Sand Prairie-Scrub Oak Nature Preserve is a 1,460-acre site located in Mason County a few miles east of the Illinois River. The site is mostly oak-hickory woodland and savanna, but sand prairie also occupies a large percentage of the area. I have been a regular visitor to this preserve since 1991. It was during one of my early visitations that I first noticed an unusual number of overwintering red-headed woodpeckers in the preserve’s mature black oak woodlands; with leaves gone from the vegetation, the woodlands are much more open, and it is possible to see quite a distance through the trees and learn the contours of the land. So even at a distance, the red-headed woodpeckers stood out against the dominant dull grays and browns of the trees. And if I missed the actual appearance of a woodpecker, it would not be too long before I would hear their distinctive calls, sometimes from several directions at once. Their bright colors and energetic defenses of small winter territories, with caches of acorns, were a welcome contrast to the silent woods and cold, forbidding winds.
Each year thereafter I looked forward to seeing the overwintering red-headed woodpeckers of Sand Prairie-Scrub Oak Nature Preserve. And I often thought of taking counts of their numbers, but was usually too preoccupied with simply enjoying the wild landscape and finding my way; the preserve has no trails, and so on my first forays I relied heavily upon the sun, or compass on cloudy days, to find my way. I also was not sure what I would do with the data if I did make counts. So I did not have a great motivation for counting.
But then in 2003 I bought a computer and discovered eBird, an Internet database managed by Cornell University’s Laboratory of Ornithology. The eBird database is used not only to store bird count data of all types and from all types of individuals, but it is used as a source of data for scientists to analyze and discover patterns and long-term trends in bird populations. For me, eBird gave a purpose to counting birds. After becoming aware of eBird, I found it difficult to visit Sand Prairie-Scrub Oak Nature Preserve and not count birds. And rather than hiking with no particular destination in mind, I began to follow a standard route over a similar time span so that my counts would not be overly biased by the amount of effort I expended. And so beginning in the fall of 2003, I made a point of systematically counting birds whenever I visited Sand Prairie-Scrub Oak Nature Preserve during the overwintering months. And I dutifully entered all of my counts into the eBird database (http://ebird.org/content/ebird/).
For the first few years, my counts of red-headed woodpeckers were fairly consistent (see accompanying figure), even given an inherent and unknown degree of variability. Then on January 7, 2007, I counted an all-time high number of red-headed woodpeckers, about twice the number (40) that I usually counted (an average of 21 per count up to October 14, 2006). But by the fall of 2007, the counts dramatically declined to a new low (13). I missed counting in 2008. And when I returned in the spring of 2009, I had trouble finding even the one red-headed woodpecker that I did record; on the February 1, 2010 count, I found none.
Of course, it has long been known that populations of overwintering red-headed woodpeckers vary with oak mast (acorns) production. They gather acorns, store them in tree holes, much of the time in standing dead trees, and then live off their acorn caches over the winter months. Years when mast is abundant tend to have higher numbers of overwintering red-headed woodpeckers in a given area than years with poor mast production, when the migrating woodpeckers fly to areas with more abundant mast. Although I made no attempt over the years to assess the availability of oak and hickory mast at Sand Prairie-Scrub Oak Nature Preserve, I would be surprised if mast production since I began counting birds at the preserve did not follow along with red-headed woodpecker numbers.
Blue jays also have a strong affinity for acorns. The figure of data shows that although blue jay counts do not perfectly mirror red-headed woodpecker counts – and I would not really expect this, given the unknown complexities usually associated with counting anything in the wild – the high count of blue jays occurred on the same date as the high count of red-headed woodpeckers (January 7, 2007), and then both declined. And this is exactly what I would expect if both species were responding to mast production.
Of course, assigning a definitive cause for a given observation is a difficult task, even when it appears to be obvious, as in this case. If I had quantitative information on mast production from 2003 to 2010, I would be in a better position to examine the possible causes behind the changes in red-headed woodpecker and blue jay counts. And there might be other factors. What, for example, about the effects of weather? Or white-tailed deer populations, a species also known to consume mast? How would these or other factors affect red-headed woodpecker populations?
What I do know for sure, and this is even borne out in my red-headed woodpecker and blue jay counts, is that the natural world is dynamic, and change is always occurring, sometimes quickly so that we notice (a crash in red-headed woodpecker populations) and sometimes so slowly that a long-term perspective is necessary and only sensitive scientific instruments may be capable of detecting the change (continental drift). This latter type of change (slow over a long time) has helped give rise to the idea that an undisturbed natural world is in a stable and balanced condition, an idea that has been profoundly discredited (for an excellent account of this charge, see the book Discordant Harmonies: A New Ecology for the Twenty-First Century, by Daniel B. Botkin, published by Oxford University Press in 1990). In fact, expectations of stability, even or especially on a global scale, are sure to be disappointed when change is finally noticed. This is especially the case when we begin counting and measuring events or organisms, where it is inevitable that change over time is what we find. And as the scale of what is being examined increases, determining the cause for change becomes orders of magnitude more difficult and less certain.
I have no idea if the year 2010 will or will not be a mast year at Sand Prairie-Scrub Oak Nature Preserve. But I will be there next fall, looking for the missing red-headed woodpeckers to return. I am quite confident that they will eventually return, but during which year, I cannot say; anymore than seven years ago I could have predicted that they would fall to low numbers during the winter of 2009-2010.
[This article has appeared in the Summer 2010 issue of Illinois Audubon magazine (pages 11 and 12), which is published by the Illinois Audubon Society (www.illinoisaudubon.org).]
NOTE: For an update on the data, please follow this link.