Wednesday, March 02, 2011

The Decorah Eagles laid their third egg today! PeggyJean posted this nice closeup.

In his book "Oology and Ralph's Talking Eggs" Carrol Henderson discusses egg shape. According to Henderson, there are four different general shapes: elliptical (including round/spherical), pyriform, oval, and subelliptical. There are short, medium, and long categories within each, so 12 basic shapes are possible. Bald eagles lay oval eggs that are about the size of a tennis ball, as you can see here. This is the most common shape (Henderson refers to it as the default shape) that are probably most easily formed in the egg as the egg passes through the oviduct.

So how are eggs formed and marked? Some more Henderson:
"The fertilized ovum and yolk are released from the bird's ovary and then enter a portion of the oviduct called the magnum. There the egg white, or albumin, is added around the yolk. In the next portion of the oviduct, called the isthmus, soft inner and outer membranes enclose the egg white. Finally the egg, blunt end first, enters the uterus for an extended period of time where the shell is formed. The pigments (porphyrins) that create colors and patterns on eggshells are produced by the breakdown of hemoglobin from ruptured blood cells...Pigment oozes through the lining of the oviduct and onto the forming shell. Depending on how the egg is moving or twisting in the oviduct, this process results in the creation of spots, streaks, and blotches, with most markings usually at the larger blunt end of the egg."
Of course, shape and color can also protect eggs. Pyriform eggs have a tapered shape (image found on internet). This helps the eggs pack neatly into a nest for incubation, since the pointed ends all face the center; lessens the change of the eggs falling from a ledge, since the eggs roll in tight circles, and may help emperor penguins keep their eggs located on their toes. While I think of eggs as being white, color and markings can help make eggs less visible to predators. Some birds even camouflage their eggs with down or vegetation to hide them.  

In Decorah, the male and female eagle will take turns incubating their three eggs until they hatch on roughly April 1st (it could be a little later, since incubation takes roughly 35 days). This will keep the eggs warm and help protect them from predators like raccoons. The parents will roll the eggs - something we've already seen - pick at the nest, doze, and eat. They will be very busy once the young eagles hatch!
Here's a video of the third egg being laid. Thanks to the many Raptor Resource Project forum members who collect and post photographs and videos.