Irruptions

Although northeast-central Mississippi didn’t get much icy precipitation out of the much-hyped winter storm system, temperatures dropped considerably—enough, apparently, to urge the neighborhood wild birds into a kind of foraging frenzy the following day.  In our backyard, White-throated and Song Sparrows, Rusty Blackbirds, Purple Finches, Hermit Thrushes, and other species busily searched for food underneath the oak leaves that line the ground.  I thought back to winters in northwestern Arkansas, where, in contrast to Mississippi, snows are a regular occurrence.  I saw my first American Tree Sparrows during one of those winters—my first winter in Arkansas, when there was a record-setting blizzard (https://www.weather.gov/tsa/weather_event_2011feb1) that dumped several inches of snow in late January and early February.  The tree sparrows were feeding with Dark-eyed Juncos on the snow-covered ground.  It was really neat to see.

One of my photos of American Tree Sparrows (with Dark-eyed Juncos).

I remembered that I hadn’t submitted this record to eBird, the Cornell Lab of Ornithology’s online bird database (to which I am slowly, steadily adding my older birding notes), so I pulled up my computer to do that.  The eBird checklist that loaded for that region and season had American Tree Sparrow flagged as “rare,” which surprised me for a second, because I knew plenty of other people in Arkansas who had seen the species.  But then I remembered: American Tree Sparrows are often an irruptive species (http://www.tnwatchablewildlife.org/details.cfm?displayhabitat=grassland&sort=aounumber&typename=GRASSLAND%20AND%20SHRUB&uid=09081113200239105&commonname=American%20Tree%20Sparrow), meaning that they move farther south, in larger numbers, when food sources up north are too low.  Like many ecological concepts, bird species irruption is all too easy to oversimplify, but the gist of it is that some species of trees have boom-and-bust seed-producing cycles, with some years—i.e., the mast years—yielding particularly large crops.  Birds that feast and raise lots of young on a bountiful crop one year may find themselves in danger of starvation the next year.  What is there to do in such a situation?  Look for food elsewhere, generally.  American Tree Sparrows aren’t the only ones to do this, of course.  Red-breasted Nuthatches, Pine Siskins, Red Crossbills, and Evening Grosbeaks are some of the best-known examples of irruptive species.  But tree sparrows are on the list, and the winter of 2010-2011 was apparently one of the ones in which they invaded the south in larger numbers than usual.  I hadn’t really realized it before, but I was lucky to be in Arkansas during a major irruption year when I could observe this species in my “yard” without much effort!

Satisfied with this information, I uploaded a couple of my tree sparrow photos to my checklist, submitted it, and, to confirm my theory, went to look at the eBird species graphs for American Tree Sparrow in Arkansas.  Examining the line graphs of abundance from various years, starting with 2010-2011, yielded some interesting results.  The eBird site currently allows you to compare up to five separate years in the abundance graph for a single species.  As you can see here, in Arkansas in 2011, American Tree Sparrow numbers shot up dramatically.

Not my graph!  Credit to Cornell Lab's eBird.

https://ebird.org/

Because species may have very different distributions and dietary preferences/requirements, they are not all on the same cycle when it comes to irruption.  Red-breasted Nuthatches in Mississippi, for instance, showed a population peak in the 2012-2013 winter, a smaller one in 2016-2017, and another one again this winter, 2018-2019.  (The graph shows Mississippi populations only, and I didn’t include 2019, since we’re only a month into the year.)  

2010-2014, eBird

https://ebird.org/

2014-2018, eBird

 https://ebird.org/

The Red-breasted Nuthatch that arrived in the backyard last October.

If you look at the distribution maps on eBird, some of the same general patterns can be detected.  It’s a bit subtler on the distribution maps than on the line graphs, but the dark purple spots of the maps—the areas with a greater frequency of sightings—are definitely more noticeable in the southeastern United States in the years 2012 and 2013, which were the years of one of the best recent irruptive seasons for Red-breasted Nuthatches.

Red-breasted Nuthatch distribution graphs modified from eBird (https://ebird.org/).

Predicting when the next irruption will occur is relatively straightforward, I suppose, but only time will tell just how extensive it will be.  In the meantime, overwintering wild birds, irruptive and non-irruptive alike, aren’t wasting a second in their daily pursuit of finding enough food to survive the cold and bleak conditions.  Spring is on its way, though, so they will be traveling back to their northern nesting territories fairly soon, where, hopefully, they will find a plethora of food sources awaiting them.