Bird Aggressive Defenses

Many small birds furiously harass predators, careful remaining just beyond the enemy's grasp. This harassment appears to make the larger bird (or other predator) uncomfortable enough to leave the immediate vicinity.

Since diving at a predator does entail some risk, many small birds join in what is known as mobbing. Mobbing birds surround a predator, calling incessantly from nearby branches, the ground or the air. Even if a predator is not driven away, mobbing is probably beneficial because it is safer to at least know the location of the predator.

Chickadees are enthusiastic mobbers, giving their buzzy calls as they flit from one perch to another around a predator. Their behavior usually attracts nearby birds of other species to join in the commotion. For an owl trying to sleep quietly at midday, all the attention and noise may be disturbing enough to cause it to find another spot for its perch. Bird-watchers can take advantage of the mobbing behavior of many songbirds, giving a lispy call (pish-pish-pish), which draws birds in for a close look at the trespassing predator.
Although it is unlikely to harm the Merlin, the constant harassment by this Black-billed Magpie will be enough to drive the predator away, eliminating it as a potential risk.