Entries Tagged as 'Migration'
December 7th, 2014 · No Comments
December 7th, 2014 · No Comments
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]
December 7th, 2014 · No Comments
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]
December 7th, 2014 · No Comments
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]
January 19th, 2014 · No Comments
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]
January 19th, 2014 · No Comments
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]
January 19th, 2014 · No Comments
One of the best-known quotations from the Greek philosopher Heraclitus is translated as “Everything flows, nothing stands still”. We always expect change. Of course, the certainty of change applies to birds.
Bird populations inevitably vary. Sometimes birds increase locally because particularly favorable conditions occur. For instance, a gypsy moth outbreak provides lots of food for insect-eating birds and the nesting success of such birds is higher than normal. Other events like the 1998 ice storm can wreak havoc on local bird populations.
Bird populations can also change on much larger geographic scales. A widespread outbreak of avian conjunctivitis in the early 1990’s resulted in the death of many House Finches in the eastern United States. The clearing of bottomland forest in the Deep South certainly was a major contributor to the extinction of Bachman’s Warbler. Global warming is causing significant shifts in the range and population size of most North American birds.
For migratory birds, changes in population sizes can be difficult to understand. According to data from the Breeding Bird Survey, Wood Thrushes show a steep decline in the three northern New England states over the past 40 years. Why? Could it be events happening in New England during the breeding season? Could it be destruction or degradation of wintering habitat in Central America? Could it be events happening during migration? Change may be certain but understanding why change occurs is often hard to understand.
One of most spectacular changes in abundance of a Maine bird concerns a shorebird called the Red-necked Phalarope. Red-necked Phalaropes breed at high latitudes on the tundra in both the New World and Old World. On the breeding ground, phalaropes largely feed on mosquito larvae and other aquatic insects in thaw ponds on the tundra. The phalaropes feed in an unusual manner by swimming rapidly in a small circle, drawing prey items to the surface of the water.
To capture the small prey, phalaropes take advantage of the high surface tension of water. By rapidly opening its upper and lower bills with the tips in the water, a phalarope causes water to flow up the bill, following a surface tension gradient right into the back of the mouth. The unsuspecting prey items are drawn into the mouth with the water become lunch. A personal conveyor belt!
Red-necked Phalaropes can be found on inland bodies of water during migration. Thousands can be found on saline lakes in western North America on their way to wintering grounds. These hardy birds winter at sea where they feed on zooplankton, the collection of small crustaceans and other invertebrates found near the surface of the ocean feeding in turn on single-celled algae (phytoplankton). Phalaropes are well adapted to feed on the plankton with their long, thin bill.
Forty years ago, millions of Red-necked Phalaropes stopped in the lower Bay of Fundy during their fall migration. The most spectacular concentrations were in the vicinity of Lubec and Eastport, Maine. In particular, the phalaropes were found in the roiling waters between Campobello Island, N.B., Deer Island, N.B. and Eastport. These oceanographic conditions produce the Old Sow, the largest whirlpool in the Western Hemisphere.
In late August of 1976 and 1977, two million phalaropes were estimated to be feeding in this small area. What a spectacular site that must have been! In 1983, 300,000 phalaropes were counted. But thereafter, the phalaropes disappeared. We don’t know where the phalaropes went.
In recent years, phalaropes have been seen in the Eastport-Deer Isle region although in much smaller numbers than the heydays of the 1970’s. The phalaropes seem to occur where large numbers of their favored prey, a small crustacean called Calanus finmarchicus, are abundant. It is possible that the Calanus population crashed in the area in the early 1980’s, forcing the phalaropes elsewhere. On a global scale, Red-necked Phalaropes seem to have a stable population.
[First published on September 15, 2013]
August 29th, 2013 · No Comments
After a wet and cool June, the warm temperatures in July convince us that summer has finally arrived. However, birds operate on a different calendar. Tree Swallows and Barn Swallows can be seen by the tens and even hundreds perched on utility wires with migration on their minds. Least Sandpipers, Greater Yellowlegs and Short-billed Dowitchers are appearing on coastal mudflats. The fall migration has begun. Perhaps, a less confusing term would be post-breeding migration but I think the phrase fall migration is here to stay.
Some ornithologists have estimated that five billion birds in North America migrate south every year. In today’s column, we will consider the why and when of the fall migration.
To begin, we need to recognize two types of migrating birds. First, we have species that breed locally but winter to our south. We can call these species migratory breeding birds. Second, we can see species that breed to our north and winter to our south. We only see these birds, called passage migrants, during their migration to and from their breeding grounds. Various sandpipers and Snow Geese are examples of passage migrants through Maine
You may wonder why Tree Swallows depart southward from Maine when the summer weather is just starting to become glorious. The answer is food. The need to migrate is not impelled by temperature but rather by lack of food. Given sufficient food, birds are capable of tolerating markedly extremes of temperature. The abundance of flying insects, on which the swallows depend, is beginning to decline. The reduction in food necessitates an August departure for most of our swallows.
Cuckoos, warblers and vireos rely heavily on caterpillars and other insects, which feed on the leaves of trees and shrubs. The abundance of these insects is sufficient to allow leaf-gleaning birds to stay in Maine well into September. After the first killing frosts of autumn, leaf-eating insect abundance declines markedly and our warblers and vireos are forced to migrate south. Except for Yellow-rumped Warblers and Palm Warblers, most of our warblers will depart by the beginning of October.
A number of our migratory breeding birds are seedeaters. Seeds from herbaceous vegetation can be found through the fall until a snow cover accumulates. White-throated Sparrows, Chipping Sparrows and Rose-breasted Grosbeaks can longer well into October.
The movement of passage migrants begins in July with the arrival of post-breeding shorebirds. Birds breeding above the Arctic Circle have a narrow window of opportunity for breeding. Insect and fruit abundance in the Arctic is amazingly abundant during the time of the midnight sun but rapidly shortening days and cooling temperatures take their toll on food availability for birds.
The Semipalmated Sandpiper, a species I have studied on its migration, provides a typical example of the migration of sandpipers. Semipalmated Sandpipers arrive on their Arctic breeding grounds in late May or early June. Both the female and the male incubate the eggs and tend the young.
Before the young sandpipers can fly, the females will begin their fall migration. They are followed a week or so later by the males. The young are left on the tundra, barely capable of flying but obviously able to find food and avoid predators. They begin their migration about a month after their parents have departed. the juveniles inherit the instinct to migrate and find the way to their South American or Caribbean wintering grounds without the benefit of a guide.
I encourage you to get out this summer and to enjoy the fall migration! The post-breeding migration is much more protracted than the spring migration. Spring migration is characterized by an urgency to get back to the breeding area and secure a good territory and mate. The fall migration is more leisurely, lasting into November when the last of our sparrows and hawks depart.
[First published on July 19, 2013]
August 29th, 2013 · No Comments
As the most mobile of all vertebrates, birds pose a challenge to ornithologists seeking to understand the where and why of bird movements. Banding captured birds is a time-honored technique. It is, however, fundamentally inefficient because a banded must be recaptured to get an endpoint for its movement. Furthermore, a Common Redpoll banded in Maine and recaptured in New Jersey may have taken a circuitous route from one point to the other.
Radio-transmitters can be used to track bird movements. A transmitter and antenna are attached to a captured bird. Each transmitter emits a unique frequency. Using scanners, field workers get a fix on a bird and by triangulation determine the position of the bird at a particular time. Typically, the transmitters are only effective over distances less than a mile. Although these transmitters are miniaturized, they are still too heavy to place on most songbirds and other small birds.
Satellite-transmitters are similarly attached to birds but their signals can be identified all ove the world. It is possible to follow the movements of an albatross or Osprey with a satellite-transmitter from a computer desktop. But like radio-transmitters, satellite-transmitters are not small enough to place on smaller birds.
In today’s column, I want to concentrate on a relatively new method of determining bird movements that can be used on smaller birds. The device is called a geolocator and is brilliant in its simplicity.
A geolocator is a light-recording device with a computer chip to collect sunrise and sunset data daily along with the time of day and date of each event. It is easy to figure out where the bird is on a particular day from that information. As an example, if the sun comes up at at 4:56 AM EST and sets at 8:31 PM on June 15, I must be in Rangeley, ME. An area further south will have a shorter daylength and an area further west will have a later sunrise and sunset.
These geolocators are quite small, weighing as little as 0.5 gram (a penny weighs three grams). Thus, these devices can be put on birds as small as vireos and large warblers. The devices are mounted on the rump, attached with a harness that runs around the upper legs of a bird.
The trick, of course, is to recapture the bird and recover the geolocator. In most of the work with geolocators done so far, ornithologists take advantage of the fact that migratory birds show a high degree of fidelity to their breeding sites. Thus, a Tree Swallow can be tagged with a geolocator one April and then recaptured in the same area the following April with daily data on sunset and sunrise for every day it wore the geolocator. Pretty cool! The investigators download the data and get a day-by-day map of the movements of that bird.
The most recent issue of the ornithological journal, The Auk, has a series of articles on remarkable discoveries using geolocators. Here are a couple of examples. Geolocators show that Tree Swallows from a range of breeding areas use southeastern Louisiana as a stopover area during fall migration to Central America. The technology shows that Red-eyed Vireos have a much slower migration than most neotropical migrants. Spring migration from South American takes about 45 days, only 13 of which are spent flying.
To me, the most amazing result of geolocator research concerns seven Arctic Terns that were banded on the breeding grounds in the Netherlands. This species hold pride of place as the the longest-distance migrant. But these seven birds showed the migration is even more impressive than formerly thought. These birds migrated after breeding south along the west coast of Africa, eastward to Australia (to New Zealand in one case) and then southwest to Antarctica for the austral summer. The movements for one year were over 55,000 miles! One year!
[First published on June 23, 2013]
August 29th, 2013 · No Comments
The wonderful spectacle of spring migration is coming to an end with the arrival of the last warblers, thrushes and cuckoos. As usual, it’s been a delightful three months of arrivals and departures.
Species arrive on their own schedules. We know that Red-winged Blackbirds and Common Grackles will be among the first spring migrants in early March, the first Eastern Phoebes will arrive in early April and early May will bring Ruby-throated Hummingbirds.
Why do our various species of migratory breeding birds arrive at different times of the spring? Ultimately, food is the answer.
Migratory breeding birds, especially males, are anxious to arrive on the breeding grounds as soon as possible to stake out a nice territory and attract a mate. But arriving before there is food to eat can be fatal.
Red-winged Blackbirds and Common Grackles do fine in early March because they can subsist on seeds. Common Loons, Ospreys, and Belted Kingfishers must have fish so arriving after ice-out is a must. Phoebes and Tree Swallows depend on flying insects that only emerge in April. Warblers and vireos rely on caterpillars and other leaf-eating insects that only emerge after the deciduous trees leaf out in early to mid-May.
The explanatory power of food availability can be applied to the eight species of woodpeckers that commonly occur in Maine. Six of these species do not migrate at all. The most common of these are Downy Woodpecker, Hairy Woodpecker, Red-bellied Woodpecker and Pileated Woodpecker. All feed in the stereotypical woodpecker way of drilling into wood to expose the galleries of insects. Using their ridiculously long, barbed tongues, these woodpeckers harpoon the insects. Carpenter ants are a favorite of Pileated Woodpeckers.
The Black-backed Woodpecker and American Three-toed Woodpecker are resident but uncommon woodpeckers in Maine. They favor burned-over areas, concentrating on the beetles that attack newly burned wood. These two woodpeckers tend to forage just beneath the bark scales for their meals.
The food of all of these species is available year-round. These woodpeckers have no need to undertake arduous migratory journeys.
We do have two species of woodpeckers, the Northern Flicker and Yellow-bellied Sapsucker, that depart in the fall and return back to us in April. Their dietary preferences necessitate a departure for more moderate winter climates.
Flickers do much of their foraging on their ground, particularly hunting for ants. Nearly half of a flicker’s diet comes from ants. Flickers also catch insects like butterflies and beetles on the wing. A dense snow pack, hibernating ants and a lack of flying insects force Northern Flickers to withdraw from Maine each fall.
Yellow-bellied Sapsuckers feed by creating shallow holes, called sap wells, in the bark of trees. The sap that oozes into the wells provides food for the sapsuckers. The sap is a fluid carried in phloem cells of the tree, just beneath the bark. This fluid is rich in carbohydrates, particularly sucrose (table sugar) sapsucker is not unlike a vampire, exposing the phloem cells and drinking the sucrose that oozes out. Unlike most woodpeckers, a sapsucker has a tongue that is brush-tipped, perfectly adapted for lapping up sap.
The sap wells in the phloem are usually rectangular in shape. I am sure that you have seen these sap wells before, arranged in neat rows parallel to the ground. A sapsucker tends its sap wells daily, making sure they continue to ooze sugar by enlarging the area of the well.
Sapsuckers do supplement their diet with insects. Foraging for insects is especially important when parents are feeding nestlings. The young sapsuckers need protein to grow.
In the winter, the phloem freezes solid, depriving sapsuckers of their favored food. Once again, the lack of available food in the winter forces sapsuckders to migrate south.
[First published on May 26, 2013]