Attending a Dissection

Whenever I think I know a lot about a subject, something happens that makes me realize how little I actually know. I had a moment like last Tuesday when I went to the American Museum of Natural History to watch William Mauck III, the ornithology-laboratory supervisor at AMNH, dissect, clean and stuff a raven before it entered the collections at the museum. Mauck normally would prepare the bird alone in the lab, but this week he did the work in front of one of AMNH’s afterschool classes and a few guests like myself.

Raven pre-dissection. Photo credit: Sandra Lewocki

Anatomy is one of my favorite subjects, and I know a little about birds, so I expected what I would see during the dissection to be familiar. Mauck skinned the raven and showed us the bird’s now skinless abdomen, and I saw what I expected - large pectoral muscles and some fat stored in the clavicular region. After we all got a good look, Mauck proceeded to cut open the abdomen to get tissue samples and see what the stomach contents were.

First cuts into the raven. Photo credit: Sandra Lewocki

As he sliced open the body, all of us were shocked by what we found.

Raven being skinned. Photo credit: Sandra Lewocki

Six eggs were inside our raven! The eggs were in varying stages of development. The most developed egg had a thin shell around it while the other five did not. Since the bird had been in the freezer for so long, the eggs were hard and we got to see them clearly, which is rare. Even Mauck, who prepares birds all the time, was surprised. He said that normally you don’t see many eggs, and if you do, they tend to pop into a gooey mess right away.

The most developed egg and its shell. Photo credit: Sandra Lewocki

I knew birds laid eggs, of course, but I never thought about how that happened. Mauck explained to us that bird genitalia is completely different than that of mammals. Both male and female birds of most species reproduce, urinate, defecate, and, if female, lay eggs through the same opening called the cloaca.

During mating season, the cloaca swells, and male birds temporarily store sperm in their cloaca. While bird courtship is notoriously long and involved, actual intercourse is short, taking about a minute. During mating, the male bird balances on top of the female, who moves her tail out of the way, allowing the male to briefly touch his sperm-saturated cloaca to hers, just long enough for sperm transfer. (Aquatic bird anatomy and intercourse is slightly different.) The sperm enter the female’s sperm-storage tubules and will then fertilize eggs as ovulation occurs. Birds may mate several times during the season to increase chance of fertilization as only1-2% of sperm that enter the female’s cloaca make it to the sperm-storage tubules.

The raven and its eggs. Photo credit: Sandra Lewocki

Female birds only have one ovary and oviduct, usually the left one, with the exception of raptors which have two. No one knows why they have only one, but it means most birds can only lay one egg per day. Once the females have sperm to fertilize their eggs, eggs develop in an assembly line. First the newly-fertilized egg is released from a follicle of the ovary to the oviduct. In the oviduct, layers of yolk are formed to provide food for the young embryo. Then the egg goes to the isthmus where the shell membranes are formed. Once that’s done, the egg moves to the uterus where the hard calcified shell is made. Colors or patterns are also added in the uterus. Finally the egg travels out the cloaca and is laid.

Egg development in the body is extremely fast, taking about 24 hours per egg. Most of the embryotic development happens outside the mother’s body, after the egg is laid. Laying eggs rather than carrying their young inside the mother’s body works well for birds since it keeps the mother from becoming too weighed down to fly.

Diagram by blog author

The raven we dissected in the AMNH classroom had one large egg with a thin shell and several eggs in earlier stages of development without shells. The raven died at a bird rehab facility near Berkeley, California of a broken wing before it could lay the eggs. The bird’s life ended, but the dissection doesn’t mark the end of the story or the end of our study - the skin and tissue samples will be kept at AMNH for researchers to learn from, both now and in the future.