Entries Tagged as 'Migration'
Abundant evidence exists to show that the earth has been warming over the past century. This evidence includes the melting of the polar ice caps with the consequent rise in sea level, earlier leaf-out dates for trees and bushes, earlier ice-out dates of lakes and the northern range expansion of various species.
The spring arrivals of migratory breeding birds are also sensitive to a warming world. With leaves emerging earlier, caterpillars become active earlier, providing earlier food for warblers and vireos. Nectar for hummingbirds should be available earlier.
A number of studies have documented earlier arrivals of migratory breeding birds. Bird clubs in the Worcester, Massachusetts area and in the Ithaca, New York area have been compiling first arrival-dates of migratory birds for over 100 years. The data clearly show earlier arrivals in recent years for nearly all species of migratory birds frequenting those areas.
In 1994, I began a citizen-science project to monitor the arrival dates of over 100 species of Maine migratory breeding birds. Each volunteer is asked to note the first arrival of each species along with the location of the sighting. This on-going project has taught us much about the nature of spring bird migration across the state. We know have over 55,000 arrival dates in the dataset. The collective contributions of so many volunteers have made this project possible.
We know over the period 1994 to 2014 that some bird species arrive a bit later in cold springs and a bit earlier in warm springs. However, Maine does not have a continuous record of arrival dates to rival those of the Ithaca and Worcester bird clubs.
Maine however did have a bird organization, the Maine Ornithological Society, active around the turn of the 20th century. This organization published the quarterly Journal of the Maine Ornithological Society (JMOS) from 1899-1911. The journal regularly published arrival dates of Maine birds along with censuses of birds. Danny Kipervaser, Scott Lilley and I collated the arrival date data from the JMOS to compare to contemporary arrival dates. Our expectation was that birds should be arriving earlier now compared to then. Our predictions missed by a mile.
We only had sufficient data from the JMOS for 80 of the species I track in the current arrival date project. Of those 80 species, only nine are now arriving earlier in modern years. These included inland Common Loons, Great Blue Herons and Red-winged Blackbirds, all of which need open water in their lake or marsh habitats. The other six species were American Woodcock, Ruby-throated Hummingbird, Tennessee Warbler, White-throated Sparrow, Rose-breasted Grosbeak and Indigo Bunting. Most of these species arrive between four and eight days earlier now.
The most common result was to see no significant change in arrival dates across the time span. Fifty-one species are arriving at the same schedule now as they did a century ago. Quite a contrast with the Worcester and Ithaca records.
Remarkably, 20 species are now arriving significantly later than they did around the turn of the 20th century. These species include seven aerial insectivores (swallows, nighthawks), three warblers, four sparrows and Bobolink.
What’s going on here? I see two possible biases. First, the amount of forests we have in Maine now is far greater than in 1900 when farmland was more extensive. As a result, habitat for the sparrows and Bobolinks that seem to be arriving later were almost certainly more abundant then than they are now. Thus, the chances of seeing an early arriving bird were higher a century ago.
Second, the JMOS birders were likely more in tune with nature than we are. Outside on foot or on horse-drawn carriages, these observers would be less likely to miss a first arrival. Despite our fine optics, the JMOS ornithologists might well have been keener observers than we are.
[Originally published on May 16, 2015]
Tags: Migration · Weather
Purple Finches are regular if erratic visitors to our feeders. The gorgeous males with their deep red breasts and heads can only be confused with male House Finches. A male House Finch has brown streakings on the flanks with less extensive red (more reddish-orange) on its head.
Female Purple Finches are mostly brown and white. The breast has dark streaks. A bold white stripe lies just above the eye; this white supercilium is absent in female House Finches.
Have you noticed that male Purple Finches are usually outnumbered by females at your feeder? Not so fast. First-year males are dead ringers for female Purple Finches. You really have to have them in hand to tell them apart by examining the wear of the primary coverts and the shape of the tips of the outer tail feathers. Some of those streaked Purple Finches at your feeder are first-year males.
Purple Finches belong to the suite of irruptive finches popularly called the northern finches. Purple Finches breed across the northern tier of the U.S. from Maine to Minnesota and across the southern tier of the Canadian provinces. A breeding population also occurs west of the Cascades and Sierra Nevada from California to British Columbia.
Strong southerly irruptions occur every other year in this species. These biennial irruptions are thought to be driven by variation in the production of cone crops of the conifers on which the Purple Finches depend.
To gain some insight into these movements of Purple Finches, my wife Bets Brown and I analyzed all of the banding data on Purple Finches from the Bird Banding Laboratory over the period of 1921 until 2008. Over 745,000 finches were banded over this period and almost 20,000 of those banded birds were subsequently recaptured (or in a few cases, found dead).
We were particularly interested in three questions. During irruptions, do birds from one area like New England move straight south or do they spread across the continent (Purple Finches can be found throughout North America)? Do Purple Finches show fidelity to breeding sites? Do Purple Finches show fidelity to wintering sites?
The analysis of banding data presents many challenges. Banding effort is never constant either across space or time. Most of the banding records come from the period 1960-1985. Banding effort varies greatly among states and provinces. Nevertheless, some general patterns can be discerned.
Birds banded in VT, NH and ME were re-encountered broadly but mostly in the eastern United States, curling eastward through the Gulf Coast states. A few reached Texas and modest numbers occurred in MI, ONT, WI and MN. A similar pattern emerged for birds originally banded in NY.
We also analyzed birds first banded in PA, NJ and NC. These areas are south of the breeding range and were banded in the winter. Re-encounters of these birds occurred mostly in the the New England and the eastern provinces. The re-encounters are consistent with the data from birds originally banded in New England.
Moving to to the Midwest, birds originally banded in MI, WI and MN also were re-encountered broadly but most irruptions were due south.
Only 275 Purple Finches banded in the Pacific states or BC were re-encountered. However, a consistent pattern was that those birds migrate west of the Cascades and Sierra Nevada.
We did find some evidence of breeding site fidelity across as many as five years. Some wintering site fidelity was evident as well over periods of one to six years. Uneven banding efforts prevent us from knowing how prevalent such fidelity is.
I’ll end with the most impressive distance between captures. A Purple Finch banded in Maine in 1966 was subsequently captured two years later in Texas, a distance of 1792 miles.
For a copy of our paper, visit http://bit.ly/1BntjMb
[Original published on March 29, 2015]
Tags: Identification · Migration · Species Accounts
We have certainly had no shortage of storms this winter. Perhaps you have wondered how birds can deal with the cold, the wind, and the snow to survive such challenging spells of bad weather. The research done on birds’ ability to anticipate storms has provided some insights but much remains to be done. In today’s column, I’ll discuss the results of two recent papers that shed light on the effect of impending storms on bird behavior.
The first paper by Henry Streby of the University of California and colleagues was published earlier this year in the journal Current Biology. The researchers had 20 Golden-winged Warblers fitted with geo-locators in April of 2013. These small dataloggers continuously record light-levels and time. From the data, a researcher can track the longitude and latitude of a migrating bird. Birds must be captured so that the data from the geolocator can be read.
In April of 2014, Streby and colleagues tracked the arrival of nine of these marked birds back to their mountainous breeding territory in northeastern Tennessee from wintering areas in South America. The birds arrived in Tennessee between April 13 and April 27, 2014.
Between April 27 and April 30, a massive storm that spawned over 80 tornadoes developed over the middle of the United States. The eastern Tennessee Golden-winged Warblers were able to detect the impending storm well before it arrived. What did the birds do? All nine of the warblers vacated their breeding grounds. Five of them did return after their storm and their geolocators were resampled. All five of these birds took evasive action to avoid the storm.
On April 27, tornadoes were being generated from Kansas to Texas. By the following day, the storm was less than 100 miles from the Golden-winged Warblers’ breeding area. The storm was quite powerful when it arrived in northwestern Tennessee, generating winds of over 100 mph.
But the wind posed no problem for the five warblers. They had moved to Florida’s Gulf Coast, beyond the range of the storm. One of the birds even flew to Cuba.
How did the birds know the tornadoes were coming? The authors believe that the birds were detecting infrasound, sound whose frequency is far too low for humans to hear. Tornadoes generate infrasounds that are propagated through the ground.
Storms are usually associated with low-pressure systems so falling barometric pressure could be a cue that a storm is approaching. A recent paper by Creagh Breuner and colleagues examined this phenomenon. The paper was published in 2013 in the Journal of Experimental Biology.
When humans hear that a blizzard is approaching, we see a run on groceries, batteries and candles. Shouldn’t birds prepare for inclement weather as well? The Breuner team addressed this question by studying White-crowned Sparrows on the breeding grounds at high altitude in Montana. Spring snowstorms are frequent.
In particular, the researchers searched for a relationship between falling barometric pressure and behaviors that might help the birds weather the storm. The researchers predicted that falling barometric pressure should cause an increase in mass (due increased feeding rate_, an increase in the rate at which fat is deposited and hormonal changes associated with stress. They also did experiments on captive birds by artificially lowering the air pressure and looking for changes in behavior.
The results were mixed. The authors clearly showed that the sparrows could detect changes in barometric pressure in their lab experiments. In the field, falling barometric pressure did not result in an increase in mass or in stress hormone production. The researchers did find a significant relationship between fat deposition and barometric pressure but the effect was very slight. In the lab, birds sometimes increased their feeding rate as pressure dropped but again stress hormone levels did not change. We have much more to learn about these intriguing responses.
[Originally published on March 1, 2015]
Tags: Behavior · Migration · Physiology · Weather
The irruption watch is on! Will Common Redpolls, Bohemian Waxwings, White-winged Crossbills, Snowy Owls and Pine Grosbeaks grace us with their presence this winter? Initial observations are promising as redpolls, Bohemian Waxwings and Snowy Owls have been reported widely in the state already.
These irruptive migrants are driven from their northerly breeding grounds by lack of food. Birds are remarkably tolerant of cold weather as long as they can find sufficient sustenance to keep their greedy furnaces stoked. When food on the Arctic tundra or the taiga becomes scarce, be it birch seeds, conifer seeds, lemmings or fruit, birds dependent on a particular food must move south or perish.
The Northern Shrike is another irruptive species. They are absent in Maine in some winters and occasionally common. At least a few Northern Shrikes have been seen in Maine. We’ll explore the biology of Northern Shrikes in today’s column.
The breeding distribution of Northern Shrikes spans the northern part of North American from Quebec to Alaska. When they withdraw from their breeding areas due to lack of food, they come farther south in the western United States, reaching southern Utah and Nevada. In the east, Northern Shrikes rarely occur south of Massachusetts and New York. This species also occurs in Eurasia where it is called the Great Grey Shrike.
Northern Shrikes are boldly marked birds, reminiscent of a bulky Northern Mockingbird. Northern Shrike adults have a gray back and crown with a narrow black mask through the eye. The wings are black with a prominent white patch. The tail is black with white outer tail feathers. The gray dorsal surface, white wing patches on black wings and white in the tail make it easy to dismiss a shrike as a mockingbird. The undersides are whitish-gray.
Immature Northern Shrikes look like washed out versions of adults. The upper parts are generally buff-colored; the mask is thinner and less darkly colored. The most distinctive feature is the prominent scaling on the breast and belly.
The similarity with mockingbirds ends when you check out the bill. The bill of a Northern Shrike is strongly hooked, used to kill small mammals, rodents and sometimes insects. A shrike is a hawk wannabe!
Northern Shrikes habitually perch at the top of a tall shrub or tree, appearing peaceful and docile. But when a potential prey is spotted, the shrike springs into action. A shrike usually captures insects or small mammals with its bill. Birds are pursued through the air and usually captured with the feet. Although the toes of shrike are not shaped like the talons of a hawk or owl, the feet are very strong. Friends of mine who band shrikes wear leather gloves when taking shrikes out of mistnets to avoid wounds from shrike feet clamping down on the banders’ hands.
Once a prey item has been secured, the shrike quickly kills it with a bite to the neck, severing the spinal cord at the level of the cervical vertebrate. The prey is typically impaled on thorns or barbed-wire, often left as a cache for later use. This macabre impaling behavior is the basis for the folk name of butcherbirds for shrikes.
While studying the effect of winter bird feeding on Black-capped Chickadees in a remote section of the North Woods east of Flagstaff Lake, I quickly learned when a Northern Shrike was in the neighborhood. The chickadees, woodpeckers and other birds frequenting the feeder would freeze. The cacophony of birds at the feeder halted; dead silence prevailed. Woodpeckers pressed themselves tightly against a tree trunk. The birds were perched stock-still in mortal feat. I never saw a shrike take a bird in this situation but they often captured a vole that made the unfortunate decision to emerge from the snow pack to grab a fallen sunflower seed.
[Originally published on December 7, 2014]
Tags: Migration · Species Accounts
Where did the summer go? The departure of most of our swallows and flycatchers indicates the fall migration has begun. Thr flood gates will soon be open as warblers, vireos and hummingbirds will leave us followed by sparrows and hawks. Today’s column is a potpourri of short items based on the theme of migration.
We delight in the spring arrival of migrating birds and claim them for our own. But when you think about it, the many species of birds that migrate from the tropics to nest in temperate North America spend only a minority of their time on our continent. A Ruby-throated Hummingbird nesting in Maine is here for only about three months. Migration to and from Costa Rica might require another two months or so. These hummingbirds are really Central American birds that grace us with their presence for a short time each year. The same can be said for Bobolinks in Argentina and Bolivia, Cliff Swallows throughout South America, Baltimore Orioles throughout Central America and numerous other migratory species.
Birding for songbirds during fall migration requires more effort than is needed during the spring migration. Fall migrants do not sing and have molted into their less conspicuous basic plumage. The phrase “confusing fall warblers” is so true.
Although most passerines do not migrate as a flock, migrants in a particular patch tend to gather in mixed-species flocks as they forage to fatten up for the next migratory leg. Fall birding in a forest is therefore hit-or-miss with often long periods of misses. A good trick is to find the chickadees. Migrant warblers and vireos often forage with the chickadees.
As an example, I was recently at West Quoddy State Park in Lubec with a couple of friends. We had walked over a mile with scarcely any birds. At the margin of the bog, I heard a couple of chickadees. I began pishing (saying the word pish quickly – if you don’t know the technique of pishing, do a YouTube search for pishing). As expected, the chickadees approached to investigate the source of the pishing but so did about 25 warblers. We were surrounded by Black-and-white Warblers, Black-throated Green Warblers, Yellow-rumped Warblers, a Common Yellowthroat and a Red-eyed Vireo for good measure.
We know that population numbers of many of these migratory songbirds are declining. One of the most important drivers in these declines is the cutting of tropical forests. A particular problem is cutting of timber on protected conservation land. These parks and preserves are difficult to police with limited staff and resources.
The monitoring of illegal timbering is done mainly with aerial or satellite images. Environmental managers may not get the photos for several days by which time the timber thieves have moved on.
Rainforest Connection, a start-up company in California, has developed a way to repurpose old smartphones to detect illegal timber activities quickly. A smartphone is covered in a water-proof case and powered with a solar battery. The phone is mounted high on a tree. A sensitive microphone is attached to the phone. The smartphone detects chain saw noises and gunshots. Each five minutes, the smartphone sends a packet of data to a central server. If the server detects the sound of chain saws, local enforcement officers, alerted by cell phone, can catch the ecocriminals in the act. Similarly, recordings of gunshots can aid in the capture of poachers.
Each phone can monitor an area of about one square mile. Cell phone coverage has penetrated deeply into equatorial forests throughout the world. The service plan costs for these phones are modest, only a few dollars a month in most tropical countries. Such a cost seems like a bargain in exchange for preventing the loss of thousands of dollars of wood or endangered animals. I suspect we will see the widespread use of this technology.
[Originally published on September 13, 2014]
In the last post, I embarked on an historical wild goose chase. I am tracing the development of our understanding of bird migration through the ages. The Barnacle Goose (this European vagrant was seen recently in Aroostook County) was the centerpiece of the last column. It’s name came from the medieval misconception that Barnacle Geese and barnacles are different stages of the same animal.
Humans did not really get a handle on bird migration until the 18th century, finally putting Greek myths about hibernation and transformations to rest. In 1749, Johannes Leche began recording the spring arrival dates of Finnish birds. As we will see, these types of records can be valuable in understanding migration.
The first published statement of bird migration appeared in Thomas Berwick’s A History of British Birds in 1798. Berwick disputed the prevalent notion that British swallows hibernated, writing “they leave us when this country can no longer furnish them with a supply of their proper and natural food …”.
From around 1900, local bird clubs have been collecting arrival and departure dates for migratory birds. By reading these local reports, an observer could determine that Ruby-throated Hummingbirds arrive in the Gulf Coast in early March, around April 10 in Virginia but not until early May in New England. The wave of migration of North American birds thus becomes evident through the shared observations.
We have come a long way since Lemche’s lonely records in Finland. Central depositories like ebird.org hold millions of records so the patterns of northward spread in the spring and southward withdrawal in autumn are clearly seen. If you haven’t tried the tools under the Explore Data link on ebird, give it a try.
Plotting the arrival and departures of migratory birds gives us insight into bird populations but not individuals. Do Ruby-throated Hummingbirds that cross the Gulf of Mexico and land in Louisiana continue their migration to the Midwest while those that land in Florida migrate up the Atlantic seaboard? We must track individuals to answer such questions.
Bird banding is just the tool we need to follow individuals. Audubon almost certainly banded the first birds in North America. In 1840, he tied a silver thread around the legs of several Eastern Phoebe nestlings on his farm near Philadelphia. Two of the phoebes came back the following year. Of course, he had no idea where the phoebes went to pass the winter but he clearly established the power of banding in following individual birds
The North American Bird Banding Program (NABBP), begun in 1920, facilitates the banding of birds in the United States and Canada. After extensive training, a person is provided with a Bander’s Permit and is given aluminum bands, each with a unique nine-digit number. Banders capture birds in nets or traps; identify the species, sex and age of each bird; take various body measurements; affix an aluminum band of the proper size to one of the legs of the bird; and release the bird.
If another bander captures the bird or if a banded bird is found dead, the finder contacts the biologists at the NABBP who provide the recovery data to the original bander and notifies the finder of the original date and site of the banding. Over the 94 years of the program, over 64 million birds have been banded and 3.5 million of them have been recovered. We have learned much about subpopulations of migratory species that maintain different migration routes, as well as information on fidelity to wintering and breeding sites over the years.
Even greater detail of migration routes can be gleaned from satellite transmitters mounted on birds’ backs or from small data loggers called geolocators that track a bird’s geographic position continuously. A geolocator has to be recovered to download the data, unlike a satellite transmitter. How cool is it to monitor an Osprey’s migration from your computer desktop?
[First published on November 9, 2014]
Tags: History · Migration
Some birders will argue that the fall migration beats the spring migration hands-down. Sure, spring songbirds are singing with full throat, dressed in their breeding season finery. But the spring migration is relatively brief.
The fall migration is much more protracted, spanning early August into December for different species. Post-breeding dispersal of many species leads to surprising records of vagrant birds. Storms may also displace migrants.
In recent years, Old World Geese visit New England in small numbers. A few Pink-footed Geese have graced us with their presence in cow pastures in Yarmouth. A Barnacle Goose or two visit northeastern North America each fall. The nearest breeding area for both of these species is Greenland.
We occasionally see a Greater White-fronted Goose. This widely spread species occurs mostly west of the Mississippi River in North America but also in the Old World, as far west as Greenland.
On October 13, Bill Sheehan hit the goose jackpot in central Aroostook County. He found six species of geese. Three species were not surprising: Canada Goose, Cackling Goose (a smaller version of Canada Goose, now recognized as a separate species) and Snow Goose. But he hit the trifecta of rare geese finding a Barnacle Goose, a Greater White-fronted Goose and a Pink-footed Goose. A great day!
But Barnacle Goose? It’s a peculiar name because these geese are vegetarians like other geese and rarely if ever eat intertidal animals. The explanation for the name provides a good opportunity to think about the methodology of science as we seek to better understand the natural world.
The path leading to our current understanding of bird migration is a circuitous one, with plenty of dead ends. Like most scientific inquiry, observers build on the observations of those that came before them. The notion of standing on the shoulders of earlier observers stems from at least the 12th century to a man known only as Bernard of Chartres. The metaphor is best known from Sir Isaac Newton’s quip, “If I have seen further it is by standing on the shoulders of giants.”
Humans have certainly been aware for millennia that bird abundance changes through the year. When you depend on birds as part of your diet, failure to pay attention to changes in bird numbers can influence survival. But where did the birds go?
The notion of migration is implicit in a verse of the Old Testament: “The stork in the heaven knoweth her appointed times; and the turtledove and the crane and the swallow observe the time of their coming.” In the eighth century BC, Homer wrote that cranes flee from the coming of winter.
A couple of centuries later, Aristotle wrote, “Some creatures can make provision against change without stirring from their ordinary haunts; others migrate as in the case of the crane.” He also wrote of the migration of pelicans. So far, so good. Observers surmised that some birds come and go in response to the changing of the seasons.
Unfortunately, Aristotle also wrote “certain birds (as the kite and swallow) decline the trouble of migration and hide themselves where they are.” He went to write that some birds hid themselves in hles in the ground, sometimes without their feathers. Some Greeks also believed in transformation. In Greece, the European Redstart is a common breeder, migrating to Africa each fall. The European Robin is a winter visitor to Greece. Aristotle claimed European Redstarts transformed into European Robins.
In the Middle Ages, Europeans used Aristotle’s mistaken observations to explain the arrival of Barnacle Geese each fall from their high Arctic breeding grounds as a transformation from the stalked, goose-neck barnacles found commonly on floating driftwood. We’ve come a long way since then but the history of this misstep is perpetuated in the Barnacle Goose’s name. We’ll continue our exploration of migration next time.
[Originally published on October 25, 2014]
Tags: History · Migration
Now is the time of year when I start looking closely at American Goldfinches. During the winter, females and males are difficult to tell apart. The dark on the wings is black in males and dark brown in females. The wing bars, particularly the upper one, are a bit more yellow in males. In a month, telling males from females will be easy. The bright yellow body and the black cap on the forehead leave no doubt that such a bird is a male in his summer finery.
The transformation occurs through the process of molting, the replacement of older worn feathers, pushed out by newly formed feathers from below. Molting is essential because feathers, marvelously light and strong, do wear down. These feathers must be replaced as they abrade or flight would be difficult and insulation poor.
With over 11,000 species of birds in the world, generalizations about molting are hard to make. Most birds do molt their contour feathers (their wing feathers, tail feathers and the large body feathers) twice a year. A bird usually molts all of its contour feathers in a sequenced fashion in the fall, leading to its basic plumage. In the spring, another molt occurs leading to the alternate plumage, the plumage of the breeding season. Our American Goldfinches will soon be molting into their alternate plumage. The spring molt is often with the head, body and sometimes tail feathers replaced.
The distinction between basic plumage and alternate plumage can be dramatic as in the warblers, tanagers, and Rose-breasted Grosbeaks. The two plumages are similar in other birds such as gulls, sparrows, wrens, chickadees and nuthatches. Those birds with similar alternate and basic plumages still undergo two molts a year, despite their seemingly unchanging appearances.
Molting requires a significant amount of energy. The only activities in a bird’s life that rival the energetic cost of molting are nesting and migration. Each activity pushes a bird to its limit. No bird can do two of these three activities at once. A typical pattern for a migratory bird is molt into alternate plumage (often a partial molt), migrate north, nest, molt into basic plumage, and migrate south.
Here are a couple of examples that demonstrate the energetic demands of molting. Some Peregrine Falcons breed on the arctic tundra. The short arctic summer is not long enough to allow the falcons to nest and then molt into basic plumage. After nesting, the falcons begin a molt, replacing some of their flight feathers. They then migrate to their wintering quarters, forced south by the deteriorating weather. Once in their winter quarters, they resume their molt.
Yellow-breasted Buntings have a broad breeding distribution in Europe and Asia, stretching from the arctic tundra to central China. They winter in Southeast Asia and India. Like the arctic Peregrine Falcons, the brief arctic summer does not allow enough time for the buntings to nest and then molt before migration. Those high latitude birds migrate immediately after nesting to the lower Yangtze area of China. In that moderate climate, the birds undergo a complete molt and then continue their migration south on fresh feathers. Buntings nesting in the southern part of the breeding range have plenty of time to nest and then molt before they embark on their southward migration.
Two of our local bird species transform themselves from basic to alternate plumage without molting. The dorsal black coloration of breeding Snow Buntings is actually present in basic-plumaged birds. As the winter proceeds, the back feathers of a Snow Bunting erode, exposing the black coloration along the middle portion of each feather. The black is hidden in the winter by the shingle-like arrangement of overlapping feathers. The white spangles on winter Eurasian Starlings are eroded in the same way, leading to the black alternate plumage. Ornithologists call this phenomenon molt by wear.
[Originally published on June 8, 2014]
Tags: Migration · Physiology · Reproduction
What will the winter bring? Birders frequently ask this question each fall. We know we can count on seeing our resident birds this winter like Black-capped Chickadees, American Crows and Hairy Woodpeckers. We also know that most species of migratory breeding birds are gone now but will be back next summer. You can count on seeing Eastern Phoebes, House Wrens and Yellow Warblers come the summer. Passage migrants (birds which breed to our north and winter to our south) seldom linger in Maine for the winter. Snow Geese and Semipalmated Sandpipers are two examples of Maine passage migrants.
The last category of birds, winter migrants, inspires excitement in birders. These birds breed to our north, some as far north as the arctic tundra. Some of our winter migrants like American Tree Sparrows are expected every year. But many winter migrants are unpredictable; in some years, they may be common and in other years scarcer than hen’s teeth. These birds include Snowy Owls, Bohemian Waxwings and a suite of finches commonly called the northern finches.
Why the variability? The answer is quite simply food availability. Snowy Owls are perfectly capable of making it through a winter on the arctic tundra if the lemming population is sufficient to provide food. Similarly, Common Redpolls can survive an arctic winter given sufficient birch seeds.
However, lemming abundance, birch and conifer seed production, and soft fruit production vary from year to year. When the requisite food is scarce, birds must migrate south to find food. The result is an influx of northern birds. Who is not thrilled by flocks of Common Redpolls at our feeders or Bohemian Waxwings in our fruit trees?
Ecologists refer to these incursions of birds as irruptions. An irruption is movement into a particular place, just the opposite of movement out in an eruption. We can think of Common Redpolls erupting from northerly areas when food is not available and irrupting into Maine where birch seeds may be more plentiful.
This winter is shaping up to be an irruption year for Snowy Owls. Over 20 of these magnificent raptors have been sighted in Maine already this winter, mostly along the coast. On December 1, two birders in Newfoundland saw 138 on an all-day birding trip. Keep those eyes peeled!
A couple of flocks of Bohemian Waxwing were seen in the past week. Look for these fruit-eaters at apple orchards or in stands of fruit-bearing trees or shrubs.
The irruptive finches show weak correlations in their abundances because they rely on different types of tree seeds for their sustenance. Ron Pittaway prepares predictions of irruptions each fall based on the production of various species of trees in the vast stretches of boreal forest to our north.
Pittaway reports that mountain ash produced an abundant berry crop to the north of us this fall. Therefore, we are not likely to see very many Pine Grosbeaks (fruit-eating finches) this year.
Birch and alder seeds are abundant in boreal forests to our north. We should not expect a major irruption of Common Redpolls.
Red Crossbills prefer to extract seeds from the cones of Red Pine and White Pine. Red Pine cone production is fair to good this year and White Pine production is poor. Look for occasional Red Crossbills in Maine where ornamental conifers or pines are laden with cones.
White-winged Crossbills prefer spruce cones. Good to excellent spruce production is seen in the boreal forest, extending down into northern New England and the Adirondacks. These crossbills are likely to be broadly dispersed so we should not expect high populations this winter.
Pine Siskins with their small bills rely heavily on hemlock and spruce cones. Fair numbers of siskins are expected in northern New England this winter.
[First published on December 10, 2013]
The fall migration of birds continues in high gear. Most of the leaf-gleaning insectivores like warblers, vireos, and tanagers have departed for warmer climes although a few Yellow-rumped Warblers and Palm Warblers will linger for a few more weeks.
Seed-eating migratory birds can be more leisurely about their migration. Until snow accumulates, these seed-eaters (or granivores) can find sufficient food. Sparrows are common granivores and are the main songbird migrants passing through Maine in October. Many of these birds are passage migrants, breeding to our north and passing through Maine on their southward seasonal journey.
Dark-eyed Juncos have increased in numbers in the past couple of weeks. Chipping Sparrow numbers have passed their peak. Diligent searching of sparrow flocks will often reward a birder with a view of a Lincoln’s Sparrow among the more common Song Sparrows and White-throated Sparrows. Swamp Sparrows are pretty common now too although you may have to flush them out of fields or marshes to see them well.
A treat this time of year is to see White-crowned Sparrows. These large sparrows belong to the genus Zonotrichia, the same genus to which White-throated Sparrows belong. Both of these Zonotrichia species have prominent black streaks on the head alternating with white or tan stripes. Both species have gray breasts without streaking (except for the streaked juveniles of both species). As the names suggest, the White-throated Sparrow has a brilliant white throat, bordered with a thin dark stripe on the lower side. The White-crowned Sparrow has a throat that is lighter in color than the gray breast but never light enough to be called white. There is no black border to the throat in the White-crowned either. One other feature that can be used to distinguish these two species is the color of the lores, the small feathers between the base of the bill and the eye. In the White-throated Sparrow, the lores are yellow. The lores are dark in the White-crowned Sparrow.
In both species, the light head stripes may be either white or tan. Birds of either species in their first-winter plumage (the plumage that replaces the juvenile plumage in the fall of the first year) show tan stripes. Adult White-crowned Sparrows always have white stripes on their crowns. White-throated Sparrows may have either tan or white stripes on the crown as adults. So, it’s a snap to age a White-crowned Sparrow by the color of the head stripes.
All White-crowned Sparrows migrating through Maine are passage migrants. In other words, they winter to our south and breed to our north and do not breed in our state. The only time we see them is when they move through on their spring and fall migrations.
In eastern North America, the closest breeding populations are in northwestern Newfoundland and the northern portions of New Brunswick and Newfoundland. The breeding distribution is wider in western North America, extending south from Canada into the United States in the Rocky Mountains and along the immediate Pacific coast all the way to southern California.
Some ornithologists claim that the White-crowned Sparrow is the best-studied songbird in North America. The species is suitable for as a research subject because of its abundance, its wide geographic distribution and its fearless behavior as ornithologists make observations.
The song type of White-crowned Sparrows varies markedly across its breeding range. Studies on its vocalization have contributed much to our understanding of song learning in songbirds.
Other studies on White-crowned Sparrows have demonstrated the remarkable navigation abilities of birds. Birds wintering in the San Jose region of California were captured and carried via airplane to Maryland and Louisiana. The birds were then released. The displaced birds found their back to their Alaska breeding grounds in the summer and reappeared at their San Jose wintering areas the following winter!
[First published on October 13, 2013]
Tags: Migration · Species Accounts