Irruptions

An irruption is a birding term referring to an unexpectedly large increase in the population of a bird species into an area where they are not normally seen.  Irruptions are caused by a disturbance in the natural environment of the species, many times due to a food shortage.  Depending on the root cause of the disturbance, more than one bird species may be affected. 

Mast refers to the fruit of woody plants, such as acorns, nuts and berries.  In some years, plants produce larger than normal quantities of mast, and in other years the production amount is meager.  Many species depend on this mast as their main food source.  One theory about why the amount of seed varies suggests that in years of abundant production, the number of seeds overwhelms the number of seed-eaters, and the plants have a greater chance of successful reproduction.  In years when very little seed is produced, lack of food thins out some seed-eating species and forces others to move.  Many bird species rely on mast as their main food source, and when the food becomes scarce, these species move farther afield, irrupting into new locations.

In 2020, we have already experienced several irruptions across the North American continent.  Cone-bearing trees in boreal forests in Canada produced small seed crops this year.  There is not nearly enough mast to feed the populations of birds that spend the winter in these areas.  Most of the affected species will head south into the United States, fanning out over broad areas in search of other food.  Nuts, seeds and berries from other trees may replace what has been lost, but many species will be opportunistic and eat from backyard feeders when possible.

Pine Siskin, Spinus pinus, normally move into southern Canada and the northern half of the U.S. in winter.  Overwhelming numbers have been reported by observation stations, especially along the east coast, and birds have been spotted as far south as the Gulf of Mexico.  Their normal winter range includes northern Illinois, but not in the high numbers that we are currently observing.  They are about the same size as American Goldfinch, with dark and light brown streaking covering their entire body and distinctive bright yellow wing and tail edges.  Pine siskins prefer evergreen forests, but will eagerly feed on thistle seeds from feeders.

The Red-breasted Nuthatch, Sitta canadensis, is another species seen here in smaller numbers in winter, but also heavily affected by the reduced amount of food.  This species is irrupting both north and south, with some populations heading north into Alaska, and many more individuals heading south into all parts of the United States.  As with the Pine Siskin, higher numbers of birds are being reported in our area for the second winter in a row.  These birds have blue-gray backs with rust-colored breasts and a black-capped head with a bright white stripe above the eye.  One of our smallest birds, they have a very short tail and short, broad wings on a plump body.  They prefer coniferous woods and mountain areas, but in winter, they can be seen on backyard feeders offering black-oil sunflowers seeds, suet, and peanuts.

Blue Jay, Cyanocitta cristata, annually migrates by the thousands through the Great Lakes and along the east coast heading for winter habitat in the southern U.S.  Smaller populations may remain along the migration route, but this year, we are seeing greater numbers in many places.  One of our largest feeder birds, they sport a large crested head and a broad, rounded tail.  Blue Jays have a white or light gray belly and black and white markings against a bright blue back and head.  Their preferred habitat is along forest edges, especially where there is an abundance of beechnut, hazelnut and acorns.  In backyards, they are most often found on tray feeders offering peanuts, sunflower seeds and suet.

Bohemian Waxwing, Bombycilla garrulus, may not be as familiar in our urban and suburban areas, but it is also experiencing an irruptive year.  These medium-sized birds are plump, with a thick neck and a short, squared off tail.  They are grayish brown overall with a striking black eye mask shaded along the edges in dusky orange.  The underside of the tail is rusty, and the tip is bright yellow.  Secondary wings on adults have bright red wing tips.  These birds move frequently in search of food sources, making them unpredictable to find; and we are at the south end of their typical winter range.  Keep an eye out for flocks as they are being pushed farther afield this year in search of food.

Irruptions can be an exciting event for bird watchers who are able to observe species that migrate outside of their normal range when conditions necessitate such movements.  An irruption may be difficult to identify, but if you observe larger than normal groups of any species, especially in late autumn, that is a clue that there may be an irruption in progress.  Keep abreast of current bird migration monitoring at sites such as the Finch Research Network or Bird Cast.