Updated: Apr 3, 2022
Commensalism is a relationship between 2 organisms (bird, plant or animal) where 1 organism benefits while the other organism remains unaffected.
It is often used as a replacement for symbiotic relationships, but that is not entirely correct. In a symbiotic relationship, either both species benefit, one species benefits at the expense of another or neither species benefits.
The most common example of commensalism is scavenging
In these photos, a tiger killed a gaur (bull) and left it by some bushes. Crows and mynas flocked to the carcass to eat the remains.
In this situation, the birds benefit by gaining food, while the tiger remains unaffected
Another example is when birds live/build nests on trees.
The bird benefits by gaining a home (the nest) while the tree remains unaffected.
This picture is not commensalism.
When the bird removes the ticks from the deers back, both the organisms benefit. The bird gains food (the ticks) while the deer get rid of uncomfortable itching.
Therefore, as both species benefit, this is a symbiotic relationship and not commensalism.