Midwinter Ramblings

This piece details some Noxubee County, Mississippi observations that I made many years ago.  Enjoy!

As is the case with any human activity, there are days when birdwatching reaps huge “benefits” for participants…you know, those days when something truly out-of-the-ordinary steals your attention and causes an immense adrenaline rush.  What constitutes an out-of-the-ordinary avian encounter is different for every person, but it always involves birds that are beautiful, graceful, especially fascinating, or rare—in other words, unusual.  Memorable occurrences such as an out-of-range species that provides a first record for a state or country, a trip to a wetland where thousands of waterfowl are congregating, stormy weather in spring bringing astonishing numbers of colorful neotropical migrant songbirds to a tiny patch of woods, and of course seeing a life bird all involve a feeling of intense concentration, combined with that welcome surge of adrenaline.

It’s a mild day in January.  For birders, midwinter days—as well as those of midsummer—seldom bring the intense excitement and anticipation that characterize other times of year, but this doesn’t concern me.  As I head out the back door, I notice the slight breeze and the freshness of the air; a cold front that moved in last night after the violent storms has left the temperature hovering at about 50 degrees Fahrenheit—although certainly not cold, it is cool enough for a light jacket, and chillier than it has been for a number of weeks.  The white airbrush streaks of airplane contrails mark an otherwise clear, azure sky—a far cry from the brooding grayness, rolling clouds, and fierce wind, rain and hail of yesterday.  I had originally figured that the conditions yesterday might have caused some birds to be more active today, but as I stand in the back yard, listening and watching, it crosses my mind that this will likely be one of the slower days of birding—in other words, an “ordinary” day.

Some of the first bird sounds to reach my ears are from the “regulars”—chickadees, titmice, and Carolina Wrens calling in the woods some distance away.  I walk across the soggy, gray lawn to the thicket behind the old outbuilding.  This thicket was mostly cleared out several years ago, but since then it has been allowed to grow into an impenetrable mass of tangled vegetation, the haunt of nesting Carolina Wrens, Northern Cardinals, and Brown Thrashers in summer.  Lush and green during that time of year, now the intertwined grasses, saplings, and vines are bare, gray, and brambly.  As I look closer, though, I notice that a few plants still have some green vegetation, and last year’s dry leaves dangle from the branches of some of the sapling oaks. 

A brief, scratchy call note, evoking the sound of a mechanical toy being wound, sounds from the upper branches of a large pecan tree in the thicket.  Oh, a Ruby-crowned Kinglet.  As I watch, the tiny bird flutters up to pluck small white berries from clusters growing near the pecan tree—poison ivy berries.  I follow these berry-laden branches with my binoculars, and with a sense of fascination mixed with horror, I find that the poison ivy has grown in the form of a large shrub, rather than the vines that I commonly see.  Its thick, hairy trunk leans against the old pecan tree.  Whoa.  I step away from the thicket and look at the sky, hoping to see a hawk or two.  As it turns out, one of the neighborhood Red-tailed Hawks is soaring overhead, the sun shining through the rust-red feathers of its rectrices—an unexpectedly beautiful sight.

Red-tailed Hawk

I walk down the slope of our backyard to the pond, and from there I go to the marshy ditch on the southeast side.  The “marsh” itself is home to willows, cattails, and various types of grasses, while pines and brushy woods grow on the northeast side of the pond.  White-throated Sparrows flush up in bunches in front of me and land in the woodland cover several feet away, exchanging sharp peek! calls.  They are most likely alarmed by the presence of this strange, flightless intruder.  If I’m careful, I can creep into the woods to look at these large sparrows, which are plumaged in subdued, yet striking, gray, brown, white, and yellow feathers.  Sure enough, I spot some of them as they dart in and out of the shrubbery.  A single, dry check note fills me in on the whereabouts of a Yellow-rumped Warbler, perhaps calling to its associates that are also somewhere in the woods, hidden from view.  

White-throated Sparrow

The churr churr call of a Red-bellied Woodpecker sounds faintly off in the forest.  I wonder where it is, exactly; most of the time, I see these woodpeckers close by in the yard, foraging in the old pecan and walnut trees.  Of course, this might not be one of the individuals that I see in our yard, but it easily could be—I’m sure that their territory includes much more than the few acres that comprise our property.  Barely detectable over the sound of the woodpecker are the little chipping calls of Pine Warblers moving through the pine thicket some distance away.  Pine Warblers tend to travel in small groups, and are seldom seen far from pine trees.  These warblers are too far away to locate at the moment, so I turn my attention to other things.  A small creek runs through these woods—barely a creek, actually… it appears more like a shallow ditch, filled with runoff from yesterday’s rain.  Just the fact that there is a depression, though, makes me think that the stream of water must run through here most of the time.  Both sides of the creek are lined with dense privet bushes, laden with clumps of small, bruise-colored berries.  Many naturalists hate privet with a passion; the invasive shrub is spread around in bird droppings, choking out native vegetation wherever it sprouts.  Although I’m not thrilled to see it here, the White-throated Sparrows darting in and out of its luxuriant growth seem to have quite a different opinion.

Red-bellied Woodpecker

It’s been several minutes now since I entered these woods, and I’m considering walking back out of them soon.  But wait—what is that little insect flying near the ground?  Looking closer, I see that it is a blue bottle fly.  It seems a little incongruous for one of these usually warm-weather insects to be out today—the 50-degree temperature surely takes a toll on its fragile metabolism. Not surprisingly, the fly buzzes around feebly before settling down on a dried stalk of grass.  A band of Blue Jays suddenly flies in, one of them giving a near-perfect imitation of a Red-tailed Hawk’s scream.   Then, just as abruptly as they arrived, the jays disperse into the woods.

Blue Jay

The White-throated Sparrows are starting to return to the territories from which I initially disturbed them.  They are settling in for the evening.  The sun will set soon, and it seems like a good time to turn in, so I head back to the house.  On the way, I stop to admire the faint pastel colors just beginning to appear in the sky.  It’s a nice ending to this midwinter day in the field, and a reminder that not all exciting birding experiences must be intense.  In fact, I wonder if an unending stream of amazing, adrenaline-pumping experiences would just tend to blend together, as the “ordinary” days do.  On the other hand, as self-proclaimed naturalists, finding something to enjoy in every outing should be one of our biggest priorities—and it’s rarely a difficult task to discover something worth watching in nature’s fascinating show.

Invasive Trees, Shrubs, and Vines

This is a consolidation of two of my earlier posts about invasive plants in the southeastern United States.  Invasives are simply nonnative species that cause significant harm to ecosystems.  Not all nonnative plants are invasive, but the invasives are very likely to be the ones that you notice on a daily basis!  Because the list of exotic plant species present in Mississippi alone (from this site: https://www.se-eppc.org/mississippi/) is lengthy and still increasing, I thought it made sense to focus on the invasive trees, shrubs, and vines that I commonly see in my part of the state.

Let’s start with trees.  A couple of quick definitions: although there are some exceptions, the general rule is that trees have a single woody stem or trunk, while shrubs have multiple woody stems/trunks.  Most of the trees that I regularly observe are native to my area, but, every so often, I will come across an invasive species or two—especially near urban settings.  Some commonly-seen cultivated plants, such as the ginkgo (an ancient, living fossil species that was wiped out across much of its range during the Pleistocene Ice Age) are basically innocuous, but others, such as the Bradford pear, can cause extensive damage to native ecosystems.  The Bradford pear is actually a cultivar of the species Pyrus calleryana, the Callery pear, which the United States Department of Agriculture introduced from Asia to the United States in 1916 to combat blight in the common pear, Pyrus communis.  (Obviously, common pears, along with other species that are used for food, largely benefit humans, so no one is suggesting that they should not be grown.)  Bradford pears are incapable of reproducing with other Bradford pears, but (as any fan of Jurassic Park would likely guess) this doesn’t stop them from proliferating; they just do it by cross pollinating with other calleryana cultivars, spreading this invasive species far and wide.  

One of several Callery/Bradford pears in the neighborhood.  Actually, this looks like it might be a "colony" instead of just one tree.

Chinese tallowtree, or popcorntree, is another common invasive.  Introduced to the U.S. in the 1700s, it is known for the waxy compounds found in its seeds, which have been used for candle- and soap-making.  Because the plants are toxic, few animals will eat them, so there is next to no biological control of their populations.  The USDA states that eradication is “virtually impossible,“ that the trees may begin reproducing at the age of only three years, and that they can continue breeding for 100 years– all of which make this species a particularly bad choice for North American naturescaping.  Don't grow it here!

The tallowtree tends to have a nice shape, which explains why it's so frequently planted.

Tallowtrees have heart-shaped leaves.  The seeds, which aren't in this summer photo, resemble popcorn.

In one sense, shrubs may be the stumpy cousins of trees, but their lushness more than compensates for that.  Along with tree saplings, shrubs comprise the understory layers in forest ecosystems.  However, invasive species, such as privet, don't always need to grow within wooded areas; they frequently thrive wherever their seeds have been dropped.  While several introduced privet species range across North America, the most common in the mid-southeastern region is Chinese privet.  There seems to be some conflicting information regarding when, exactly, Chinese privet was introduced to the United States—some sources say 1852, while others, including the previous link, say 1952.  The latter date might have been a typo, since the 1852 date shows up in sources such as the websites for the USDA, the University of Florida’s Center for Aquatic and Invasive Plants, and the Texas Invasive Species Institute.   

Chinese privet in the summer.

Privet leaves and unripe fruits.

At any rate, Chinese privet has become well established outside of its native range—so much that United States and regional noxious plant legislation isn’t even concerned with it and other privet species.  Eradicating privet from the U.S. is basically a lost cause.  Its fruits are eagerly consumed and widely spread by birds and other animals; it possesses tough, hardy, evergreen foliage; it forms extremely dense thickets; and, if cut, it regenerates quickly from its extensive underground root systems.  Several privet hedges in the neighborhood where I live have been cut before, but that didn't stop them from coming back with a vengeance.

  

A mockingbird and privet, photographed in Arkansas in 2013.  Birds are attracted to privet berries.

I have written about two bird nests—White-eyed Vireo and Northern Cardinal—that I observed in a thorny olive (or thorny elaeagnus) near the house.  The upper branches of thorny olives grow long and snake-like for the purpose of clinging to whatever is directly above them.  While this species—introduced in 1830 from Asia—lags behind privet in terms of invasiveness, it’s still a problem in many areas, forming spiky, impenetrable barriers.  There are a number of methods used to control thorny olives, but most of them are not particularly effective.  Essentially, even though the thorny olive does provide a certain amount of seclusion and protection for birds during the nesting season, there are much better options that don’t choke out all other vegetation in the ecosystem!

Long "tendrils" on the thorny olive.

The undersides of the thorny olive's leaves have silvery scales and sparse, tiny brown spots.

Another invasive shrub that can dramatically transform its environment is the multiflora rose (Rosa multiflora), a sometimes vinelike plant that was introduced from Asia to the U.S. in the 1860s.  Like kudzu, it was widely used for erosion control, and it also shares the ability to form impenetrable thickets.  Its thorny stems can attain heights of 10 to 15 feet, and it has attractive flowers that have five white petals with blunt or slightly indented tips.  Herbicides are effective at controlling multiflora rose, but follow-up treatments must be consistently applied to prevent germination of seeds, which can lie dormant, ready to sprout at any time, for up to 20 years in the soil.  

Multiflora rose flowers

We've looked at trees and shrubs, but what about vines?  One of the most out-of-control vines in the southeastern U.S. is so abundant that it has even had festivals created around it.  Kudzu (Pueraria montana) is a leafy, purple-flowered vine in the legume family.  Although it was originally imported to the U.S. (from Japan in 1876) as an ornamental, and was used for erosion control in the early 20th century, it has since become a noxious weed, infamous for covering entire landscapes and choking out native vegetation.  In ideal circumstances, it can grow up to a foot a day.  Controlling kudzu is difficult partly because of the stolons (or runners) that it produces, as well as rhizomes, or underground stems that produce roots and shoots; uprooting the plant so that it's incapable of forming new growth is next to impossible.  Herbicides can work if there is enough product to treat the many acres of ground that kudzu frequently smothers.

Kudzu creates odd, alien-looking landscapes.

Interestingly, one of the natural predators, the kudzu bug, was accidentally introduced into the U.S., with the first individuals recorded in Georgia in 2009.  However, because the kudzu bug tends to cause more damage to soybean crops than it does to kudzu, it can't be relied on as a biological control method.

Joining kudzu on the list of invasive vine species are two of the most widespread introduced legumes: Chinese and Japanese wisteria (Wisteria sinensis and W. floribunda, respectively).  Because these species can and do hybridize in the wild (rather like the red and black imported fire ant species), identification is often difficult.  Both species have long, compound leaves and clusters of purple or white flowers.  Given the toxicity of wisteria's fruits, native wildlife species are almost certainly not helping these plants to spread.  Instead, wisteria's quick growth and tendency to strangle and shade out native vegetation provide the explanation for its range expansion after its introduction to the U.S. in the early 19th century (1816 for Chinese wisteria, and about 1830 for the Japanese wisteria).  According to the Mississippi State University Extension Service, wisteria control is extremely difficult.

Wisteria flowers

Wisteria growing over another invasive: privet.

Another fairly horrifying invasive climbing plant is Japanese honeysuckle (Lonicera japonica).  Like all of the plants discussed thus far, Japanese honeysuckle was imported (in 1806) as an ornamental.  It has attractive and fragrant blossoms, and many people have fond childhood memories of sucking the nectar from these blooms.  Like wisteria, however, Japanese honeysuckle's thick, woody stems quickly strangle native vegetation—and, as with kudzu, biological control doesn't work.  In fact, there is no available biological control for Japanese honeysuckle.

Japanese honeysuckle flowers

Because of the limited ecological benefits and the difficulty of controlling Japanese honeysuckle, the Mississippi Forestry Commission recommends planting several native vines, instead—including crossvine, coral honeysuckle, and Virginia creeper.  That site and others, such as the Mississippi State University Extension Service, are well worth perusing for detailed information about native trees, shrubs, vines, and herbaceous plants.

In general, all of the introduced trees, shrubs, and vines described in this post flourish in their new environments due, in part, to a lack of herbivory by insects.  Native insect species didn't evolve with these plants, so they generally lack the ability to effectively utilize them.  Many studies have shown that insect communities tend to decline in the presence of nonnative plants.  A paper written by Berghardt et al. (2010) and published in the scientific journal Ecosphere (https://esajournals.onlinelibrary.wiley.com/doi/pdf/10.1890/ES10-00032.1) describes how Lepidopteran (that is, butterfly and moth) communities showed both lower population sizes and less species diversity on nonnative plants than on native ones.  Given the fact that many songbird species rely on caterpillars as a food source for their nestlings, the results of this study should definitely ring some alarm bells.  Fewer caterpillars mean less leaf damage, but also impoverished ecosystems.

Blackbird Flocks

During the fall and winter, many species of blackbirds travel in flocks, foraging in a variety of places for seeds and insects.  While some of these flocks are composed of only one species—for example, Red-winged Blackbirds—others may be mixed-species flocks.  How do you tell which species comprise a particular mixed-species group that you're observing in the field?

Although it can be overwhelming at first, looking closely at individual birds in large groups of blackbirds will give you the practice you need to identify those species much more quickly.  Here is a flock of blackbirds swarming the upper branches of three tall trees:

When you examine the flock more closely, you can see that it is made up of Brown-headed Cowbirds and Red-winged Blackbirds.  There is an obvious flash of red on the wing of a male Red-winged Blackbird, and the female Red-wings have pale "eyebrows" and are densely streaked with brown.  The Brown-headed Cowbirds—clay-colored females and brown-headed males—are also fairly easy to pick out.

Can you spot the Brown-headed Cowbirds on the wires in the following photo?  A second species is present, as well.

Here is a close-up of the two species: Brown-headed Cowbird and European Starling (the bird on the right).  Starlings are not in the same family as North American blackbirds, although they frequently flock with them.  Studies indicate that mixed-species flocking helps birds to evade predators.

Starlings have longer bills than cowbirds, and their plumage in the winter is speckled with white.

Some blackbirds, like Common Grackles, are easy to identify because of their larger size and proportionally long tails.  These features contrast strongly with those of other, smaller species.  However, even in single-species flocks, grackles' lankiness is distinctive.

Common Grackles seem too big for tube feeders!

The Brewer's Blackbird is another fairly common species in eastern North America, and its shape and proportions make it appear more like a "standard" songbird.  The males, like the one below, have glossy, dark, iridescent plumage and pale eyes, while the females have dark eyes and are browner.  The Brewer's Blackbird tends forage in open areas, such as agricultural fields.

Brewer's Blackbirds shine in the sun.

The Rusty Blackbird is a widespread species that, sadly, has experienced steep population declines—likely as a result of climate change and habitat loss—over the last several decades.  Although it has become scarce, you can still occasionally find larger flocks of these birds during the colder months.  As the name suggests, both males and females have rust-brown coloration in the winter (although males are black in the breeding season).  

Bottomland hardwood forests—or habitats at the borders of these types of woods—are particularly attractive to Rusty Blackbirds during the winter.

Notice the rusty barring and pale eyes.

While several other blackbirds and associated species occur in North America, these are the most common ones over much of the eastern region.  The next time that you're in the field, take a closer look at blackbird flocks—you may be surprised at the diversity of species that you find! 

Oak Masting

Do you see many oak trees in your area?  Did you know that there are two main types (or sections, in botanical terminology) of oaks in eastern North America?  While oaks are extremely variable, hybridize often, and can be quite difficult to identify to species, figuring out which section they belong to is relatively easy.  If they are white oaks, then they will have smooth margins on the lobes of their leaves, rounded lobes, and acorns near the ends of their branches.  Red oaks, on the other hand, will usually have a bristle at the end of each leaf lobe (which will probably be pointed instead of rounded, but there are several exceptions), and their mature acorns will be farther down on the twigs.  The reason for the difference in acorn position is that white oak acorns mature in a single season, while red oak acorns take two full seasons to mature.  Therefore, red oak branches will display additional growth past the acorns.  This feature is very noticeable once you start looking for it.

Acorns are at the tips of branches in white oaks (left) and at the earlier growth level in red oaks (right).

Oaks are perhaps the most familiar trees that produce large crops of seeds on a synchronous, highly variable basis, although they’re far from the only ones.  The mass production of seeds across an entire population of plants is referred to as masting.  Most oak species have these masting events every two to six years, and these mast production years can have significant long-term impacts on forest ecosystems.  In general, high acorn production has a positive effect on rodent and deer populations and a neutral to negative effect on nesting songbirds. 

One of the most famous studies to demonstrate the link between oak masting and rodent populations was conducted by Jerry Wolff in the Allegheny Mountains of southwestern Virginia.  Wolff (1996) surveyed populations of mice and eastern chipmunks by live-trapping and ear-tagging the animals, and recording data on their sex, age, mass, and reproductive condition.  Wolff’s data showed that populations of these rodent species peaked in the summers of 1981, 1986, 1989, and 1990, while an index of oak mast data for the area showed that the highest production occurred during 1980, 1985, 1988, and 1989.  What this indicates is that, while the rodents were enjoying an abundant food source during the high mast years, they were increasing both their chances of survival during the winter and their rate of reproduction during the following summer.  High mast years weren’t bad for the oaks, either: because there were more acorns than could be consumed by the rodents, more seeds than usual were able to germinate.

Mice, chipmunks, and squirrels will sometimes prey on eggs and young in birds’ nests, so, as might be expected, larger rodent populations can negatively affect bird species that nest either in low shrubs or on the ground.  One study in a forest near Front Royal, Virginia, showed that nest predation by rodents increased significantly following high mast years, and that Breeding Bird Survey indices for both Hooded and Worm-eating Warblers correlated negatively with high mast production years (McShea 2000).  However, it’s important to note that not all ground-nesting bird species will necessarily be affected.  For example, Ovenbird populations in Hudson Valley, New York, remained the same after increases in rodent numbers (Schmidt and Ostfeld 2003).  Schmidt and Ostfeld also found that Sharp-shinned and Cooper’s Hawks, which can feed on small mammals as well as on birds, became more abundant after high mast years—not surprising when you consider the rodent population boom that often follows a mast event! 

The links between deer populations, oak mast, and bird nest predation are a little more complicated.  Deer are more likely to produce twins after a high mast crop in the fall (Ostfeld et al. 1996), so it seems clear that acorn mast has a positive effect on deer populations.  Deer are heavy browsers of forest plants, though, and the previously mentioned study by McShea (2000) found that deer significantly reduced the understory vegetation within the forest community when their numbers were high, and that populations of white-footed mice and eastern chipmunks increased when deer populations were excluded from study plots.  McShea did not observe significant impacts on bird nesting success due to the presence of deer, but he did note that research by Leimgruber et al. (1994) discovered that, when vegetation density was high, nest predation rates tended to be low.  Therefore, even though numbers of nest-raiding rodents tend to increase when there are fewer deer in a forest, ground-nesting birds still benefit from the higher density of vegetation.

If deer populations increase after mast years, how does that affect the spread of Lyme disease?  Lyme disease is caused by a bacterium, Borrelia burgdorferi, which is mainly transmitted by the black-legged tick (Ixodes scapularis), a common parasite of deer and mice.  Ostfeld et al. (1996) noted that larval tick populations were around 10 times higher in oak forests than in other habitats after a mast year.  Additionally, deer avoided oak-dominated habitats during poor mast years, preferring instead to use forests dominated by maples and other tree species.  When the deer did this, larval tick populations also increased in those habitats. 

Yet another study (Ostfeld et al. 2001) monitored black-legged ticks, white-footed mice, and acorn production during the growing season in southeastern New York, and found that the number of B. burgdorferi-infected ticks was higher in the two years following a high mast year.  Not surprisingly, this population increase was also correlated with increases in mouse populations.  In case you’re wondering, ticks can be collected for research by dragging white cloths through study plots.  As anyone who has walked through forest undergrowth during the summer months knows, it really doesn’t take much effort to find and collect ticks!

As we’ve seen, oak mast directly and indirectly affects populations of other organisms.  But these relationships are far from one-sided; it’s important to understand that animal behavior in turn influences oak evolution.  For example, research by Steele et al. (2001) demonstrated that small mammals can strongly influence the growth and dispersal of oaks.  In that study, gray squirrels were found to prefer the acorns of white oaks to those of red oaks, because white oak acorns lasted longer in winter caches.  Also, squirrels performed embryo excision—that is, the killing of an acorn embryo by notching a seed at its apex—on their cached white oak seeds far more frequently than on red oak seeds.  This is probably because white oak acorns germinate earlier and more rapidly than those of red oaks, and squirrels can’t risk losing their cached food supply.  Of course, white oaks’ rapid germination also ensures that reproduction takes place despite some loss to squirrel caching.  It’s basically an evolutionary tug-of-war. 

Identification tip: white oak acorns (left) tend to have much deeper caps than red oak acorns (right).

On an even broader ecological scale, Blue Jays play an important role in the dispersal of oak species across eastern North America, and have helped to determine the present-day distribution of oak species across the continent.  Paleontological evidence suggests that, between around 126,000 and 11,700 years ago, Blue Jays dispersed oaks northward beyond what had been their usual range at the time (Johnson and Webb 1989).  Blue Jays can transport acorns hundreds of meters away from the source tree, and, for every acorn that they consume, they disperse about three.  Because climate change could potentially impart major changes to oak distributions in North America, dispersal of oaks by Blue Jays ultimately may help to compensate for areas that are unsuitable for oaks’ continued survival.  Hopefully, though, we won’t have to find out if that’s the case.

To sum up: Oak mast production and the presence of oaks in forest communities have many significant relationships with other species within the community, and are an essential part of the community food web.  Take a look around your area and see what sorts of oak-related ecological interactions are occurring!

References

Johnson W. C., & Webb, T. III. 1989. The role of blue jays (Cyanocitta cristata L.) in the postglacial dispersal of fagaceous trees in eastern North America. J. Biogeogr. 16:561-571.

McShea W. J. 2000. The influence of acorn crops on annual variation in rodent and bird populations. Ecology 81(1):228-238.

Ostfeld R. S., Jones C. G., Wolff J. O. 1996. Of mice and mast: ecological connections in eastern deciduous forests. BioScience 6(5):323-330.

Ostfeld R. S., Schauber E. M., Canham C. D., Keesing F., Jones C. G., Wolff J. O. 2001. Effects of acorn production and mouse abundance on abundance and Borrelia burgdorferi infection prevalence of nymphal Ixodes scapularis ticks. Vector Borne. Zoonotic. Dis.       1(1):55-63.

Schmidt K. A., & Ostfeld R. S. 2003. Songbird populations in fluctuating environments: predator responses to pulsed resources. Ecology 84(2):406-415.

Steele M. A., Turner G., Smallwood P. D., Wolff J. O., Radillo J. 2001. Cache management by small mammals: experimental evidence for the significance of acorn-embryo excision.    J. Mammal. 82(1):35-42.

Wolff J. O. 1996. Population fluctuations of mast-eating rodents are correlated with production of acorns. J. Mammal. 77(3):850-856.