Friday, February 14, 2014

Courtship, copulation, eggs, and other things romantic

Valentine's Day is here and we're all awaiting the first egg in Decorah. Given that the day is a celebration of things romantic, we thought we would discuss copulation, fertilization, and other egg-related subjects.


The video below shows Mom expressing interest in Dad. She twists her tail feathers to the side while vocalizing and rubbing against Dad. In Decorah, courtship activities begin in late fall. Mom and Dad bring sticks and branches into the nest, eventually adding soft nesting material as courtship deepens. Food is (often) shared. The eagles vocalize together and spend more time in close proximity, moving from sitting side by side to gently pecking and footing, wing and tail brushing, and body rubbing with tail twisting and vocals. Mom is letting Dad know in no uncertain terms that she is receptive and ready for copulation. A few people have commented on Dad's 'shyness'. While this is pure speculation, female eagles are larger and stronger than males. Perhaps males want to be absolutely certain females are receptive before getting too close.

Video: Mom expresses interest in Dad

Bald eagles and peregrine falcons are both birds of prey, but they have slightly different courtship rituals. In both species, the male brings food gifts and both sexes engage in nest-preparation activities. But peregrine falcons incorporate bowing, and bald eagles don't. The video below shows peregrine bowing, which starts around 28 seconds. We should start to see this courtship behavior sometime in mid to late March.

Video: the male falcon bowing


Male eagles, like most male birds, lack an external phallus. Bald eagles mate by touching or pressing cloaca together - the famous 'cloacal kiss'. The female perches and tilts forward, allowing the male to land with his feet lodged on her back. She twists and moves her tail feathers to one side so the mounted male can press and twist his cloacal opening around her cloaca, passing sperm from his cloaca to hers. Intense and rapid vocalizations will precede and accompany their mating. As we’ve seen in Decorah and Fort St. Vrain, mating is brief, intense, and takes place fairly frequently while the eagles are in season.

Video: Eagle copulation at Fort St. Vrain


So we've gone through courtship and copulation. Now the sperm needs to reach the female's ova at the infundibulum, or site of fertilization. The female is regularly ovulating, but sperm needs to be there when an ova arrives. If sperm are too early, they will die prior to the arrival of an ova. If sperm are too late, they can't penetrate the eggshell layers that form around the ova in the female's oviduct. So how can fertilization be assured, assuming both eagles are fertile? 
  • Regular mating. While eagles copulate outside of maximum fertilization, regular copulation helps assure a good supply of sperm, making it more likely they will be in the right place (the infundibulum) at the right time (when an ova arrives).
  • Sperm storage. Some of the male's sperm will enter sperm storage tubules, which determine the fertile period by maintaining sperm viability and continually releasing sperm to the infundibulum. They also help prevent sperm from being ejected when the female defecates. We mammals have reproductive, intestinal, and urinary tracts with separate openings. Eagles do not. Their cloaca serves as the only opening for their intestinal, reproductive, and urinary tracts.
  • Slow-moving sperm. Sperm move relatively slowly through the infundibulum, so that there is usually a population available to fertilize each ovum as it is ovulated.
Let's assume that sperm has reached the infundibulum and found a fertile ova, yolk erupted, waiting for fertilization. Once a sperm fertilizes the yolk, the outer egg membrane seals off the fertile egg, which prevents further sperm entry. The egg begins to drop through the oviduct, a process that lasts for two or more days. As the egg descends, it is coated in layers: eggshell membranes, cuticle layers, and finally the outer crystallization membranes that form what we think of as the eggshell. Viola - a fertile eagle egg ready for laying and incubation!

Video: An egg's journey

Although we don't know exactly how long egg-laying occurs after mating, it is generally believed that female eagles lay eggs 5-10 days after a successful mating takes place. Since love is clearly in the air at the Decorah nest, we shouldn't have to wait too long.

Have a happy Valentine's Day!


Raptor Research and Management Techniques, 2007
Raptor Research Foundation
Edited by David M Bird and Keith Bildstein

Blog by David Hancock:


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