Will Mom and DNF lay eggs?

Will Mom and DNF lay eggs? When will they lay eggs? Will those eggs be fertile? A lot of watchers have egglets and eaglets on their minds! The short answers:

  • We believe that Mom Decorah and DNF (the Decorah North Female) will lay eggs based on what we’re seeing at their nests. They are both bonded with mates who are meeting their physical and psychological needs, including meal-sharing, mutual nestwork, nest bowl preparation, territorial defense, courtship, and copulation. While we worry, they are getting ready for eggs!
  • Mom will probably lay eggs around February 20, although she could go a little later with a new mate and/or cold, dry weather. We don’t have any data on DNF, but new females have commonly tended to lay eggs in late February or early March at the nests we watch. Eggs could come as early as February 12 – less than a week! – at Fort St. Vrain, although the average first egg date there is February 15.
  • We can’t guarantee fertility, but as long as plenty of copulation is taking place, the eggs should be fertile. We’ve seen copulation at all of our nests – a very good sign of things to come!

Egg Timing

In the nests we watch in Iowa and Colorado, bonding and copulating behaviors become more pronounced and frequent after the winter solstice (see this blog for more on that topic, or this blog for a graph of daylight length rate changes). Female eagles begin laying eggs 5-10 days after productive copulation begins. This usually happens in mid-February at the Decorah and Fort St. Vrain nests. The average first egg date in Decorah is February 20th and the average first egg date at Fort St. Vrain is February 15. While we don’t have an average date for the new female at Decorah North, new females have tended to lay their first egg in late February or early March at the nests we watch. Each egg is laid about 3-5 days apart, and incubation starts with the laying of the first egg.

Bald Eagle eggs compared with a familiar object

Egg Laying

Eagles have been observed mating ten months out of the year, but they only produce eggs and sperm for a very brief period of time. As the days begin to lengthen following solstice, birds’ gonads swell and produce sex hormones. Around the end of January, our bald eagle pairs switch from infrequent copulation to frequent, highly enthusiastic copulation, putting us on notice for eggs! Watch for female eagles to foot and nibble males, vocalize at them, rub against them, and even mount them to indicate receptivity. Watch for male eagles to increase the amount of material they bring into the nest, work on the area that will underlie the nest cup, and vocalize at and with females as both sexes become more vocal. Will our eagles lay eggs? What we’re seeing at our nests points to ‘Yes’!

Egg Fertility

Will those eggs be fertile? A quick primer on egg fertilization in birds. Sperm needs to encounter an ovum at the infundibulum, or site of fertilization. If sperm are too early, they will die prior to the arrival of an ovum. If sperm are too late, they can’t penetrate the eggshell layers that form around the ovum in the female’s oviduct. So how do birds assure fertilized eggs? They:

  • Copulate regularly. Regular copulation helps assure a good supply of sperm – especially important in an animal that regularly clears its cloaca when eliminating waste!
  • Store sperm. Sperm storage tubules maintain sperm viability, prevent stored sperm from being ejected, and continuously release sperm to the infundibulum.
  • Concentrate sperm at the infundibulum. Released sperm are passively carried to the infundibulum. Their continuous release and relatively slow drift help ensure that sperm are present when an ovum arrives.

In short, fertile eggs depend on a good supply of sperm, so if our eagles are copulating frequently, we’ll have fertile eggs! We’ll know roughly 35 to 40 days after the first egg is laid at any given nest. We are very hopeful since we’ve only ever had two eggs fail to hatch at any of our bald eagle nests: one in Decorah in 2016 and one at Decorah North in 2018.

Don’t give up hope and keep your talons crossed – egg-laying should be just around the corner at all of our nests! We’ll be posting on eggs, incubation, and related topics while we wait.