In the case of owls, the loud mobbing calls of diurnal songbirds are often the first clue for the human observer that a nocturnal owl is roosting nearby. Therefore, if you learn to recognise the mobbing calls of bird species that are common in your area you will increase your chances of locating owls.
We really don’t know why small birds mob owls but there are many hypotheses to consider. Curio (1978) listed nine hypotheses as follows: silencing-offspring, selfish herd, confusion effect, move on, perception advertisement, alerting others, attract the mightier, cultural transmission and site avoidance. These have been grouped into three main “classes” of hypothesis: parental care, altruistic or selfish (Ostreiher 2003). At this point, we don’t have enough evidence to evaluate which of these hypotheses is correct and therefore detailed observations of mobbing behaviour are especially useful. Especially in situations where you can distinguish the age or sex of prey birds, your observations may be informative. In the very rare situation where the observer understands the kinship amongst mobbers, there is also potential for garnering new information (kin selection is a possible mechanism to consider in behaviour of this type).
Eastern Screech-Owl (Megascops asio) being mobbed by Blue Jay (Manitoba, Canada)
The crow family are some of the most frequent and most ardent mobbers of owls and they are capable of causing an owl to flee. This male Eastern Screech-Owl retreated into his roosting cavity when mobbed by Blue Jays (the female and large nestlings remained ensconced in the nesting cavity nearby).
Great Horned Owl (Bubo virginianus) being mobbed by American Crow (Manitoba, Canada)
Another example of a crow family mobbing owls. Crows are aggressive but tangling with a Great Horned Owl is risky business (I once observed a Great Horned Owl that was apparently fleeing from a flock of mobbing crows perform a near barrel-roll manoeuvre and grab a crow in flight, then killing and eating the crow on the ground).
Northern Hawk Owl (Surnia ulula) being mobbed by Gray Jay (Manitoba, Canada)
Pearl-spotted Owlet (Glaucidium perlatum) being mobbed buy Senegal Eremomela, Cameroon
Mobbing a day-active Glaucidium species (pygmy-owls) is probably the most risky of all. These tiny owls are lethal predators and they strike with surprising speed. Some authors have hypothesised that the false-face on the back of the head of some pygmy-owls incites mobbing and that the owls use this to their advantage.
Peruvian Pygmy-Owl (Glaucidium peruanum) being mobbed by Amazilia Hummingbird, Peru
Tengmalm’s Owl, also known as Boreal Owl (Aegolius funereus) being mobbed by Red-breasted Nuthatch (Manitoba, Canada
Of course, these are only a few examples.