The level of intelligence in birds, as a scientific inquiry, has not been as thoroughly researched as similar questions regarding primates and other mammals. However, there is a general belief that they are more intelligent, as a class, than the reptiles, and that many species are just as intelligent as mammals of comparable size. Because birds lack forelimbs with which to modify their surroundings, it is often difficult to test for intelligence as we would define it for mammals. Traditionally, biological science has maintained that most actions performed by birds that may indicate intelligence are merely ingrained instinctual behaviours and that birds are unable to learn. While parrots have the distinction of being able to mimic human speech, studies with the African Grey Parrot have shown that some are able to associate words with their meanings and form simple sentences. Along with parrots, the crows, ravens, and jays (family Corvidae) are perhaps the most intelligent of birds.