Parasitism occurs when one organism lives off another organism that gets no benefits. The parasite may steal the other’s resources (for example, their stored food), or directly feed off the organism itself (for example, their hair or blood). The parasite does not need to stay physically close to the other organism.
One peculiar form of parasitism is carried out by the cuckoo. It tricks other bird species into doing all the labour of raising the cuckoo’s own progeny.
Birds can be parasites too! Here a robin is the victim of a cuckoo’s brood parasitism (that is, the cuckoo tricks other bird species into raising the cuckoo’s own progeny). Photo: Alandmanson, CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons).