The difficulty of defining or measuring intelligence in non-human animals makes the question of avian intelligence resistant to scientific study. In general, birds have relatively large brains compared to their head size. Furthermore, bird brains have two to four times the neuron packing density of mammal brains, for higher overall efficiency. The visual and auditory senses are well developed in most species, though the tactile and olfactory senses are well realised only in a few groups. Birds communicate using visual signals as well as through the use of calls and song. The testing of intelligence in birds is therefore usually based on studying responses to sensory stimuli.