In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.
Q: Article: Have you wondered why different animals or pests have their particular colors? Colors in them seem to be used mainly to protect themselves. Birds, especially seagulls are very fond of locusts, but birds can't easily catch locusts because locusts change their colors together with the change of the color of crops. When crops are green, locusts look green. But when crops are ripe, locusts take on exactly the same brown color as crops have. Some other pests with different colors from plants are usually easily found and eaten by their enemies. So they have to hide themselves in terror for lives and appear only at night. If you study the animal life in any part of the world, you will find the main use of coloring is to protect themselves, bears, wolves and other beasts move quietly through forests. They are usually _ to the eyes of hunters, because they have the color much like the barks of trees. An even more strange act remains to be noticed. A kind of fish living in seas can send out a kind of very black liquid when it faces danger. While the liquid is over, its enemies cannot find it, and it quickly swims away. Thus, it has existed up to now though it is not powerful at all.
Answer: the protective coloration of animals and pests
A:
This passage mainly talks about _ .