I have been keeping a close watch on my birdfeeders. Not only because I love seeing the juncos and goldfinches that arrive in flocks, and the black-capped chickadees that zip around, and even the blue jays that tend to scare everyone else away, but because I am hoping for some not-so-typical visitors: red crossbills and pine siskins.
Both are year-round residents of northern boreal forests, as well as the Rockies and Pacific Northwest. It’s not unusual to see them in New England in the winter, but every few years, large numbers of them arrive in search of food. These sudden population shifts are known as irruptions. In addition to red crossbills and pine siskins, other irruptive species include evening grosbeaks, redpolls, and perhaps most famously, snowy owls, which have been frequently observed this year.
Irruptions are often driven by food. In the case of the red crossbills and pine siskins, the failure of conifer crops in the West has displaced them from their typical winter grounds. In the case of snowy owls, which spend summers north of the Arctic Circle, last year’s boom in the population of lemmings, voles and other prey led to a wildly successful breeding season. That has resulted in more birds heading south and looking for food.
SNOW BUNTINGS
SNOW BUNTINGS
But exactly how many more birds? What constitutes an irruption? “The lines blur,” said Matt Young, who works for the Cornell Lab of Ornithology’s Macaulay Library. “There's no defined threshold other than larger than usual numbers.”
For example, in the winter of 2007-08, data from Project FeederWatch, a winter survey of birds overseen by the Cornell Lab, noted that pine siskins were present at 24% of their sites in the lower 48 states and the average flock size was about 11 birds. The following year there was an irruption and pine siskins were reported to be at 50% of the sites, in flocks averaging 15 birds.
Irruptions tend to occur every few years among various species. But about once a decade there is a superflight year, when numerous bird species are on the move due to a combination of environmental forces. The Northeast had a superflight year in the winter of 1997-98, said Young, when large numbers of pine grosbeaks, evening grosbeaks, common and hoary redpolls, red crossbills, white-winged crossbills and pine siskins arrived in force.
While those species travel in flocks, snowy owls tend to be solitary. But one is all it takes to create a stir. With their ghostly feathers, yellow eyes and a wingspan of more than four feet, they have an otherworldly look about them.
Mixed flock of Red Crossbills, Pine Siskins and Evening Grosbeaks
Mixed flock of Red Crossbills, Pine Siskins and Evening Grosbeaks
One of these owls has taken up residence in the coastal town of Rye, New Hampshire and has caused such a commotion that the police are considering ticketing people who disturb the bird or trespass on private property in an effort to catch a glimpse of it.
Closer to my home, a snowy owl was reported on the campus of Vermont Technical College (VTC) in Randolph. Brian Lowe, a bird enthusiast and leader of the Audubon Christmas Day count for Orange County, said it was first spotted in November and was still in the area in mid-January.
Recently, I attended an event in Randolph and took the opportunity for a side visit to the VTC campus. I drove around slowly, looking at the big open fields that border the school, trying to guess the ideal vantage point for a snowy owl. I drove down the road to the Vermont Veterans’ Memorial Cemetery to see if I might have better luck there. The cemetery slopes up a hill and has plenty of coniferous trees that might also draw red crossbills and pine siskins. The open fields next to VTC were within sight.
It was in the low 40’s, sunny and breezy. After weeks of sub-zero nights and single-digit days, the air felt warm and welcoming. I had the place to myself and took pleasure in walking around without a hat or gloves. Every so often I scanned the trees with my binoculars. I didn’t see a single red crossbill, pine siskin or snowy owl. But I loved the idea that I could have.
And still might.
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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, why does it happen 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.
Goldfinch flock
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 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:
Snowy Owls
Pine siskins
Snowy Owls
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 songbirds and finches irrupt, they often do so in massive flocks, banding together in areas where winter food is plentiful.
Evening Grosbeaks
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 spreadmore easily and birders should take extra precautions to watch for illness. Cleaning bird feeders and bird baths 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. At the same time, birders may want to investigate ways to save money on birdseed and other top winter foods, such as suet, to feed so many hungry visitors.
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.
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