storing sperm in storage tubules, and concentrating sperm at the infundibulum, or site of fertility. But how long does it take Mom to lay an egg once her first yolk has been fertilized? The short answer: approximately two days (48-50 hours). Note that our oviduct times were derived using the ratio between bald eagles (~48 hours) and domestic chickens (~25 hours). The actual times could be slightly different.
Step One: I’m Ready!
According to Professor Tim Birkhead, there are no known cases of copulation-induced ovulation in birds. As we’ve seen at all the nests we watch, female birds signal their fertile status by their interest in (or their insistence on) copulation. In response to an internal clock and the presence of a mate, a female’s yolk swells until it ruptures the follicle that produced it, releasing a ripe yolk into her oviduct.
How long does it take for a bald eagle’s yolk to swell? Our observations suggest about eight to ten days. For comparison, it takes four to five days in small birds like great tits and white-crowned sparrows, six to eight days in larger birds like ducks and pigeons, 10-13 days for large gulls, and up to 16 days in some penguins.
How many follicles are swelling inside a female eagle each season? In domestic hens, each follicular cohort (follicles that will become eggs) numbers six to twelve follicles, and follicles are selected roughly every 24 hours. We know that Mom Decorah’s annual cohort measures at least three. Since it takes roughly 48 hours for her to lay an egg, follicle number two must rupture about the time egg number one is laid. Follicle number three experiences a delay of about 48 hours, giving Mom’s ‘egg-machine’ a chance to rest and resupply!
Step two: The infundibulum! Length of stay: ~15 minutes (T – 47+ hours)
While the name infundibulum (funnel) suggests a passive process, the newly released microscopic yolk stays in one place while the infundibulum flows around it. If Mom and DM2, Mr. North and DNF, and Ma and Pa FSV have been copulating regularly, and if females have been storing sperm, hundreds or thousands of sperm are in place and ready to fertilize yolks! Fertilized or not, the infundibulum seals the yolk after roughly 15 minutes and it proceeds on its journey down a female eagle’s oviduct.
How does the yolk stay in the center of the egg? The infundibulum’s seal, or chalaza layer, forms the egg’s first layer of albumen, aka egg white. The ends of this dense chalaziferous zone twist with other proteins to create two filaments as the egg spirals through the magnum. These filaments will eventually anchor the yolk to the egg’s hard calcareous shell, keeping it in place.
Step three: The magnum! Length of stay: ~5 hours (T – 42.5 hours)
The yolk makes its next stop in the magnum, where it receives another coating of albumen. The albumen is secreted by special cells in the magnum wall that absorb water and proteins from the female eagle’s bloodstream. The albumen will cushion the developing embryo and provide much of the protein needed for its development. Remember DM2/Mr. North/Pa FSV’s food gifts? Female eagles need all the water, protein, and calcium they can get to produce viable eggs!
Egg in cross section
Step four: The isthmus! Length of stay: ~3 hours (T – 39.5 hours)
The yolk moves into the isthmus next. It receives a little more albumen and its inner and outer soft shell membranes, which (like an eagle’s feathers, beak, and talons) are made of keratin – another protein! The inner shell membrane provides a point of contact for the chorioallantoic membrane that develops in the first three to four days of an embryonic eagle’s life. Both membranes help protect the porous egg from bacterial contamination and keep water from escaping too quickly. The membranes sit closely together during the egg’s trip through the oviduct, but separate after the egg has been laid.
Step five: The shell gland! Length of stay: ~39 hours (T – 15 minutes)
The yolk, wrapped snugly in its jacket of albumen and shell membranes, moves into the shell gland or uterus. Water and minerals are pumped into the developing egg and a hard calcareous shell is formed around it.
Our female eagles are removing a lot of calcium from their bodies to produce the egg shell. While I couldn’t find figures for bald eagles, hen chickens remove about 25 mg of calcium from their blood every 12 minutes during active egg shell formation. Since bald eagle eggs are significantly larger than chicken eggs, it seems very likely that they are removing more calcium than that. Anything that can’t be derived from dietary sources will be obtained from special bones in their skeletons – another reason that food is extremely important right now!
Step six: The vagina and egg labor! Length of stay: roughly 15 minutes!
We finally have an egg! The microscopic yolk has been wrapped in membranes, plumped up, and surrounded by a hard shell. While we don’t know whether Mom, DNF, or Ma FSV were aware of the egg before, they are certainly are now! Let’s use Mom as an example. Contraction of a powerful sphincter muscle causes her egg to rotate in her muscular vagina and enter her cloaca pointed-end first. Her powerful vaginal muscles and full-body contractions eject the egg through her cloaca and into the waiting egg cup. Two days after Mom’s first egg was started, it emerges after her brief egg labor and she lays down for a well-deserved rest! Her second egg is just beginning its journey!
Does female eagles feel pain during egg labor?
We often get asked whether female eagles experience pain or discomfort, especially in the final phase of laying. Whether or not any given eagles knows her follicle burst, egg laying is an energetically expensive process. Here’s what we saw in 2018 at the Decorah nest. Two or three days before Mom laid egg number one, we started seeing her on and around the nest more. Dad brought several meals to her and she spent a long period of time in the nest the morning before the egg was laid. While her soft chirps were lovely to hear, she was obviously experiencing some discomfort. It was a relief to all of us when she finally laid the egg!
Our female eagles will incubate their first egg 35 to 37 days before it hatches. While males do their share of incubation, incubation gives females the time they need to rest and build up her reserves. Sweet eagle dreams to all our eagle Moms!
Curious about what egg-laying looks like? Here’s a great video from 2018
What happens when a bird loses its nest and mate with an egg in the pipeline?
We often get asked what happens when a bird’s season is interrupted. We haven’t seen it at any of our bald eagle nests, but we have seen it in several peregrine falcons. In 2017, an interloper laid an egg in the nest box at Dairyland Alma before the returning female ousted her, and Newman spent a lot of time courting and copulating with St. Louis Girl before Michelle showed up and kicked her out. While there isn’t much information about it, I would guess that any eggs in the pipeline get laid somewhere. But if follicle stimulation is about more than just lengthening daylight hours – which it appears to be, at least in wild birds, which don’t tend to lay eggs absent a mate, copulation, and a nest – losing a mate and territory may shut follicle stimulation and yolk production down. This would prevent additional eggs from forming: a wise strategy given how energy intensive egg production is!
Does it feel like the parts of a female bird’s reproductive system were named by committee? Technical language is daunting enough when it fits together nicely! But people have studied birds, especially domestic chickens, for a long time. Not everything we are describing now was named at the same time by the same people, which can lead to an odd combination of words.
- Infundibulum is derived from a16th century Latin word for “pour in” (or funnel): infundere.
- Chalaza is derived from a Greek word that means “small knot”: Khalaza
- Magnus is a Latin word that means “great”. The magnus is the largest part of the oviduct.
- Isthmus is derived from a Greek word that means narrow neck of land: isthmos.
Things that helped me learn and write about this topic!
- Diagram from the paper Sperm storage in the female reproductive tract in birds, Sasanami T, Matsuzaki M, Mizushima S, Hiyama G – J. Reprod. Dev. (2013). Taken from Open i, US Department of Health and Human Services, https://openi.nlm.nih.gov/detailedresult.php?img=PMC3944358_jrd-59-334-g001&req=4.
- Egg cross section derived from The Avian Egg: Alexis L. Romanoff, A.J. Romanoff, 1949. Image credit poultryhub, although it appears in other places as well: http://www.poultryhub.org/physiology/the-avian-egg/. If you own this image and do not want it posted or need the credits changed, please contact me: [email protected]