Scientists have argued for long over whether birds show real intelligence. Some scientists believe birds have abilities including communication and different kinds of memory. In some unusual cases, their abilities seem better than those of humans.      Irene Pepperberg, a birds expert, did research about a grey parrot named. Griffin. Ms. Pepperberg says Griffin can arrange objects in order of size. She says the talking bird also can combine words in the right order. For example, he will combine words when asking for a piece of food. The researcher says experts had thought that only humans and other mammals with large brains have the ability to combine objects and words. She believes that bird brains have the ability to understand that complex  tasks must be done in the correct order.      Some birds have other memory skills. For example, they collect and store thousands of seeds in autumn, and find them later in winter. Alan Kamil and Alan Bond of the University of Nebraska are studying the memories of birds called jays and nutcrackers. Their experiments show that these birds use natural objects to find the seeds they have stored.      It's said some birds can learn as many as 2,000 different songs. Songs may have developed as a way for birds to communicate with other birds.      According to Verner Bingman of Bowling Green State University in Ohio, birds must have a special guidance system in their brain. He says that understanding how a bird's brain operates may help us better understand how a human brain processes information.      However, some scientists do still believe birds do many things that appear to be acts of thoughtful intelligence. In most cases, they say, their behavior is simply instinctive , no reasoning power directing it. What is the main subject discussed in the passage?
The intelligence of birds. Irene Pepperberg's study on birds. The difference between humans and birds. Ways for experts to train birds.