Thursday, February 26, 2015

On the Mobbing of Owls

The photos in this blog post show six owl species from five genera being mobbed by potential prey species. Although only one individual of each prey species is visible in the photo, other individuals were present during the mobbings and sometimes multiple species were present. Mobbing is when prey species, usually in groups, harass a predator using loud calls and possibly also stereotypic movement sequences (that can recruit other birds to join the mobbing party). Note that mobbing is an interaction between predator and prey and is different to a competitive interaction as may occur between an owl species and a hawk species for example. Mobbing sometimes succeeds in driving a predator away from an area, which is an obvious benefit, but it also comes with risks. These risks may be higher when mobbing certain predators. Mobbing nocturnal owl species during the daytime is probably less risky than mobbing diurnal raptors and is sometimes successful in driving the owl away from a roost site; however birds that mob owls are sometimes killed by the owl they are mobbing. Although many owls seem disturbed by mobbing, some owls might benefit. One of the several competing hypotheses as to why certain species have large “false face” markings on the back of their head is to evoke a mobbing response that subsequently provides either a prey assessment opportunity or a hunting opportunity. Mobbing is not restricted to owls of course, nor is it restricted to nocturnal owl species (one of the photos above shows a day-active Northern Hawk Owl being mobbed). Sometimes recently fledged juvenile owls get mobbed and if they are knocked from their perch during the mobbing the result can be fatal (the owlets are vulnerable on the ground).

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)    
Mobbing a day-active owl is also risky business but this nimble Gray Jay got the owl's attention but did not encounter any problems, nor did the Northern Hawk Owl budge from the perch. Although I have seen Gray Jays mobbing Northern Hawk Owls on relatively few occasions, I have never seen them mob Great Gray Owls and I suspect that may be because Great Gray Owls are so specialised in the prey selection that they almost never take birds.

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 
Owls are mobbed by more than just songbirds as this Amazilia Hummingbird demonstrates. This is another case of one of those highly–feared pygmy-owls being mobbed by nervous prey species.

Tengmalm’s Owl, also known as Boreal Owl (Aegolius funereus) being mobbed by Red-breasted Nuthatch (Manitoba, Canada 
In North America, chickadees and nuthatches are amongst the most frequent of mobbers and they seem to understand each other’s alarm calls and readily mob owls together.
Of course, these are only a few examples.
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