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.

September Bird Identification Quiz

As we move into the early fall, lots of birds will begin migrating.  Some have already started, and there are also many young-of-the-year individuals that are just now experiencing independence from their parents -- a double-edged sword, since large numbers of immature birds do not survive the hazards of their first year. 

With all of that in mind, let's look at some species that can be seen in the Southeast during the early fall!

1.  This species is a year-round resident across much of its range, but there are some areas where it is seen only during migration.

2.  This bird closely resembles another species that is often found in the same regions.

3.  Birds in flight sometimes present more of a challenge to observers!

4.  A similar species may cause some confusion.

5.  This bird has been expanding its range eastward in recent years.

Answers:

1.  Reddish Egrets are almost exclusively coastal.  In the central portions of the Gulf Coast, they can be found during the spring and fall months, and they are fairly common breeding birds in Mexico, Texas, and Florida, Cuba, and the Bahamas.  Their active style of foraging involves running and wing-flapping.

The Reddish Egret occurs in both a dark morph and a white morph.  Although adult dark-morph Reddish Egrets have the signature rust-colored heads and necks, immature birds, like the one in the photo, are entirely a blotchy gray.  They also have dark bills, while their parents' beaks are half pink and half black.

2.  Although Cooper's Hawks are more likely to conceal themselves in dense foliage, they can sometimes be seen hunting right out in the open.  The adult hawk in the photo has gray upperparts and reddish barring on its breast; immature birds will be brown above and brown-streaked below.  

The Cooper's Hawk resembles the smaller Sharp-shinned Hawk, but a number of field marks help to distinguish these two species.  For one thing, the Cooper's Hawk's tail is proportionally longer, and usually has a rounded tip with a broad, white edge.  The Sharp-shinned Hawk has a shorter tail with a squared or slightly notched tip.  Sharp-shinned Hawks also have more of a hooded appearance than their larger relatives, which have dark "caps."

3.  The Turkey Vulture is such a common bird that its easy to overlook them in favor of rarer and flashier species.  But this species is fascinating, in no small part because of its large olfactory bulb, which gives it the ability to locate carrion by the smell.

Turkey Vultures' V-shaped wing posture and silvery wing linings are useful field marks for distinguishing them from the shorter, stockier Black Vultures, which hold their wings flat and have pale outer primary feathers.  You may see both Turkey and Black Vultures soaring on thermal currents, although the Turkey Vulture is far more migratory than its smaller relative.

4.  When its polka-dotted tail is not in view, the Yellow-billed Cuckoo (colloquially known as the rain crow) is a little trickier to identify.  However, the yellow mandible and reddish wing feathers are diagnostic.  The more northerly Black-billed Cuckoo has a bill that is either black or, in juveniles, grayish.  Both of these species are highly migratory, breeding in North America and wintering in South America.

You may find cuckoos in thickets and woodlands this autumn, especially if there are plenty of tent caterpillars—popular prey items—in the area!

5.  The Scissor-tailed Flycatcher is declining across much of its range, so the range expansions aren't necessarily indicators of a strong population.  I have observed these birds on Dauphin Island during the fall migration, and they are still common across a sizable portion of the south-central United States throughout the breeding season.  The streamer-like tails and salmon-colored underparts of the adult birds are unmistakable.  Immature individuals have shorter tails and yellow underparts.

Several years ago, I was one of several birders who got to see a Scissor-tailed Flycatcher nest in north-central Mississippi!

Late Summer Bird Identification Quiz

For southeastern North America, this is basically late summer.  With that in mind, let's have a look at some birds that may be observed at this time of year!

1.  The plumage of this first bird might be confusing for some birders.

2.  Hint: This species was featured in an earlier quiz.  It is a bold and sometimes aggressive bird.

3.  Watch for these near water!

4.  A familiar species may seem unfamiliar if the environment doesn't provide enough clues.

Answers:

1.  I'm used to seeing Little Blue Herons with solid gray-blue adult plumage, white immature plumage, or an intermediate blue-and-white patchiness.  This individual was mostly blue, but the small patches of white on its belly and back were reminiscent of the plumage of the Tricolored Heron, a coastal species that occasionally strays inland.  This Little Blue Heron lacks the Tricolored Heron's white feathering on the front of the neck, and it doesn't show any yellow on the bill or in front of the eye, as Tricolored Herons would in the late summer.

For comparison, here's a Tricolored Heron in August:

2.  With its pugnacious attitude toward birds and other animals several times its size, the Eastern Kingbird lives up to its classification as one of the tyrant flycatchers.  It frequently perches high in snags and other vegetation, and its bright white underparts, dark upperparts, fairly thick bill, and moderate proportions are distinctive.

3.  The White Ibis is a wading bird that occurs over a wide range in southeastern North America and Central America.  The photo I used is tricky, as it doesn't really show the distinctive curvature of the birds' red bills!  The white plumage, black wing tips, and long legs help to clinch the identification, though.  White Ibises are experiencing population declines in Louisiana and a few other coastal areas, but they appear to be increasing farther north.  This probably bodes well for the birds' adaptability, but conservationists and birders should still keep an eye on those crucial, vulnerable wetland regions.

4.  Juvenile birds of all species may be seen in the late summer, and American Robins, which tend to nest around human dwellings, are some of the most frequently observed.  Unlike their parents, young American Robins have lots of speckling and barring on their plumage.  However, their "red" breasts and white facial markings are still noticeable.  This individual added to the potential confusion by flying down to a water source in a small woodland.

More Frogs and Toads

American toad (photo taken in Arkansas)

Summer is ramping up, which makes this a great time of year to look for frogs and toads.  I will examine species in the families Ranidae (or the true frogs), Bufonidae (the true toads), Microhylidae (the narrow-mouthed toads), and Scaphiopodidae (the spadefoot toads).

Of course, habitat determines which families of frogs that you are likely to encounter in a given area.  Moist wooded areas and gardens are good places to find members of Bufonidae, the true toads.  These mostly terrestrial amphibians possess neurotoxin-producing parotoid glands, so if you ever touch a toad, just be sure to keep your hands away from your eyes and mouth!  Toads are otherwise harmless.  (Well, unless you're a bug.)

One of the most abundant toad species in Mississippi is the Fowler’s toad (Anaxyrus fowleri).  Its distinctive call is a somewhat raspy, unmusical bleat (which is similar to that of the Woodhouse's Toad, the species in the linked audio file).  The American (Anaxyrus americanus) and southern toads (Anaxyrus terrestris) are two of the other common species in the state, and both have slow, drawn-out trills that you may hear in the evening.  The oak toad (Anaxyrus quercicus) is a fairly common but hard-to-find amphibian in the southernmost third of the state.  It produces a mellow peeping sound.  Yet another possible find in southern Mississippi is the Gulf Coast toad (Incilius valliceps).  Its call is lower in pitch than that of the southern toad.

Spadefoot toads are a separate family that have a hard, sickle-shaped spade on each rear foot that they use to burrow into sandy soil.  The eastern spadefoot (Scaphiopus holbrookii) is the only species that occurs in the southeastern United States.  It is widely distributed but local in Mississippi, and the call is a very loud, throaty sound. 

If you hear a whiny, high-pitched nasal bleat while walking around a pond or weedy ditch, try not to be too alarmed.  It’s not a half-submerged goat, but an eastern narrow-mouthed toad (Gastrophryne carolinensis), the plump, small-headed Microhylid that is also sometimes called the eastern narrow-mouthed frog. 

When you think of frogs, you probably also think of water, and there are several frog species that spend their lives around lakes, ponds, streams and marshes.  These are the true frogs, or the family Ranidae.  One of the most abundant and attractive Ranids is the southern leopard frog (Lithobates sphenocephalus).  The leopard frog, which can be green or brown, is named for the many dark spots that cover its back and legs.  The call of this species is a rolling series of clucks and croaks.  A frog that appears fairly similar to this one is the pickerel frog (Lithobates palustris).  The spots on this brownish frog are square, and its call is a low-pitched snore. 

If you're near a pond and hear a sound similar to the thump of a rubber-band, you’ve probably heard the bronze frog (Lithobates clamitans clamitans), a medium-sized brownish subspecies of the green frog.  In the northeastern corner of Mississippi, the greenish version is present instead of the bronze frog.

The largest Mississippi frog is the American bullfrog (Lithobates catesbeiana).  Nearly everyone is familiar with this huge frog that produces a deep sonorous rooum-rooum similar to that of a bull.  And if there’s a frog that sounds like a bull, then it shouldn’t be terribly surprising for there to also be one that sounds like a pig!  The pig frog (Lithobates grylio) occurs in the southern portion of the state and closely resembles the bullfrog.  Its snout is more pointed, however, and the fourth toe doesn’t extend past the webbing as it does on the bullfrog.  Its call is a series of deep hoglike grunts. 

There are a few other Mississippi frogs that are very local or rare, including the gopher frog (Lithobates capito), the Mississippi gopher frog (Lithobates sevosus), and the river frog (Lithobates heckscheri).  Notably, the Mississippi gopher frog is a federally endangered species found in only a few locations on the coast.  Its rarity is a reminder of how fragile ALL amphibians and their habitats can be, so be sure to tread carefully in the frog environs that you may visit.


Sources:

The IUCN Red List of Threatened Species, http://www.iucnredlist.org/

USGS Frog Quiz, https://www.pwrc.usgs.gov/frogquiz/index.cfm?fuseaction=main.lookup

Finding Summer Treefrogs and Cricket Frogs

Mississippi is home to quite a few taxonomic families of frogs and toads, including Ranidae, or the true frogs (such as bullfrogs and leopard frogs), individuals of which are largely aquatic and can be found in ponds, lakes, and streams; Bufonidae, or the true toads; Microhylidae, or the narrow-mouthed toads; Pelobatidae, or the spadefoot toads; and Hylidae, or the treefrogs, a group that encompasses the chorus frogs (including the spring peeper and the upland and Cajun chorus frogs), as well as many other species that can be heard calling right now.  Let’s take a look at some of these hylids.

Treefrogs are distinguished by the sticky pads at the ends of their toes, which enable them to climb.  They are typically found in trees and shrubs near sources of water.  In the southeastern U.S., one of the most common species is the American green treefrog (Hyla cinerea).  As its name suggests, this frog is typically bright green.  However, like other treefrogs, it is capable of changing color to adapt to changes in temperature and environmental surroundings.  It usually, but not always, has pale stripes running along its sides, and its skin is very smooth.  The call is a nasally quank that carries for a good distance.  Large choruses of this frog can be heard in almost any wet, wooded area.

Another common species is the squirrel treefrog (Hyla squirella), a small frog with a raspy call that is reminiscent of a squirrel’s barking sound.  This extremely variable species may appear almost any color from bright green to dusky brown, depending on environmental conditions.  At first glance, a bright green specimen of the squirrel treefrog might be mistaken for a green treefrog.  However, the squirrel treefrog is smaller and always lacks the pale lateral stripes of the green treefrog.  Squirrel treefrogs were formerly restricted to the southern half of Mississippi, but have recently expanded their range northward. 

The next three species to be discussed, the common gray, Cope’s gray, and bird-voiced treefrogs, can be visually separated from the green and squirrel treefrogs by virtue of their bumpy skin.  Like all treefrogs, they are more likely to be heard than seen, so it is still a good idea to learn to recognize them by their calls.

As their name suggests, gray treefrogs are frequently mottled gray in color, although they may also be various shades of green or brown—again, depending on temperature and habitat.  The two species of gray treefrogs, common gray (Hyla versicolor) and Cope’s gray (Hyla chrysoscelis), look identical and are impossible to distinguish from each other in the hand.  This is due to the fact that one species, the common gray treefrog, is thought to have diverged from the other, the Cope’s gray treefrog.  Interestingly, common gray treefrogs are tetraploid, meaning that they have four pairs of chromosomes (i.e., genetic material)—twice the usual number.  Tetraploidy is fairly common in plants and invertebrate animals, but it is not as frequently observed in vertebrates.  While common gray and Cope’s gray treefrogs cannot be reliably distinguished from each other by sight, they can be easily identified by sound.  Both species have low-pitched trills, but the Cope’s gray treefrog’s call is faster and more snore-like than the drawn-out trilling of the common gray treefrog.  If you are a Mississippian, though, it's possible that you will not need to distinguish between the two, since the common gray treefrog may occur in only the northernmost sliver of the state: http://maps.iucnredlist.org/map.html?id=55687.  The Cope's gray occurs statewide: http://maps.iucnredlist.org/map.html?id=55448.

Bird-voiced treefrogs (Hyla avivoca) look like smaller versions of gray treefrogs.  One quick way to identify them in the field is to look at the ventral portions of their legs; unlike gray treefrogs, which have bright orange skin in this area, bird-voiced treefrogs have greenish-yellow.  You should have no trouble with identification if you hear the frogs, though; the whistling, bird-like call of the bird-voiced treefrog is drastically different from the sounds of gray treefrogs.  Also, while you could find gray treefrogs in almost any type of forest, bird-voiced treefrogs tend to be most common in swampy woods, especially those containing cypress and tupelo trees.  This is one of my favorite frog species in the state.  

The pine woods treefrog (Hyla femoralis) and the barking treefrog (Hyla gratiosa) are two species found in the deep southeastern portions of the U.S.  Unfortunately, I have had few encounters with either, and have heard pine woods treefrogs only in forests on the coast.  However, a friend and mentor of mine has heard that species as far north as Newton County, Mississippi, so it is certainly possible to find them fairly far inland.  They have strange calls—perhaps best described as sounding like hyperactive Morse code messaging.  The call of the barking treefrog, on the other hand, is throatier in sound than the green treefrog’s call.  Choruses of this species bring to mind—well, for me, anyway—hundreds of bouncing rubber balls.  The barking treefrog frequently has dark spots and is also noticeably bumpy-skinned, making it easy to distinguish from green treefrogs.  In the hand, pine woods treefrogs could be mistaken for squirrel treefrogs, but their inner thighs have rows of pale spots, unlike those of squirrel treefrogs.

The tiny, aptly-named cricket frogs are not technically treefrogs, as they belong to the genus Acris.  Nevertheless, they are hylids, and, like their relatives, they can commonly be found in vegetation near water.  Unlike treefrogs, cricket frogs tend to stay very low to the ground.  The southern cricket frog (Acris gryllus) and the northern cricket frog (Acris crepitans) are the two most common species in the state.  They are similar in appearance, and their insect-like calls are also a bit difficult to distinguish from each other.  However, the southern cricket frog has a raspier sound than the northern.  Both of these tiny frogs can be found in grasses and herbaceous vegetation near water. 

So much for the adult hylids—but what about the earlier life stages: eggs and larvae?  Well, if you peer into small ponds, woodland pools, roadside ditches, or potholes on old dirt roads, you might be lucky enough to see these.  Frog eggs are in clusters, while toad eggs are in strings.  Tadpoles may take a month or more to fully metamorphose.  May the summer months be hopping!

Sources

The IUCN Red List of Threatened Species, http://www.iucnredlist.org/

USGS Frog Quiz, https://www.pwrc.usgs.gov/frogquiz/index.cfm?fuseaction=main.lookup

Species Profile: Cope’s Gray Treefrog (Hyla chrysoscelis), Savannah River Ecology Laboratory, http://srelherp.uga.edu/anurans/hylchr.htm

Species Profile: Pine Woods Treefrog (Hyla femoralis), Savannah River Ecology Laboratory, http://srelherp.uga.edu/anurans/hylfem.htm