Answer:
Biologists have tried to distinguish the living things from non-living things and suggested eight characteristic features unique to living organisms.
The eight characteristics are: cellular, contain heredity material, perform metabolism, can reproduce and produce progenies, maintain homeostasis, sensible to the environment, show growth and development and can adapt to the environment.
Since, bacteria, birds and trees show this unique characteristic like they all are made up of cells, contain genetic material and others, therefore, are considered living organisms.
Viruses are considered non-living beings as they do not show the cellular characteristics like lack organelles, lack metabolism and cannot reproduce on their own.