Eagles, ‘menopause’, and a new mate at Xcel Energy’s Fort St. Vrain plant

Has Ma FSV entered ‘menopause’? This blog was going to be focused on eagles and ova, but Elfruler, our original DE lead mod and a long-time chronicler of bald eagle nests, noticed that the male had a band on his right leg, not his left. A new eagle has replaced Pa FSV. If you’d like to learn more about eagles and ova, please read on (TLDR: Ma FSV has not entered menopause). Thanks to Elfruler for her observations and Donna Young and the folks on the Fort St. Vrain cam page for the caps.

Eagles and Ova
Avian Reproductive System, From Handbook of Bird Biology, Second Edition

Avian Reproductive System, Handbook of Bird Biology

Like mammals, female birds hatch with a fixed number of ova in their ovaries. How many do bald eagles have? We don’t know, but Waha Thuweeka (a.k.a. Bill Voelker), director of the Sia breeding project, told us that Golden Eagle Micah was 52 years of age when she laid the egg that produced her daughter Waipi. He wrote:

“2020 was our fiftieth year of conducting culturally-based field observations of Golden Eagle populations that occur in the corridor of our historic Numunuh (Comanche) migration route used when our People left the high country of the Rockies and worked their way onto the Southern Plains. During the course of the past 50 years we have identified and verified, through unique plumage or anatomical idiosyncratic features, several individuals in the wild that have bred successfully in the same territories in excess of twenty-eight years!”

Golden eagles lay one to three eggs year year. If a female Golden Eagle laid an average of two eggs per year for 28 years, she would lay 56 eggs.  The number of ova a female bird is born/hatches with differs between species and genera. We have a better idea with falcons (100 – 120+ on average) than we do with eagles, although eagles vary depending on biogeographic region. Members of Spizaetus, a genus of tropical eagles, may have only 54 – 60 ova based on egg production records from long-term breeders and post-mortem examination of breeder adults. These eagles usually lay just one egg per clutch and don’t breed every year, so isn’t surprising that they might develop fewer ova than their prolific northern counterparts.

How many eggs have our eagles laid?

We have excellent data on three nests: Decorah, Decorah North, and Fort St. Vrain.

  • Decorah Eagles: Mom is eighteen years old. She has laid 38 eggs to date: two eggs in 2008 and three eggs every year since 2009, with one failing to hatch (2016) and one breaking (2019). If she keeps laying three eggs per year, she has six years to go before she reaches 56 eggs, although she will likely well keep laying well beyond that. There are very few studies of free-living birds over the course of their entire reproductive lives. This underscores the importance of observing this year’s eaglets, even if Mom and DM2 choose the yonder-yonder nest. Counting eggs and years helps answer questions about how long females can breed and how many ova they have.
  • Decorah North Eagles: We don’t know how old DNF is, but we don’t think she has been breeding very long. Assuming that 2019 marked her first year, she has laid six eggs to date.
  • Xcel Fort St. Vrain Eagles: We think Ma FSV started nesting in 2007. Why? In 2006, the female eagle laid her first egg on February 17. But in 2007, she laid her first egg on March 3. Over the next few years, we saw her schedule advance until she was laying around February 16 every year: a pattern we’ve observed at our peregrine falcon nests as well. If we are correct, Ma FSV has laid 39 eggs since she began breeding in 2007. That’s a lot of eggs but not, based on what we know, nearly all the eggs she could lay.

The new male at Fort St. Vrain has a band on his right leg, not his left. This is a more common protocol: with falcons, the federal band always goes on the right leg and the auxiliary band goes on the left. I’m a little surprised that he is banded, since eagle banding isn’t common and Colorado ended its eagle banding program well over over ten years ago. I believe that eaglets were last banded at Fort St. Vrain in 2007. 

So new mates push the lay date back?

They do! We’ve documented it in peregrine falcons and bald eagles. Let’s take a look at our eagles:

  • In 2009, her second year on site, Mom Decorah laid egg #1 on March 2. She advanced to February 25 in 2010, to February 23rd in 2011, and to February 17 – her earliest date! – in 2012. She usually lays three eggs a year.
  • In 2019 and 2020, DNF (Decorah North) laid her first egg on February 21. She laid egg #1 on February 16 this year. She usually lays two eggs a year.
  • In 2007, which we think was her first year on site, Ma FSV laid egg #1 on March 3. She advanced to February 28 in 2008, February 27 in 2009, and February 17 in 2010. She usually lays three eggs a year.

Why do new mates push lay dates back? In some cases, mate succession happens when one bird pushes another off territory early in the breeding season (note: we don’t know that this happened at FSV). If the interloper is a female, the resident male will need to bring her into condition for eggs. If the interloper is a male, he’ll need to bring the resident female into receptivity. Even if they accept one another immediately – and falcons often do! – the schedule will frequently fall back. Ma and Pa FSV had been a pair for fourteen years. He was most likely her first mate and it might take her (and us) time to adjust to somebirdy new. The two are bonding and working on the nest: a very good sign for future eaglets.

Mrs. North, the first female eagle at the North nest, had a very unpredictable egg clock! We only watched her for three years, but during that time, her earliest egg arrived on February 19, 2017 and her latest arrived on March 11, 2016: almost a month later. I didn’t include her data because we don’t know how old she was, how many eggs she had laid over the course of her lifetime, or how long she and Mr. North had been breeding.

Did you know?
  • The oldest known wild bird in the world is Wisdom, an Albatross on Midway Atoll. She is 69 years old and laid an egg on December 3, 2020. Albatross lay one egg per year. https://www.fws.gov/refuge/Midway_Atoll/News/Where_Is_Wisdom.html
  • The oldest known wild bald eagle died at 38 years old, after being struck by a car. Adult male temperate zone eagles can produce fertile semen until the end of their lives, although we didn’t have up-to-date breeding records for 629-03142.
Things that helped me learn and write about this topic
  • Jim Grier and Waha Thuweeka, aka Bill Voelker. Both men were an invaluable source of information and help on a topic I know very little about. Follow this link to learn more about the The Comanche National Ethno-Ornithological Initiative: http://www.comancheeagle.org/home.html.
  • Nesting records. Nest cams are an excellent, non-invasive way to get accurate, long-range records of egg-laying, egg hatching, and fledge. With all we know, there is so much left to learn!