Zoroastrian tradition holds that demons enter bodies upon death. The only way to purify these bodies is to give them to the birds.
The outermost ring is for men, the second ring is for women, and the innermost ring is for children.
The bodies remain there for about a year.
Over the course of that year, the corpses are baked by the sun and picked at by birds.
The bones are then placed in the ossuary pit below.
That's where they ultimately rot and become one with the soil.
The towers are kept away from surrounding villages.
It may seem like a bizarre practice to us, but is feeding your dead to the birds any weirder than burying them in the ground? Examining funeral rites gives us a fascinating look into the human race's obsession with death.
Source: http://www.creepybasement.com/tower-of-silence/