When a living cell acquires another organism that eventually becomes a part of this cell, this process is known as primary endosymbiosis.
When an organism that has already undergone primary endosymbiosis engulfs another eukaryotic organism then this process is known as secondary endosymbiosis.
Secondary endosymbiosis has been known to occur in plastids because of which they are surrounded by two membranes. This bilayer is present because one of the layers is the original outer membrane of the organism and the other membrane is that of the organism that has been engulfed.
The reason is because of the theory "the path of least resistance." The bird is not touching anything else, so it doesn't become our fried birdie dinner.
