Blood Swallow, a kind of bird's nest, is dark, fluffy and bloodshot, and pink. Therefore, it is called "Blood Bird's Nest", and its quality is slightly inferior to that of Bai Yan. There are different opinions that bird's nest is blood red. One statement: When the nest built by Jin Siyan for the first time or two is taken away, it will drive to another nest. At this time, not only are there few hairs on their bodies, but there is even less saliva. Only when they fly to the seaside, they take seaweed, moss and other plant fibers bite by bite and mix them with a small amount of saliva, and this kind of bird's nest is dark red in color. Another saying: Blood bird's nest is dyed blood red because it is moistened by blood red water oozing from the cracks in the rock. Thirdly, the red color of blood swallow comes from the food eaten by Jin Siyan, such as branches and leaves, which makes the bird's nest blood red. Although there are different opinions about the formation of "blood swallow", one thing is certain, that is, its nutritional value is not as rich as that of Bai Yan.