There are two ways of documenting the migration patterns of birds, firstly the animal has to be captured. 

Mist nets are erected with large spans, difficult to detect when flying allowing ornithologists to gather bird life. 

>The first and most common way of documenting birds is tagging. Technological models have been trialed and tested but the original system has never been seen as the more efficient and easier method. 

>The second which requires a lot more technology, is related to the physical condition of the bird. Taking one feather can identify the origin of the bird through the variations in ratio of the stable hydrogen isotopes across the latitude.