The dog follows a track. And in general a track consists of three parts
1) the individual scent of the animal
2) Blood and other “parts” of the animal like bones, hair etc
3) “destruction” of the ground
Each animal has its own smell. No animal smells like the other and the dog is able to distinguish between them. That is why a tracking dog is able to follow the right one, even if it is part of larger group.
Blood and other parts
Blood and other parts that might be at the shooting sight or within the track are not important for the dog. They are more important for the tracker. The kind of blood, where it is found, the way it falls to the ground etc. shows, where the animal is hit and which organs are wounded or destroyed. This is important for the tracker to get an idea, what to expect during the track.
“Destruction” of the ground
This is the most important of all three parts. To understand why, it is important to have a closer look on what happens, if an animal puts his foot on the ground and we have to have in mind, what the dog´s nose is able to smell. If a foot touches the ground it is pressed into the ground. How far depends on the consistency of the ground and the weight of the animal. What now happens is, that the foot in combination with the animals weight crush the soil, plants and bacteria. That causes a smell that is different from the smell around the footprint. When the animal moves forward small parts of the ground stick on its feet. These small parts are brought into the next foot-print. This is the reason, why the dog is able to recognize in which direction the animal is moving, even if it went back in its own track for hundred yards or more, what they sometimes do. On a muddy surface, we can see these footprints but the same happens on hard surfaces like streets, which are often used by wounded animals.