Thursday, December 22, 2016

Nesting Chronologies

We were asked about the nesting chronologies of bald eagles. While nest timing can very from region to region (Florida, for example, is quite different from Iowa), mark your calendars as follows!
  • In the nests we watch in Iowa and Colorado, bonding and copulating behaviors become more pronounced and frequent after the winter solstice. 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 Norths have a slightly later chronology and probably won't lay eggs until mid-March.
  • Each egg is laid about 3-5 days apart, and incubation starts with the laying of the first egg.
  • Eagle eggs begin hatching roughly 35 to 37 days after they are laid. This usually begins in late March in Decorah and Fort St. Vrain, and mid-April at the North nest. Hatch can take more than 24 hours for any given egg.
  • Eaglets spend 75-80 days in the nest before fledging. This usually happens in mid to late June at the Decorah and Fort St. Vrain nests, and early to mid-July at Decorah North.
More on the subject! 
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 after the winter solstice (see this blog for more on that topic, or this blog for a graph of daylight length rate changes). Between solstice and egg-laying, watch for female eagles to foot and nibble males, vocalize at them, rub against them, and even mount them to indicate receptivity. Males will 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. Listen for the unique and wonderful sound of the eagles vocalizing together!

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.
Incubation starts immediately after egg laying begins in mid-February to early March. Eggs shouldn’t get too cold, but they also can’t get too hot, or the embryos will die. Adults sit on the eggs when they need heat and get off them when they need to be cooled. Both parents have a brood patch, a natural thinning of the abdomen feathers caused by hormonal changes, where their skin is in direct contact with the eggs to transfer warmth. Since incubation starts from the time the first egg is laid, eggs will hatch about 3 days apart in the order they were laid. Both the male and female take turns incubating, but the female, being larger, takes the longer incubation periods overnight. It is believed that her larger body weight makes her a little more tolerant to cold - important during a long inactive spell!

Once eggs start hatching in late March to mid-April, it can take longer than 24 hours for any given eaglet to complete hatch, although it doesn't tend to take that long in the nests we watch. We will announce hip or Hatch In Progress watch on Facebook, Twitter (@RaptorResource), and our website (

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