Of Little Acorns do Blue Jays Grow?

An interesting little still-life from today:

White Oak (Quercus alba) acorn caps, Van Patten Woods Forest Preserve,
Lake County, IL, 1/17/2014

Why acorns? Back to that in a moment.

The most noticeable bird (partly because they're so noisy) in the woods today was this:

Blue Jay (Cyanocitta cristata), Illinois Beach State Park,
Lake Co, IL  5/4/2012

This year Blue Jays have been particularly easy to find here in Lake County. This isn't always the case -- Waukegan CBC numbers, since 1980, have varied from 9 to 276, while Evanston CBC numbers have varied from 7 to 159. The correlation between the two counts, by year, is highly significant -- low years are low for both counts, and high years are high for both counts. (1) What causes these? While weather would certainly be a possibility, this year has been a rather hard winter so far, and it appears to be a very good year for Blue Jays. So that's probably not it. The other favored explanation for winter abundance, of course, is food. And that's where the acorn caps come in -- acorns and other nuts are a favored winter food for Blue Jays. And oaks and hickories show an interesting bit of natural history -- they tend to vary considerably from year to year in how many nuts they produce. Even more interestingly, the trees in a given region all tend to vary the same way -- a good year for one tree is usually a good year for most of the trees around them.

The argument usually given for why this happens goes like this: predators love to eat acorns, hickory nuts, etc. If we put out the same number of nuts every year, then the predator population will quickly grow to the point that none of our offspring will survive. On the other hand, if we spend several years just saving our energy, then put out a whole big bunch of acorns all together, then the predators won't be able to build up, and we'll overwhelm them with numbers so that some of our offspring are bound to survive. This makes sense, although I continue to wonder about the signals that trigger this united front -- is it just weather conditions over the course of the summer, or is something else going on that we're not seeing?

In any case, a good Blue Jay winter in Northeast Illinois probably implies a good mast year. (Mast is a general term for fruits and nuts produced by trees.) Another indication of this is that Red-headed Woodpecker numbers tend to vary in very similar ways to Blue Jays -- good Blue Jay years are usually good Red-headed Woodpecker years, etc. (1) Interestingly, in Missouri, this doesn't seem to be the case -- Blue Jay numbers tend to be independent of mast production, although Red-headed numbers do vary with mast production. (2) Perhaps they're far enough south that Blue Jays manage to find other winter foods in sufficient abundance to winter regardless, while up here they can't do it without acorns?

Other species show similar responses -- mice and chipmunks in Virginia showed increased winter survival and offspring survival in good mast years (3), and Black Bears in New Mexico showed dramatically decreased birth rates (down 60%) and recruitment rates (down 70%) after particularly poor acorn crops. (4)

This straightforward story does have a potential problem, though, at least in the eastern United States. One of our most noble hardwood trees, the American Chestnut, largely disappeared in the early 20th century as the result of an introduced fungus, the Chestnut Blight. Like the European Chestnut, the American Chestnut was an important mast tree. Unlike our oaks and hickories, it's production was reasonably constant from year to year. (5) Since it made up roughly 25% of the eastern hardwood forests, it's relatively sudden loss not only decreased the available winter food supply for a lot of species (and their predators, by extension), but it apparently drastically increased the annual variability, which must have had impacts we haven't even guessed at yet.

Of course, we're well outside of the Chestnut's historic range, so the blight probably hasn't had any effect here that we could detect. And that means that the Blue Jays and Red-headed Woodpeckers have probably been making decisions on where to spend the winter by looking for those little acorn caps for as long as they've been here for us to wonder about.

(1) National Audubon Society (2010). The Christmas Bird Count Historical Results [Online]. Available http://www.christmasbirdcount.org [2014]

(2) Smith, Kimberly G., and Todd Scarlett. "Mast production and winter populations of red-headed woodpeckers and blue jays." The Journal of wildlife management (1987): 459-467.

(3) Wolff, Jerry O. "Population fluctuations of mast-eating rodents are correlated with production of acorns." Journal of Mammalogy (1996): 850-856.

 

(4) Costello, Cecily M., Jones, D. E., Inman, R. M., Inman, K. H., Thompson, B. C., & Quigley, H. B. "Relationship of variable mast production to American black bear reproductive parameters in New Mexico." Ursus (2003): 1-16.

 

(5) Diamond, Seth J., Giles, R. H., Kirkpatrick, R. L., & Griffin, G. J/ "Hard mast production before and after the chestnut blight." Southern Journal of Applied Forestry 24.4 (2000): 196-201.