Ovaries:
- Birds: Most birds have only a left ovary due to the degeneration of the right ovary during development. The ovary produces ova (egg cells) and female sex hormones.
- Mammals: Mammals have two ovaries, located on either side of the uterus. They produce ova and sex hormones such as estrogen and progesterone.
Oviducts:
- Birds: Birds have a pair of oviducts, one leading from each ovary to the cloaca (the common opening for the digestive, urinary, and reproductive systems).
- Mammals: Mammals have a pair of Fallopian tubes, which are the oviducts that connect the ovaries to the uterus. These tubes serve as the sites for fertilization.
Fertilization:
- Birds: Fertilization occurs internally when sperm from the male bird passes through the cloaca and travels up the oviduct.
- Mammals: Fertilization usually occurs internally inside the female's reproductive tract, specifically in the Fallopian tubes.
Eggs vs. Live Birth:
- Birds: Birds lay eggs after the fertilized ova move along the oviduct and become covered with layers of albumen (egg white), shell membrane, and finally a hard calcium carbonate shell. Embryo development and hatching occur outside the female's body.
- Mammals: Most mammals give birth to live young after the fertilized ova, called zygotes, develop into embryos and implant in the uterus, allowing for the exchange of nutrients and oxygen through the placenta. Some mammals, such as the platypus and echidna, lay eggs and are referred to as monotremes.
Uterus:
- Birds: Birds do not have a uterus in their reproductive system.
- Mammals: Mammals have a well-developed uterus, an organ with a thick muscular wall, where the fertilized ovum implants and grows into an embryo and later a fetus during gestation.