Blood swallow: It is a kind of cave swallow. Jin Siyan built a nest on the rock wall of the cave. Minerals in the rock wall slowly tunnel into the bird's nest through the contact surface between the bird's nest and the rock wall or through the water dripping from the rock wall. When iron is in the majority, it will show some irregular and faint rust red, so it is called blood swallow.

Because of the uncertainty of environmental factors, this kind of bird's nest will also use metal substances, which will increase the burden on the body and kidneys and is not recommended. At present, the blood swallows on the market are generally artificially dyed and are not recommended to be eaten.