Why is bird's nest soup red?

It is red because of blood mixed into the spit of the swift. As nests are harvested, the swift has to rebuild its nest and this takes a physiological toll on the bird (because the eggs, which are not yet laid great tax the bird's system).

Some time ago people believed that a red nest was created by an overworked Swiftlet that had built its nest partly with blood. Nowadays, people believe that the red nest is created by a Swiftlet that has eaten seafood or a food substance which contains a high volume of iron. Research has proved both of these beliefs to be untrue. The red nest (red blood) is not created by the Swiftlet, but by the surrounding environment.

Cave nests turn red in colour due to the mineral content and the wet surface of the cave which seeps into the nest. This polluting of the nest by cave water significantly reduces the nutrient levels of the nest. The red appearance of a house nest is achieved through the unethical dying of white nests. The harvest is deliberately delayed to let the gas from the Swiftlet's droppings react with the nest. The gas emitted by the Swiftlet is called Ammonia. Ammonia is a compound found in fertilizer and detergent.

Long term consumption of bird's nest which has been contaminated in this way will have adverse effects on personal health. The consumption of toxic chemicals has strong links to cancer and other major illnesses. This is the reason all bird's nest experts advise expectant mothers and the elderly to avoid the consumption of red nests.
The above is based on researches and reports released by Pureness Bird's Nest