Some birds can cause quite an irruption

It occurred to me while lying in the recovery room at St. Mary’s Hospital in Rochester, Minn., (carpal tunnel surgery — too much keyboarding through the years) two weeks ago Thursday and several hours after the Tribune’s deadline for the outdoors page that week (‘Oh, no!’ or something along that line) that in writing about backyard birds, I had used a term not necessarily familiar to all readers, especially those with limited interest in birding.

The word was “irruptive” and I failed to define it while describing a couple different species birders might see this fall and winter. The term “irruptive” describes various species of northern-wintering birds that don’t always show up this far south, or, if they do, they do so in relatively small numbers.

During an irruption, which is caused primarily by low food availability, the population density of one or more species of birds changes suddenly and sometimes dramatically.

For example, pine siskins, a small member of the finch family, routinely show up in central Iowa in small numbers, but on years when there is a shortage of their primary food source — conifer seeds — larger than normal numbers of pine siskins will “irrupt” and travel outside their regular northern wintering range in search of food.

“They’re an annual winter bird that show up here in small numbers,” said Steve Dinsmore, an avian ecologist in the Department of Natural Resource Ecology and Management at Iowa State University. “But every few winters, there is a major irruption.”

Dinsmore said when pine siskins that can’t find enough conifer seeds show up in places such as Iowa, they shift their focus to birdfeeders where they feast on Nyger and sunflower seeds, the favored food choices of goldfinches, as well.

Doug Harr, president and chief operating officer of Iowa Audubon, said pine siskins are showing up in good numbers all across Iowa this fall.

“I’ve had a flock of eight at our Ogden feeders, more than we’ve ever had in this little prairie town,” he said.

Another member of the finch family that’s an annual “regular” irruptive species is the purple finch, which, because of similarities in its appearance, is sometimes confused with the house finch, a species that’s a year-round resident of Iowa. Dinsmore said purple finches always show up in the state in at least small numbers.

“Their numbers are less variable from year to year than most other irruptive species,” he said.

Purple finches prefer conifer seeds and mountain ash berries, but when they travel south into Iowa, Dinsmore said, they are most attracted to sunflower seeds at feeders.

Another regular irruptive species familiar to many birders is the red-breasted nuthatch, a smaller cousin of the common white-breasted nuthatch, a year-round resident of Iowa.

In their boreal homes in the north, these perky little birds prefer conifer seeds, especially from small-coned species. But if one shows up at your feeder here this fall and winter, it will be after sunflower seeds, peanuts and suet.

“Red-breasted nuthatches have been seen in good numbers this fall, although some are just passing through on their way further south,” Harr said. “But many are also still being reported at feeders and in evergreen trees, so we will likely see quite a few this winter.”

One regular irruptive species not likely to visit your yard this time of year unless you live in the country is the rough-legged hawk, a raptor that prefers to nest on cliffs in the tundra during the summer before traveling afar during the winter in search of ample food supplies.

“There are always a few, but they can be fairly common some winters,” Dinsmore said.

Rough-legged hawks feed primarily on small rodents in the winter, such as mice and voles.

Another occasional visitor from the far north that prefers open spaces where it can hunt is the snowy owl, a raptor Dinsmore describes as a “rare irruptive species.”

“We get at least a few most winters, but a major irruption occurs less than once per decade,” Dinsmore said.

Fortunately for avid birders, Dinsmore and Harr both said this winter may be an exceptionally good one for seeing snowy owls.

“National Audubon just posted an article … about snowy owls and the fact that this could be a really big winter for them in the states,” Harr said. “Several have already been seen in Iowa, but while Audubon’s story relates to a large population (of snowy owls this year), they fail to mention possible tundra food shortages, which could also be a partial cause (for the irruption).

“Some of the owls seen in Iowa so far are in poor shape or have even died.”

Like the rough-legged hawk, snowy owls feed on rodents, probably mostly meadow voles in Iowa, Dinsmore said.

Three other rare irruptive species to Iowa worth mentioning are the Bohemian waxwing, red crossbill and white-winged crossbill.

The Bohemian waxwing, not to be confused with its more common cousin the cedar waxwing, is “very rare to absent most years, and during an ‘invasion,’ there may be 105 reports statewide,” Dinsmore said.

Bohemian waxwings feed on fruit, mostly ornamental trees, such as highbush cranberry, in towns in Iowa, he said.

When it comes to the crossbills, which are members of the finch family and feed on a wide range of conifer seeds, Dinsmore said there is typically “at least one sighting every winter of each species, but occasionally there are major irruptions. Irruptions are usually one or the other species and not both at the same time.”

This fall has been an “excellent” one for red crossbills and “mediocre” for white-winged crossbills, Dinsmore said.

One more finch that’s showing up this fall in places not accustomed to hosting it is the common redpoll. This light-colored bird has brown stripes, a yellow beak and splashes of raspberry coloring, including its namesake red stripe or “poll” on the crown of its head.

“They are being reported in more places than normal — usually just in northern Iowa but now in several central-Iowa locations,” Harr said. “Overall, this is likely to be a good winter for northern invasions.”


Todd Burras can be reached at


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