Learning Birds - Topography application has three different passerine and gull illustrations with the birds field marks and feather groups identified. The application also contains a glossary of field marks and feather groups contained in the illustrations. You can use the illustrations to learn the different field marks, test your knowledge of the field marks or as a reference of the different field marks of a bird. Birders deal with feathers and knowing the different feather groups and field marks of a bird can be one of the most important tools a birder can have when trying to identify a bird by sight.