Survival in the wild often depends on the predator NOT knowing the location of the bird. In addition, the absence of song in the male canary stops courtship and mating, insuring only the fittest birds reproduce. When male canaries don't sing they are either molting or have a health problem. Canaries normally sing year-round except during the molting period and a short time thereafter. Song stops because of the physical and nutritional stress from growing up to 2,000 new feathers.
Canaries actually announce problems to their owners by being quiet.
When this happens, see your veterinarian. In captivity, the most common abnormality to affect singing in male canaries is nutritional deficiencies. As with any other animal, canaries require balanced diets, but the seed diets traditionally offered to canaries are lacking in vitamins, minerals and proteins. Further, neither canaries or other birds living in the artificial confines of captivity have the instinct or ability to pick out a balanced diet from an assortment of seeds, minerals, vitamins and supplements placed in the cage. It is common for canaries in homes to sing well for 2-3 years and then to have a prolonged molt and never return to full song.