Urban Jungle

Crocuses display their thermonasty

Crocuses open their flowers as temperatures rise and close them when the mercury falls, sometimes repeating the action several times during the day. The tendency is known as thermonasty, a trait they share with tulips.

A closed flower will protect the precious pollen within from snow and rain, conserving it until warmer weather brings out more bees and other pollinators.

A temperature rise of only 0.36 degrees is enough to begin to open a crocus.

SOURCE: Journal of Experimental Botany

Advancing mats of invasive lesser celandine crowd out many native spring wildflowers, but common blue violets -- native, but weedy in their own right -- manage to pierce the alien yellow carpet to add some complementary color.

One reason that lesser celandine is effective in overwhelming its native counterparts is because it emerges earlier. Land managers who wish to curb its spread sometimes use that trait against it, by spraying herbicide on the leaves in late winter, while it's the only plant exposed.

SOURCES: Plant Conservation Alliance; National Park Service

In a community garden beneath the flight path of jets roaring out of Reagan National, a song sparrow repeatedly shouts its claim to territory and enticement to mates. A nearby leaf blower joins the chorus. Song sparrow songs occupy a prominent spot in the city's spring soundscape, adding bright icing to a thick cake of background noise.

"Suitable breeding habitat is not simply comprised of space and food, but also an auditory opening," says Stephen Yezerinac, an ornithologist at Bishop's University in Quebec. "Humans are altering all of the above in urbanized areas, and birds are being affected."

While at Reed College in Oregon, Yezerinac and student William E. Wood examined the effects of urban noise on the song sparrow songs of Portland. They recorded 28 birds in areas with various levels of background noise, carefully measuring frequency (pitch) and amplitude (volume; the amount of energy invested in making a sound).

Birds in the noisiest spots were more likely to cede some of their lower frequencies to background noise, while their songs' higher-frequency notes remained constant.

Listen to a local song sparrow compete with an airliner and a leaf blower:

SOURCE: "Song sparrow (Melospiza melodia) song varies with urban noise," The Auk, 2006

Song sparrows are notable for numerous regional variations in plumage, size and song. Individual birds seem to embody that plasticity with a repertoire of five to 13 songs.

Wood and Yezerinac suggest that urban sparrows may be evolving higher-pitched songs. Many factors are at work:

• Birds may be choosing to sing songs that contain higher notes, or are raising pitches to stand out above the noise.
• The regions of the birds' brains associated with song undergo partial renewal each spring, which may cement into the bird's head certain songs tailored for a noisy territory.
• Birds may learn modified songs, or may not be able to hear low notes, or they may simply drop low notes that aren't effective.
• Females may be more likely to choose males who sing in higher frequencies, and are known to select males that show a proficiency for learning.
• If birds sing more loudly in response to noise, they will use more energy, which could lead to either shorter, more efficient songs or diminished vigor.

In early May, sticky sweet nectar oozes out of the flowers of tulip trees and splatters onto leaves, branches and car windshields.

Also known as tulip poplar and yellow poplar, the tree is not really a poplar, but a magnolia, with flowers resembling tulips.

In a couple of weeks the blooms will fade and collapse, leaving behind a slender cone that will mature in the autumn and release seeds in the winter.

Nectar collects on one of the flower sepals, right. Its bright, sweet flavor is followed by a dry, slightly bitter aftertaste.

The flowers are a big draw for hummingbirds and honeybees. A 20-foot tree can produce eight pounds of nectar, which bees are able to convert into four pounds of dark amber honey.

William Bell, who keeps 15 hives of bees in Isle of Wight County, Va., says that in a good year "a hive can produce 60 to 70 pounds of good spring honey," derived predominantly from tulip trees. "It's got a very sweet, bold taste."

SOURCES: National Forest Service; Nuby Run Bees

Insects have begun to eat holes in this tree's leaves. A favorite of livestock, cottonwood leaves are rich in protein, offering more amino acids than many common grains.

Cottonwood seeds:

A poplar lottery

Drifting on breezes and riffles, cottonwood seeds dot the sky and fleck local waterways. The odds that any one seed will germinate are remote.

Also known as Eastern poplar, a single cottonwood tree can release more than 25 million seeds, each suspended by a frizzy mass of cottony fibers that can transport the seed far from the tree.

Seeds set sail at about this time of year, when water levels are dropping, revealing freshly scoured gravel bars, sandbars and stream banks, ideal spots for young cottonwoods.

The fluff must settle onto the right surface soon. Seeds are viable for only one to two weeks, and they stand a chance of germinating only if they fall onto sunny, moist, exposed soil.

Lucky seeds sprout within a day, after which they grow as much as a quarter of an inch in their first 24 hours. To survive long enough to be saplings, seedlings need constantly moist soil with abundant sunlight. Most seedlings are either trampled and eaten by herbivores, overrun by exotic invasive plants, beaten down by rain, swept away by floods or -- if they survive long enough -- gouged by winter ice.

Young saplings can tolerate inundation, but a drought will kill them. They devote much of their energy to sinking roots down toward the water table, piercing the soil by as much as a meter in their first year.

Once saplings have tapped a guaranteed source of water, they can grow 30 to 50 feet in five to 10 years, at which point they reach maturity. Female trees can then begin to disperse a multitude of their own fluffy long shots.

A cottonwood seed (actual size and enlarged four times) discards its parachute soon after landing. A million of these would weigh less than three pounds.

SOURCES: "Biology of Populus and its implications for management and conservation," by Reinhard F. Stettler, et al.; U.S. Forest Service

In every garden a shrew

Young short-tailed shrews are leaving the nests where both parents raised them this spring.

Born in late March and reared in an underground nest of shredded vegetation, a shrew pup and its half-dozen siblings will reach sexual maturity before summer. Shrews breed through September.

Highly abundant in cities and suburbs but seldom seen, short-tailed shrews thrive in vegetable and flower gardens, where they cruise through an elaborate network of passageways at the soil surface.

Shrews eat as much as three times their weight every day. Their diet includes earthworms, insects, slugs, nuts, berries and the shrew equivalent of big game: mice and voles, both of which are larger and heavier than they are.

A shrew's limited sense of smell and sight is offset by its use of echolocation. As with bats, shrews emit a series of ultrasonic clicks that bounce back, enabling them to navigate through dark spots and locate food.

A short-tailed shrew uses it sharp, red-tipped teeth to deliver a poisonous bite that incapacitates its prey. Shrews tend to hoard their food and can store enough stunned snails and beetles to provide a chance at surviving the winter.

Humans are too large to be paralyzed by the toxin in a shrew's saliva, but a small wound from a shrew bite can remain painful for days.

SOURCES: University of Michigan; National Museum of Natural History

Black and white and mean all over

The yearly onslaught of Asian tiger mosquitoes has begun, and these pests will torment gardeners, picnickers and porch-sitters until the next hard freeze.

Unlike native mosquitoes, which feed at dawn and dusk, invasive tiger mosquitoes bite at all hours, often employing the stealth tactic of feeding behind our knees and elbows. They bite quickly. Swat at them and they seem to vanish.

About five days after filling her tank with your blood, a female will lay eggs on a hard surface just above a source of water. When rain raises the water level, the larvae hatch and begin to feed on decaying plant matter in the water. In a week or so, those "wigglers" will have transformed and taken flight as blood-hungry females and nectar-sipping males.

People can reduce their neighborhood's swarm by eliminating standing water in gutters, buckets and overflow dishes beneath potted plants. Frequent replacement of birdbath water is critical.

Tiger mosquitoes are potential vectors for dengue fever, viral encephalitis, dog heartworm, West Nile virus and chikungunya fever.

Water gardeners are wise to stock their backyard ponds with Eastern mosquitofish, native minnows with a voracious appetite for mosquito larvae. The small fish have difficulty surviving the winter in pools shallower than three feet, but they can be kept alive through the cold months in a sunny indoor tank filled with pond water and a fistful of aquatic plants.

SOURCES: Centers for Disease Control and Prevention; University of Florida Institute of Food and Agricultural Sciences; D.C. Fisheries and WIldlife Division