Every winter, backyard birders look forward to possible bird irruptions that will bring new and welcome guests to their feeders. But what exactly is an irruption, and what does it mean for birding?

Defining an Irruption

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 Thomas Malthus.

While one or two vagrant birds of northern species may appear at southern feeders in any year, an irruption is characterized by large numbers of unexpected birds. Depending on the species, irruptions may occur in cycles from 2-10 years, or they may be much more unpredictable.

Causes of Bird Irruptions

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. Different birds of prey may also irrupt when the seed crops are poor and cannot support the necessary rodent populations for raptor food sources.
Other causes for bird irruptions include unduly harsh cold or severe weather that may force birds to find more temperate wintering grounds, or overbreeding that may further deplete even plentiful food supplies.

No matter what the cause of the irruption, however, it is difficult to predict where or when irrupting species may appear.

Birds that Irrupt

Many bird species found in boreal and other northern habitats can irrupt if circumstances require it. Species that commonly irrupt include:

• Pine siskins
• Bohemian waxwings
• Evening grosbeaks
• Boreal chickadees
• Purple finches
• Pine grosbeaks
• Common redpolls
• Red crossbills
• White-winged crossbills
• Northern shrikes
• Red-breasted nuthatches
• Hoary redpolls
• Varied thrushes
• Great gray owls
• Snowy owls
• Rough-legged hawks

The exact circumstances that can prompt an irruption for any of these species will vary, as will the scope of the population shift and where the irrupting birds appear south of their normal ranges. When several species irrupt to the same region in one year, it is referred to as a superflight.

Irruptions in Your Backyard

Many birders welcome irruptions as an opportunity to see unexpected birds at their feeders. When large numbers of birds appear, however, diseases can be spread more easily and birders should take extra precautions to watch for illness. Cleaning birdfeeders and birdbaths regularly is essential to help keep irrupting birds and normal backyard flocks healthy.

Large numbers of some irrupting birds may also bully or intimidate other backyard birds, restricting their access to feeders and monopolizing the seed. Birders can change the types of feeders and seed they offer to discourage such behavior, or they can put up extra feeders to welcome all the new guests to their yard. Spacing feeders out will minimize territoriality and aggression while ensuring that all birds can enjoy the buffet.

Bird irruptions can add excitement to winter birding. While birders should never count on an irruption, it can be a pleasant surprise when northern birds appear at southern feeders.

Photo – Pine Siskin on Feeder © Greg