|MadSci Network: Zoology|
Thanks for your question concerning the fertilization of reptile and bird eggs. I am sorry that I took so long to respond.
The fertilization of the egg and the shelling/laying of the egg in both reptiles and birds are somewhat independent processes. The fertilization process is similar to that of mammals. In reptiles, the male inserts his sperm into the female's cloaca (a chamber into which both the reproductive and excretory systems empty) via a copulatory organ. Birds achieve internal fertilization without the use of a copulatory organ through a process sometimes referred to as cloacal apposition. From the cloaca the sperm travel to the oviduct where they encounter and fertilize an egg.
As the egg passes through the oviduct, various coatings and membranes are added. A shell has been added once the egg reaches the end of the oviduct (or the uterus in birds.) Finally the egg is laid. In birds this process happens one egg at a time, while in reptiles several eggs can be shelled and laid at once.
Usually the timing of these two processes coincide (through hormonal and behavioral cues) such that the eggs are fertilized before they start the shelling/laying process. It is rare that an egg is not fertilized. (There is almost always enough sperm to go around!) However, in the event that an egg is not fertilized, it may still undergo shelling and laying. This is common in domesticated fowl. Thousands of years of artificial selection have produced birds which will shell and lay unfertilized eggs throughout the year.
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