Red-headed Strangers

Common Redpolls, a sometimes winter visitor to Montana.

This winter, bird watchers from across Montana (and most of the northern states) have reported large numbers of red-capped, Common Redpolls. These are birds that summer in the boreal forests of northern Canadian and typically winter in southern Canada.

During a typical winter, there may only be a few scattered reports of Redpolls in Montana. Not this winter. This sudden appearance in large numbers every few years makes these guys an "irruptive" species. ("Irruption" refers to breaking in, while "Eruption" refers to breaking out, so to speak.)

"Irruptive" typically refers to bird species that normally spend their winters in one geographic region, only to show up some winters in different regions and in unusually high numbers. It's sort of an all-or-nothing proposition for birdwatchers.

Irruptive species tend to be seed eaters, and a review of 30 years of bird and seed numbers pointed towards two main irruption triggers: variation in winter seed crop size, and increasing bird populations. Winter weather was not a major factor in bird irruptions.

One group of irruptive species seems to require a combination of both triggers - a seed crop fluctuation and a population increase. A large coniferous seed crop one year often leads to a population increase due to higher winter survival. When the increased population is confronted by a poor seed crop the following autumn, large numbers of the birds seek a different place to spend the winter. That usually means they keep migrating farther south than normal. This group includes five irruptive species in the west: Pine Grosbeak, Evening Grosbeak, Pine Siskin, Bohemian Waxwing, Red-breasted Nuthatch.

The second trigger for irruptions is an increasing population size that is independent of seed crop failures. Irruptions tend to occur only when a good seed crop the previous year leads to a population increase. The density of wintering birds seems to be more important than the available seed crop. Black-capped Chickadees fall into this group, along with the Redpolls that were enjoyed almost everywhere in Montana this winter.

Inside the irruptions: eBird is an interactive web presence (designed and managed by the National Audubon Society and the Cornell Lab of Ornithology) where birders of all abilities can enter their bird sightings. One of its many features is a nifty tracker that allows you to search irruptive reports by bird species and location.