Bird reproduction is a sexual process, with both a male and female necessary for producing offspring. Male birds have a pair of sex organs called testes, that produce sperm, and female birds have a single ovary, that produces eggs, on the left side. Once the male fertilizes the eggs of the female, the female lays the fertilized eggs. The number of eggs the female will lay caries by species. A clutch of eggs may have as few as only one egg to as many as twenty eggs. Birds that live in high altitude areas tend to lay more eggs than those in lower altitudes. Eggs in large clutches tend to be smaller than those in larger clutches, although this is not a hard and fast rule. Regardless of the size of the clutch, female birds generally lay only one egg per day, usually in the early morning hours. The developing cell of the egg, the yolk, is covered by layers of jelly-like albumen, two layers of membrane, and a hard shell. The albumen feeds the growing bird embryo, while the membranes and shell support and protect it. These fortifying and protective layers develop around the yolk as it moves from the hen’s ovary out of her body. The egg must be fertilized before the shell, the last layer, is added to the egg. Once the hard shell is on the egg it cannot be fertilized. Eggs can be as small as 1/3 of an inch long, in the case of hummingbirds, or as large as the 13-inch long eggs that the now-extinct elephant birds from Madagascar once laid. Although hummingbird eggs may seem extremely small, most small birds lay eggs that are very large in proportion to their bodies. In the case of a hummingbird, the average egg is around 15% of the bird’s total body weight. Larger birds may lay eggs that are only 2-5% of their body weight. Just as the size of the eggs vary by species, so too do the shape and color of the eggs. Birds that nest on the ground tend to have eggs that will be camouflaged in the grassy areas in which they nest. Similarly, birds that nest in outcroppings of rocks high off the ground will have eggs shaped so that they roll in tight circles if they fall out of the nest, to prevent them from rolling off the ledge. A vital part of bird reproduction is incubation. Most bird eggs are incubated by either the mother or father, who sits on the eggs in the nest to keep them at the optimal temperature for development. Incubation periods vary by species of bird and can last anywhere from around 10 days to around 75 days.