Why Are Red-breasted Nuthatches Irrupting?

There can be no doubt that this year is an irruption year for Red-breasted Nuthatches. Sitta canadensis isn’t just irrupting out of its far northern home but exploding southward, with reports in every southern state except for Florida, including birds on the outer banks of North Carolina, on Grand Isle, Louisiana, in a suburb of Atlanta, and on the Gulf Coast of Texas. Bird bloggers from Wisconsin to Massachusetts have noticed the irruption and blogged about it. But just how much have Red-breasted Nuthatches shown up since their irruption started in August? Check out this graph adapted from eBird that shows the frequency of sightings in the United States in 2012 – Red-breasted Nuthatches are showing up on birders’ checklists just over sixteen percent of the time!

frequency of Red-breasted Nuthatch sightings in the United States, January-September 2012

But why are the nuthatches irrupting? The usual understanding of why irruptions of birds – from Red-breasted Nuthatches to Snowy Owls – occurs is that there is a shortage of food. No lemmings left in the north? Birders are delighted by Snowy Owls. No cones from which Red-breasted Nuthatches gather seeds? Birders across the United States are delighted by the “yank-yank” of Red-breasted Nuthatches on sixteen percent of their outings.

Red-breasted Nuthatch at Jones Beach State Park, New York

Ron Pittaway’s famous winter finch forecast includes some some non-finch irruptives, including Red-breasted Nuthatch. This year’s forecast, which, as always, is Ontario-centric, says the following about red nuts:

A widespread irruption of this nuthatch beginning in mid-summer indicated a cone crop failure in the Northeast. Most will leave the eastern half of the province for the winter, but some will probably remain in northwestern Ontario where cone crops are much better.

 No cones, no nuthatches – they irrupt! But could the widespread nature of this irruption and the sheer number of birds indicate that something else is going on? The cones that nuthatches are dependent upon are apparently notorious for having a boom-and-bust cycle. If they are a bust this year, as Pittaway indicates that they are at least in Ontario, then last year it is likely that they were booming. This article discusses the heavy cone crop in Maine last year on red spruce, balsam fir, and larch, and Ron Pittaway’s forecast from last year said “cone crops are excellent and extensive across much of the boreal forest and the Northeast” and that nuthatches had “very little southward movement.”

Red-breasted Nuthatch at Jones Beach State Park, New York

In December of last year Clare Kines wrote a post here on 10,000 Birds about the cause of the massive Snowy Owl irruption many birders were lucky enough to experience. He explained that the irruption was not just caused by the drop in lemming populations that are the main prey of Snowy Owls but by the huge number of lemmings that were around just prior to the bust:

Snowy Owls produce large broods of up to 14 chicks on years when lemmings are plentiful. A brood of nine chicks will be fed something in the order of 1500 lemmings from the time they are hatched until they are independent.  Remember that friend of mine that saw 17 breeding pairs of Snowy Owls in a single valley? If they each raised nine owlets, the owlets alone would have eaten over 25,000 lemmings from that valley.

So this was a good summer for lemmings, a very good year. They were everywhere, including a couple of them that inhabited a cage in my house for a while (they were released back where they were caught). They actually make very good pets, becoming very tame quite quickly. And because it was a good year for lemmings it was a very good year for Snowy Owls, who produced large broods.  As these broods have grown into juvenile birds there is more and more competition for fewer lemmings and they begin to disperse farther and farther afield, resulting in the irruption that many of you are enjoying now. I think that you’ll find that most of the Snowy Owls you are seeing are juveniles.

Do nuthatches have the same boom-type population burst because of good conifer crops? Yes and no, or at least, that is what I theorize. Red-breasted Nuthatches rely on seeds from conifers as a staple when they winter in the far north. Young birds are fed on what all young birds are fed on – lots of protein. Bugs, caterpillars, and other assorted creepy-crawlies are what young nuthatches get from their parents. But when winter came to the far north last year their diet, like their parents’ diets, changed to seeds from cones. And because cones were abundant nearly everywhere in their range, they had plenty to eat. And because they had plenty to eat they did not have to travel great distances to find food, which is the most dangerous undertaking a bird, particularly a young bird, experiences. That means there were a lot more nuthatches around this spring to breed and that means that there are lots and lots more nuthatches to irrupt across the United States right now.

Thank a pine cone. Or a spruce cone. Or a larch cone.

Red-breasted Nuthatch at Van Saun Park, New Jersey

I don’t want to understate my lack of scientific credentials, which is total. Also, I don’t really trust that eBird data for previous irruption years – which seem to indicate that this year is a much larger irruption than past years – is entirely accurate, simply because many more people are using eBird now than in past years and the number and quality of checklists entered into eBird is up. Anecdotal evidence seems to indicate that this irruption year is larger than most but I would like to see some rock-solid empirical data. If anyone has anything further to add, or wants to scold me for my ignorance, please feel free to have at it in the comments.

Red-breasted Nuthatch jumping for joy

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