Bird is one of the most abundant and widespread groups in the world. With many specialized structures, such as plumage, air sacs, and hollow bones, most bird species have got flight ability to adapt various niches. Therefore, bird can migrate between wintering and breeding ground, which is usually a long distance, and it's called migration. Bird navigation is important in migration and is a complex process, which attracts many scientists to dig in how bird finds its way. Since 1873, Charles Darwin has ever mentioned that bird might take the method of dead reckoning on a long-distance migration like human, but at that time no one made further progress. Until 1950s, Kramer firstly found that Common Starling (Sturnus vulgaris) can respond to solar azimuth via mirror test. From then on, many experiments revealed that at least four navigation mechanisms are used in bird migration via more than nine external factors. They are: (i) celestial navigation, celestial clues (e.g. solar azimuth, star position, and polarized light) are used during migration period. (ii) Olfactory navigation, odor distributing in the air forms odorous gradient map or mosaic map which can be detected, or can activate directly certain mechanism to navigate. (iii) Auditory navigation, infrasound (0.05 Hz) produced by mountains and rivers generate sonic gradient map. And (iv) magnetic navigation, geomagnetic field can be detected via magnetic materials or chemical magnetoreception to find correct directions. Although many scientists approve that magnetic navigation may be the main mechanism to orientate and navigate, bird has never taken just one mechanism to migrate. Indeed, many species also use the other three mechanisms to calibrate direction, for example, Savannah Sparrow (Passerculus sandwichensis) can use polarized light to calibrate the magnetic compass at both sunrise and sunset. Different external clues correspond to different sense organs, so various brain areas should deal with information from different navigation mechanisms. The hippocampus participates in spatial perception and manages anything about celestial navigation via the tectofugal visual pathway and the thalamofugal visual pathway. The piriform cortex (CPi) is the main area to receive stimulation from olfactory bulb and determines how to migrate after receiving olfactory clues. Nervous systems of magnetic navigation include two parts which are trigeminus system and Cluster N. Despite the controversy whether there are some magnetic materials on bird, many experimental evidences have proved that magnetic materials detecting geomagnetic field involve to Trigeminus system. Cluster N, however, is an active area when bird migrates at night and it has an important role in transferring information from chemical magnetoreception to the hippocampus. As illustrated above, navigation mechanisms can get full information from many clues, and then, different brain areas trade off those and co-operate each other to make an elaborate map. Bird navigation involves the receptors to environment and the response of nervous system, so many issues are still maintained. The exact mechanism will be revealed with the new techniques and model animal applied. © 2017, Science Press. All right reserved.
