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Large irruption year brings red-breasted nuthatches to Arkansas

Arkansas Natural Heritage Commission - Friday, December 21, 2018

An irruption is an irregular winter migration or phenomenon in which unusually high numbers of a bird species travel away from habitat within their normal winter range. Typically, irruptions are seen in areas that are much farther south than the birds are usually known to travel, catching the attention of birders and scientists. Red-breasted nuthatches (Sitta canadensis), ordinarily a year-round resident of Canada and the northern U.S., are experiencing a record-breaking irruption year this winter and have been seen as far away as Los Angeles, Calif. and Tampa, Fla.


Why is this year’s irruption so large?

Irruptions are hard to study because it is hard to predict when they will happen or where the birds will go. Small irruptions are typically cyclical, based on the availability (or lack thereof) of cones produced by an area’s conifers. In years and areas where cone crops are scarce or absent, bird species leave their normal winter range and leave in search of food. This year, we are seeing an uncharacteristically large irruption of the red-breasted nuthatches because cone crop failures across significant portions of their normal winter range resulted in a lack of their primary winter food (conifer seeds).

However, it is known that species affected by masting, like the red-breasted nuthatch, common redpolls (Acanthis flammea), and evening grosbeaks (Coccothraustes vespertinus), are usually the birds experiencing an irruption year. Masting occurs when a tree species produces a large number of seeds across thousands of miles of forest in the same year. Birds gorge themselves on all of the extra food, breed early, and produce more offspring than usual. This can result in a population explosion.

Typically, a masting year is followed by one or more years of cone crop failure (conifers will produce little to no seeds). When autumn comes, there isn’t enough habitat or food for the large number of offspring. Many birds are forced to leave the area in search of food and habitat, traveling farther and farther from their normal winter ranges. These events occur every few years within a subportion of the bird’s range. This cycle of masting, followed the next year by cone crop failure, results in segments of the overall population irrupting south to find food. In contrast to this well understood pattern, this year there is widespread, uniform failure of cone crops across much of the bird's range. Far greater numbers of birds are moving south and into areas not normally (or often) observed.

Why do trees have a masting year?

The true cause is unknown. However, we do know that masting customarily occurs one to two years after a warm, dry spring that has followed a poor seed production year.

What else can cause bird irruptions?

Other cycles of food excess that occur over an even larger time scale have been known to cause irruptions. In the 1970s, there was an outbreak of the spruce budworm (Choristoneura fumiferana) in Canadian forests, which destroyed a large number of spruce trees. In addition, the birds in the area gorged on the extra budworms. Evening grosbeaks and purple finches (Haemorhous purpureus) experienced a population explosion. Due to the increase in the population of these birds and the decrease in spruce trees, both bird species experienced a winter irruption.

Similarly, in the winter of 2013-2014 snowy owls (Bubo scandiacus) had an irruption year. Scientists theorize that it was caused by a large increase in the summer population of their favorite food, lemmings (a type of rodent). The snowy owl population exploded and that winter, snowy owls were seen in Arkansas and as far south as the Bahamas.

How do scientists know about bird irruptions?

Long-running, large-scale citizen science projects have been a major source of information about bird irruptions, and the extent and timing of bird movements. Citizen science projects involve everyday citizens in scientific observations, under the direction of professional scientists. Bird counts are one of the many popular citizen science projects. Audubon’s Annual Christmas Bird Count, the longest-running citizen science project in the world, is currently in its 119th year. Other leading citizen science projects that gather data on birds include: Cornell Lab of Ornithology and National Audubon Society’s Great Backyard Bird Count; Cornell Lab of Ornithology’s and Bird Studies Canada’s Project FeederWatch; and the U.S. Geological Survey’s North American Breeding Bird Survey. Thousands of scientific studies have used data from these projects, including the National Audubon Society’s “Birds and Climate Change Report.”

Another benefit for those studying irruptions is the real-time citizen science website (and smartphone application) eBird, which is managed by the Cornell Lab of Ornithology and the National Audubon Society. eBird users submit their bird observations through a web-based interface. The interface and applications help birders to maintain their personal bird records, explore observations of others anywhere across the globe, and contribute to science and conservation.

Data from these citizen science resources increase scientists’ knowledge of bird migration, including irruptions, and may help us understand climate change. They can provide data about the changes in weather patterns and the frequency and occurrence of masting events. Because irruption events directly relate to seed production, it may provide scientists with information on how climate change is affecting the homes and breeding grounds of billions of North American birds.

What does an irruption year mean for me?

In a non-irruption year, you would not be as likely to see the red-breasted nuthatches in Arkansas during the winter. This year, the likelihood of seeing one at your feeder or in your neighborhood is much greater due to synchronous cone crop failures over a larger than normal area.

What should I look for?

Red-breasted nuthatches are small birds that have a black and white striped face, slate-colored back, and a rusty-colored underside. They get their common name from the way they use their beaks to pull back pieces of tree bark in order to stash seeds. You can also listen for their distinctive “toot, toot, toot” vocalization, which sounds like a small tin horn. Keeping your bird feeders full will increase your likelihood of seeing these birds (especially if you live in an area of pine habitat) and help the birds out as well.

Photos:

Top left -- Red-breasted nuthatch (Sitta canadensis). Photo by Peter Pearsall, U.S. Fish and Wildlife Service.

Middle left -- Snowy owl (Bubo scandiacus) taken in Little Rock, Ark. during the 2013-2014 winter irruption. Photo by Bill Holimon.

Bottom right -- Red-breasted nuthatch (Sitta canadensis). Photo by Dave Menke, U.S. Fish and Wildlife Service.



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