Egg embryo is adequately developed it hatches, i.e. breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to crack, pip, or break the eggshell or covering. The largest recorded egg is from a whale shark, and was in size. Whale shark eggs typically hatch within the mother. At and up to , the ostrich egg is the largest egg of any living bird, though the extinct elephant bird and some dinosaurs laid larger eggs. The bee hummingbird produces the smallest known bird egg, which weighs half of a gram (around 0.02 oz). Some eggs laid
Preening (bird) sail-shaped secondaries of the drake Mandarin duck) or distinctive color (such as the ) on the bird. Preening may also be performed as a displacement activity. In some cases, it is done in place of another activity that birds are strongly motivated, but unable, to do. In one study, black-headed gulls which were prevented from incubating a full clutch of eggs (by the removal of eggs from their nest) responded by preening and nest building—both displacement activities. When all three eggs in their regular clutch were removed, the gulls showed a significant increase in the amount of time they spent