We’re getting a lot of questions about Mom Decorah and eggs. When will she lay eggs? Will she lay three eggs again? We don’t want to count Mom’s eggs before she lays them, but we have some answers (and some speculation) to your questions!
Will a new mate equal later eggs?
Let’s start by taking a look at Mom’s egg laying history…
| Date |
First Egg |
| 2018 |
February 21 |
| 2017 |
February 20 |
| 2016 |
February 18 |
| 2015 |
February 18 |
| 2014 |
February 23 |
| 2013 |
February 17’ish |
| 2012 |
February 17 |
| 2011 |
February 23 |
| 2010 |
February 25 |
| 2009 |
March 02 |
Eggs in 2016
As the table above shows, Mom’s first egg (that we have a date for, since she laid eggs in 2008 but we don’t have a date) gradually moved earlier after her first season with Dad (the one exception is addressed at the end of this post). While it doesn’t always happen, a new mate (male or female) often pushes egg-laying a little later in the eagles and falcons we watch. Sometimes it happens for very obvious reasons, like mid-season mate replacement. Let’s say that female falcon A and male falcon B are nesting together on a cliff in mid-March. Falcon A has begun the process of egg production: her egg follicles are swelling, she’s gaining weight, and she’s copulating like crazy! Unfortunately, she’s killed or displaced by female falcon C, who is producing plenty of sex hormones, but no eggs. B doesn’t waste any time courting C. His bowing, gifts of food, and copulation will trigger her to begin egg production, but their egg-laying schedule will be pushed back by as much as or more than ten days. Absent other factors, it should float earlier in the years to come.
But later egg-laying can also happen in new couples that don’t experience mid-season mate replacement. Both Mom Decorah and Ma FSV laid the first eggs of their lives in early March (March 2, 2009 for Mom Decorah and March 3, 2007 for Ma FSV) but moved earlier as the years progressed. However, Mom had just achieved sexual maturity when she bonded with Dad, so she didn’t have an established egg-laying schedule. Now that she does, a new mate might not change things – especially since UME-2 has already attempted to copulate with her three times that we know of. We are eagerly waiting to see what happens in 2019…talons crossed!
Can new mates equal less eggs?
We’re going to start by ruling out immature mates. While we haven’t heard of immature bald eagles nesting, we very occasionally see it in the falcons we monitor. When immature falcons lay eggs, they usually lay one or maybe two infertile eggs. Falcons and eagles may also lay fewer eggs in their first mature year of breeding. But once they’ve established a pattern and absent other factors like declining fertility, the birds we watch usually lay the same amount of eggs year after year, regardless of who they are mated with.
Will wild female birds of prey lay eggs in the absence of mates? They will not, which I think is part of what makes this question interesting. Lengthening photoperiods get sex hormones flowing, but sex hormones alone don’t cause yolks to swell and burst their egg follicles. While we don’t know all of the details, a male eagle is required for egg production to begin, although he doesn’t seem to have much of an impact on the number of eggs his mate produces. More about this later.
Fortunately, Mom has accepted a new mate well before egg-laying begins. With UME-2 there to help bring her into receptivity, we are hopeful she will lay three eggs. Again, we’re crossing our talons and eagerly waiting to see what happens!
What are the signs of egg-laying onset?
Mom Decorah initiates copulation in 2018
We will be looking for copulation to really ramp up! In 2018, copulation went from casual to very frequent – and very determined on Mom’s part – on February 10. She laid her first egg eleven days later. As Mom becomes gravid with eggs, she’ll start spending more time on and around the nest. She may appear drowsier and will appear dumpier or larger as her body swells with the water she needs to produce eggs.
We’re glad that UME-2 is trying to copulate with Mom, even if he’s still working to get the hang of it. At least so far, signs are pointing to a productive nest in Decorah in 2019! We invite everyone to watch with us and plan to roll out some tools to let users document observations while we all wait to see what happens.
Did you know?
- While wild ‘floaters’ – adult birds of prey that don’t have a territory or mates – don’t lay eggs, egg laying has been documented in solo captive female birds, including wild caught educational and falconry birds. But are they really solo? They have territory – their mews, pen, enclosure, etc, and their physical and emotional needs are met by their keepers, who provide food, stimulation, and companionship. In short, keepers of birds can become their birds’ mates as far as the birds are concerned, which can lead to egg production even absent copulation.
- New mates aren’t the only thing that can influence egg-timing. If we have cold, dry weather this year, look for eggs on the later side! In 2014 – a very cold, dry year – Mom didn’t lay eggs until February 23. Drier weather results in higher levels of evaporative and cutaneous water loss, which makes it harder for female bald eagles to accumulate the water reserves they need for egg production: https://raptorresource.blogspot.com/2018/03/why-did-mom-and-dad-lay-eggs-later-this.html.
- Once Mom’s first yolk has been fertilized, it will take her about two days to lay her first egg. She should have a two to three-day period between egg one and egg two, and a three to four day period between egg two and egg three. This delay gives Mom’s ‘egg machine’ a chance to rest and resupply! Specifically, she may need to rebuild water reserves before the last yolk is able to swell enough to burst its follicle. More on that here: https://raptorresource.blogspot.com/2018/02/how-long-does-it-take-mom-to-lay-egg.html.
- It was a little annoying to search ‘bald eagles’ and ‘sex’ (not to mention embarrassing to my browser history) and find a lot of references to bald eagles mating in flight or mating through their feet. While talon-locking and whirling can be part of bald eagle courtship, mating doesn’t happen in flight and feet have nothing to do with sperm transfer! More on that here: https://raptorresource.blogspot.com/2014/02/do-bald-eagles-mate-in-flight.html.
The Raptor Resource Project