Tag Archives: migration

Red (white and blue) carpet for migrant birds

They’re forest beacons, glowing come-hithers to migrant birds. These gene catapults, each carrying a plant’s hope for another generation, are ripening this week in Sewanee and across the eastern US. Not coincidentally, thrushes and other south-bound birds are also on the move (see BirdCast for the feathered forecast near you).

Some fruits from the woods near Sewanee:

 

Major bird kill at Sewanee’s Memorial Cross

A scene of bird carnage at Sewanee’s War Memorial Cross on the campus of the University of the South (photo credits: Sandy Gilliam):

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The cross is a tall white structure, brightly illuminated all night by spotlights. It stands on an overlook on the edge of the Cumberland Plateau in Tennessee, surrounded by hardwood forests, and visible from many miles away in the valley below. The night of the kill — 26/27 September — was cloudy and birds got caught in the dome of light, circling circling circling until they killed themselves by smashing into the out-sized reconstruction a Roman instrument of execution. Over 130 birds were killed.

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Catbirds

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Sora rails and American redstart

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Yellow-billed cuckoos and wood thrush

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Pied-billed grebes, nighthawk, and nightjar

Many bird migrate at night, avoiding predators, staying cool, and navigating by the stars. Bright electric lights disorient them and many are killed each year in collisions with communication towers, buildings, and other artificially lit structures. The numbers killed in this way are hard to know, but one recent review estimated “hundreds of millions (building and automobile collisions), tens of millions (power line collisions), millions (power line electrocutions, communication tower collisions)”.

Data from ebird.org (an online database of millions of bird sightings) reveals the migration patterns of birds. The following graphs are from the “Central Hardwoods” and “Appalachian Mountains” of Tennessee (to run your own analyses, click on “explore data” at ebird). The height of the green bars on the graph indicate abundance. Each species has a different seasonal pattern (some are here only in the summer, others surge through in migratory pulses, and many are winter visitors). For many migrant songbirds, mid-September through early October are peak times for movement through our region. These migrants are often very abundant in the forests around Sewanee: every bird taking the overland route from the northern US or the the vast Canadian boreal forest is winging through the southeast.

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Within a few hours of the discovery of the dead birds, staff from the University’s offices of Environmental Stewardship & Sustainability and Physical Plant Services were developing a plan to rework lighting to avoid future kills during bird migration. Many thanks to Nate Wilson, Sandy Gilliam, Greg Rollins, William Shealey, and colleagues for their rapid and proactive response.

Reminder of what flies over unseen on autumn nights

A sora, dead on the road outside the post office in Sewanee, Tennessee. These are wetland birds of the north. A dry road gutter in a town built in the southern forested uplands is a far cry from the sora’s usual marshy home.

img_20160912_133918895 This is not the first dead rail that I’ve seen on Sewanee’s roads during the autumn. Their nocturnal migratory flights carry them over the Cumberland Plateau. Perhaps they are lured then confused by the “security” lights that festoon our small downtown area. An early morning driver must have struck this bird as it wandered the road.

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Those stubby wings are well adapted to movement in the dense vegetation of marshes. They’re also powerful enough to carry the birds — those that escape our lights and tires — over the Gulf of Mexico to their wintering grounds in Central and South America.

Listen as you walk in the evening: autumnal migrants are streaming through the skies every night, calling as they fly. Chirp, tzup, zzip, the sound of hundreds of thousands of memories of wetland, forest, and prairie, winging bird-thoughts south, away from the leading edge of winter. We’re hearing part of the landscape’s mind in motion. On the roads, in the morning, tiny flecks of lost understanding.

 

 

Continental gyre of birds.

A paper published this week by scientists at the Cornell Lab of Ornithology uses eBird data to map the migratory paths of birds in the Western Hemisphere. eBird is a free online checklist program used by tens of thousands of bird watchers. The database now has millions of observations from across the world, most of them in North America. This latest study used 6.1 million eBird checklists to find the “center” of bird species’ ranges through the year. The summary below shows, for 118 species, how these centers move. Each point is one species, tracked through checklists over a year.

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A pattern emerges: birds tend to “loop” clockwise, migrating north in the spring overland through Central America and Mexico, then flying south in the autumn over the Gulf of Mexico and the Atlantic. These loops are caused by prevailing winds, especially the northeast trade winds over the ocean. Turn, turn, turn: sung by the Byrds.

To my eye, deviations from the pattern are just as fascinating. Why do some species veer so far east? Why do some linger in the south, then zip north in a rush? A map with each dot labelled by species lets us peer into these mysteries.

To write honestly and with conviction anything about the migration of birds, one should oneself have migrated. Somehow or other we should dehumanize ourselves, feel the feel of feathers on our body and wind in our wings, and finally know what it is to leave abundance and safety and daylight and yield to a compelling instinct, age-old, seeming at the time quite devoid of reason and object.” — William Beebe

 

Red maple: the burn begins, warblers drawn to the heat

winter_no_AprilGentle, domesticated plants are singing springtime songs, lifting gardens with flowers and newly emerged leaves, but the forest is wintry, especially in the uplands. Mountain slopes may glow with ephemeral wildflowers and buckeye saplings, but the rolling tabletop of the Cumberland Plateau seems little changed from January.

Red maple trees are the exception. Oaks and hickories have their buds clamped shut, but red maple blooms are out. From a distance these trees seem to stand in a shroud of carmine smoke. Each tiny bloom is  wine-red, standing like a small flame at the tip of a long, twiggy taper. Many of these flames have already matured and fallen, so my feet to move, for a few moments, through a dust of fallen embers as I pass below the trees.

Not to belabor a point, but these trees have rather variable sexual systems. Red maple flowers are usually either male or female, although a few blooms are both. Individual trees carry all male, all female, or mixed collections of flowers. On mixed trees, single branches will usually grow just male or female flowers. Richard Primack studied a small population of these trees and has written an interesting discussion of how the red maple breeding system fits within the diversity found within the whole Acer genus.

Click on images for captions and a slideshow.

The flowers scattered across our trails are almost all males. Once they have shed their air-borne pollen, their work is over and they become food for worms. (Brave Percy undoubtedly walked among them during his sojourn in Sewanee; the photos above are from a trail close to his haunt at Brinkwood.) The female flowers intercept floating pollen and will, over the coming months, grow the maple’s distinctive samaras or “helicopter fruits.”

Along with these emerging flowers come insects, scraping and sucking and chewing the newly emerged vegetation. And along with the insects: birds. Black-throated green warblers, just back from Central America, are congregating in the maples. I counted three of the warblers in one tree; all were steadily working from one flower to the next, pausing to hurl a short song to the forest, then getting back to work, beak to bloom.