What is the meaning and proper usage of the term "irruption" in bird behavior?

Question

The author of a question on this site wondered if they had possibly seen a snowy owl.

A quoted source in this answer to that question used the word irruption in part of a sentence about where the owls spend the winter.

Another answer to the same question also used the word irrupt. Here is the pertinent part:

Snowy owls sometimes irrupt southward in substantial numbers (unpredictably), so it will be interesting to see if this is the beginning of a big winter for snowies.

I'm interested in the definition and usage of irruption, and what it means in the context of bird behavior and bird watching.

I understand that birds, including owls, have migration patterns. Is irruption a migratory behavior? If not, what makes it different?

Is it something that happens only with owls, or with other birds too? Can a layman tell by observing a bird if it is one that irrupts, and, if so, whether or not it's in a period of irruption?

Answer 1

I am not sure where you live, Sue. But, as a birder in NE Ohio, I watch this website in the fall as do a lot of other birders:

http://www.jeaniron.ca/2017/wff17.htm

This predicts the amount of food seed-eaters will have in Canada, which can give some indication of what northern birds will come to Ohio.

Now, it isn't an exact thing as this current fall/winter is showing. The forecast was that there was plenty of food up north. But, we are already having a number of reports of pine siskins and crossbills. Crossbills are usually very uncommon in NE Ohio. But, there are at least two flocks being reported right now. One is west of Toledo in Oak Openings, and the other is near a reservoir just south of Mansfield.

We are also experiencing a number of snowy owls all along the Lake Erie shoreline and in some inland sites like airports and farmlands. It is thought that a number of these owls are youngsters who have a tough time up north compared to the adult/more experienced owls. But, there are some adults around right now.

I will add something else that I learned from a number of organizations that do bird banding. It is difficult to determine if birds like blue jays and chickadees are visitors from Canada to Ohio seeking food (irruptions) or they are just local birds that are wandering since we have those species year-round. There is really no way to tell unless the birds have been banded and are recaptured. (Banding is a low-return activity. Only a fraction of banded birds are ever recovered. But, it is one of the only way to try to track bird movements. GPS/satellite trackers are coming down in size and cost. They may be the future of tracking the smaller songbirds. They are used in some larger birds now like hawks and cranes.) Birds like the siskens, crossbills and others like evening grosbeaks are easy to label as they don't normally live here in Ohio. So, it's a pretty good bet that they are from the north when we see them.

Answer 2

TLDR: Its when a massive amount of birds suddenly migrate all at once to a area they aren't usually found in.

An irruption is a dramatic, irregular migration of large numbers of birds to areas where they aren’t typically found, possibly at a great distance from their normal ranges.

This type of population shift can also be called Malthusian growth, in recognition of the population studies and analyses done by English cleric and demography scholar Thomas Robert Malthus.

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Several factors can lead to irruptive years for different birds. The most common cause is a lack of food in the birds’ normal wintering grounds; famine can force large numbers of birds to seek more plentiful habitats until seeds, flowers and insects return in the spring. Birds that feed on the seeds and catkins of birch, maple, pine, spruce and hemlock trees often irrupt when those types of trees have poor seed crops.

Source

Ornithologists generally concur that irruptions are triggered by food shortages, such as failure of the coniferous cone crops over a large geographic area. Analysis by ornithologists Carl Bock and Larry Lepthien of many years of Audubon Christmas Counts indicate that a synchronization of seed crop failures in some high-latitude tree species leads to southward irruptions of species normally dependent on those seeds.

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Diurnal and nocturnal raptors that feed on small mammals with cyclic population fluctuations constitute another group of irruptive species which also eat foods that fluctuate from year to year in boreal regions. Among North American species, Rough-legged Hawk, Northern Goshawk, Snowy, Great Horned, and Short-eared Owls are known to irrupt periodically. Two main cycles are recognized in boreal small mammals: a four-year cycle among tundra and grassland rodents, and a ten-year cycle that characterizes snowshoe hares.

Source