gradient. Nests of loosely arranged sticks, rocks, or dirt are cooler in temperature at the bottom where the egg contacts the nesting material.Such nests also act as humidity regulators by allowing rain to drain into the bottom sections of the nest so that the eggs are not in direct contact with the water.. As the water that collects in the bottom of the nest evaporates, the water vapor rises and is heated by the incubating bird , which adds significant humidity to the incubation environment. In artificial incubation programs,aviculturists remove eggs from the nests of parrots and incubate them under laboratory conditions. Most commercial incubators heat the eggs fairly evenly from top to bottom,thus ignoring the bird's method of natural incubation, and perhaps reducing the viability and survivability of the hatching chicks. When incubators are not used, aviculturists sometimes suspend wooden boxes outdoors to use as nests in which to place eggs. In areas where weather can become cold after eggs are laid, it is very important to maintain a deep foundation of nesting material to act as insulator against the cold bottom of the box. If eggs rest against the wooden bottom in extremely cold weather conditions , they can become chilled to a point where the embryo can no longer survive.Similarly, these boxes should be protected from direct sunlight to avoid high temperatures that are also fatal to the growing embryo. Nesting material should be added in sufficient amounts to avoid both extreme temperature situations mentioned above and assure that the eggs have a soft, secure place to rest. 21. What is the main idea of the passage? (A) Parrot eggs are difficult to incubate in laboratory conditions. (B) Aviculturists struggle to replicate natural incubation methods for parrot eggs. (C) The survival of parrot chicks depends on humidity and temperature control. (D)Wooden boxes are an effective alternative to natural nests for parrot eggs. 22. According to the passage, how do nests in the wild help regulate humidity? (A) By allowing water to evaporate from the top of the nest (B) By using nesting material that absorbs water (C)By letting rainwater drain to the bottom, where it evaporates and creates humidity (D)By maintaining a uniform temperature in all parts of the nest 23. What does the word "gradient" in the first paragraph most likely mean? (A) A process of incubation (B) A consistent change in temperature 24. Where in the passage is the information about how nests protect eggs from direct contact with water?