Irruptive Species

December 2014

Slideshow December 3, 2014

What is an Irruptive Species?

An irruption is the sudden change in the population density of an organism. In North American birds, irruptions often refer to the movement of northern-wintering species to the south in years of low food availability. You can recognize irruptive movement patterns at your feeders: some winters you may see a species at your feeders in great numbers, but in other winters they don’t show up at all.

Irruptive Species Gallery

  • Common RedpollThe Common Redpoll is a small, brown-streaked finch with a yellow bill and a red fore-crown. It breeds in the northernmost regions of Canada and Alaska. Erratic winter movements are generally thought to be associated with the food supply; when you notice lots of redpolls at your feeders, it is likely that their natural food supply in the boreal forests is running low that winter. Common and Hoary redpolls show a great diversity of plumage variation: some are rosy-hued all over, others have clean white breasts, others are heavily streaked with brown.  One diagnostic feature, however, is their little black chin, which distinguishes them from other finches such as house and purple finches.Photo © Yves Déry

    Common RedpollThe Common Redpoll is a small, brown-streaked finch with a yellow bill and a red fore-crown. It breeds in the northernmost regions of Canada and Alaska. Erratic winter movements are generally thought to be associated with the food supply; when you notice lots of redpolls at your feeders, it is likely that their natural food supply in the boreal forests is running low that winter. Common and Hoary redpolls show a great diversity of plumage variation: some are rosy-hued all over, others have clean white breasts, others are heavily streaked with brown. One diagnostic feature, however, is their little black chin, which distinguishes them from other finches such as house and purple finches.
    Photo © Yves Déry

  • Pine SiskinWhen seeing Pine Siskins for the first time, they can be hard to identify. Sometimes they mix with flocks of American Goldfinches, which are similar in size, shape, and behavior, making them hard to notice unless you look closely. When seen alone, they look very much like female House Finches. Pine Siskins have brown streaks like House Finches, but they are smaller and have small bills. Pine Siskins also have a touch of yellow in their wings, but that can be hard to see, especially at a distance.Photo © Laura Erickson

    Pine SiskinWhen seeing Pine Siskins for the first time, they can be hard to identify. Sometimes they mix with flocks of American Goldfinches, which are similar in size, shape, and behavior, making them hard to notice unless you look closely. When seen alone, they look very much like female House Finches. Pine Siskins have brown streaks like House Finches, but they are smaller and have small bills. Pine Siskins also have a touch of yellow in their wings, but that can be hard to see, especially at a distance.
    Photo © Laura Erickson

  • Evening GrosbeakEvening Grosbeaks look a bit like giant goldfinches - yellow bodies, black wings, but they are many times the size of a goldfinch and they are becoming much less common. The Evening Grosbeak is an irruptive migrant, occasionally moving out of its boreal and montane breeding areas to winter at lower latitudes and elevations. These yearly fluctuations have been documented by participants of Project FeederWatch, allowing researchers at the Cornell Lab of Ornithology to track changes in the abundance and distribution of the species. According to a recent study, reports of Evening Grosbeaks at FeederWatch sites declined by fifty percent between 1988 and 2006. Photo © Mike Wisnicki

    Evening GrosbeakEvening Grosbeaks look a bit like giant goldfinches – yellow bodies, black wings, but they are many times the size of a goldfinch and they are becoming much less common. The Evening Grosbeak is an irruptive migrant, occasionally moving out of its boreal and montane breeding areas to winter at lower latitudes and elevations. These yearly fluctuations have been documented by participants of Project FeederWatch, allowing researchers at the Cornell Lab of Ornithology to track changes in the abundance and distribution of the species. According to a recent study, reports of Evening Grosbeaks at FeederWatch sites declined by fifty percent between 1988 and 2006.
    Photo © Mike Wisnicki

  • Red-breasted NuthatchUnlike Evening Grosbeaks, Red-breasted nuthatches are doing just fine - their populations have been stable or increasing for the past several decades.  Nonetheless, they may not be common at your feeders every winter, because in years of plenty, they may stay in their northern coniferous forest habitats instead of moving south.  Red-breasted nuthatches can be distinguished from their white-breasted relative by their smaller size and conspicuous chestnut-colored breast and flanks. Photo © Ron Kube

    Red-breasted NuthatchUnlike Evening Grosbeaks, Red-breasted nuthatches are doing just fine – their populations have been stable or increasing for the past several decades. Nonetheless, they may not be common at your feeders every winter, because in years of plenty, they may stay in their northern coniferous forest habitats instead of moving south. Red-breasted nuthatches can be distinguished from their white-breasted relative by their smaller size and conspicuous chestnut-colored breast and flanks.
    Photo © Ron Kube

Monitor Irruptive Species!

Join Project FeederWatch and help scientists monitor irruptive species movements.

This Month’s Articles

Irruptives Seen By You! Participant Photo Slideshow

Slideshow December 29, 2014

Find The (Irruptive) Flock

From Our Data December 18, 2014

The Patterns of Irruption

Take a Look December 10, 2014

What is an Irruptive Species?

Slideshow December 3, 2014

5 Comments

  1. Robyn G says:

    If I understand your definition correctly – by far the largest irruptive species where I live – in NE Florida – is the Canadian goose. At some point within the last decade or so – large numbers of them decided it was more fun to live here full time than flying thousands of miles between their normal summer and winter habitats. Can’t say I blame them. But they’re really making a mess of things everywhere here. From our golf courses to our retention ponds in HOAs to lots of other places.

    Anyway – if you’re going any work on this at Cornell – please email me and I will forward your info to our golf course and HOA. Both are spending a fortune dealing with this new “irruptive” species here and would probably be glad to participate in experimental research. Robyn

    • K Moon says:

      Hi, just browsing the blog and scanned your comment, albeit a year and change after you posted initially.

      An irruption normally carries with it a connotation of a temporary or short-term event that occurs only sporadically. Contrast this with ‘migration’ which is something a population does regularly, or in the case of your geese–a third item.

      In the case of the Geese it is almost certain that they are actually expanding their range with no real intention on the part of your local ‘group’ of returning to their more traditional regions once the pressures at home let up. (Irruptions are usually food or weather related and everyone goes home once things improve). Expanding and shrinking ranges are a normal part of the meta-life cycle of all species. We humans have the distinction of being able to induce–or at least catalyze–these changes. What we do with those changes is up to us to handle well as you imply with the note regarding the HOAs and golf courses. I wish you luck with any research or future with the geese!

  2. Dianemarie Yates says:

    For us here in North-Central Arkansas the irruptive species this winter is the American Robin. Seen in flocks up to 400 birds, they take many minutes to pass overhead. When in the forest they sound like the cacophony of a tropical rain forest. On the highway we must often slow down as they pass low in front of us by the dozens or more. Common Grackles are a close second but it is not uncommon when these are present to see them in large numbers, often forming patterns in flight that look like bait balls in the ocean. Pine siskins are back though not in the numbers we saw a couple years ago.

  3. Spruill Kilgore says:

    Flocks of robins have been in central FL for a few weeks.

  4. Spruill Kilgore says:

    Flocks of robins have been in central FL for several weeks.

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