Entries Tagged as 'Migration'
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
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]
Tags: Migration · Species Accounts
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]
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]
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]
Tags: Migration · Species Accounts
Shorebird migration continues apace through Maine. I can’t help but marvel at the tremendous migrations many shorebirds undertake. Semipalmated Sandpipers depart from the Bay of Fundy and fly non-stop over the ocean to the mudflats around the mouth of the Amazon River in Suriname. The Pacific Golden Plover flies non-stop from Alaska to its wintering grounds in Hawaii (a nice winter vacation!). The champion is the Bar-tailed Godwit which flies non-stop from Alaska to New Zealand, a distance of over 7,200 miles.
Unlike most land birds, shorebirds tend to congregate at particular food-rich stop-over areas during their migration. At these stop-overs, the birds can feed gluttonously to put on sufficient fat to fuel their long migratory flights. Visiting a stop-over area at the right time of year allows a birder to be wowed with large numbers of shorebirds. But this staging behavior is fraught with peril as well should an environmental disaster like an oil spill despoil the habitat.
The migration of Red Knots involves a series of stop-over areas as the birds move from their wintering areas at Tierre del Fuego (the southern tip of Argentina) to the Canadian arctic. The most important stop-over area for this species in the New World is Delaware Bay. The majority of Western Hemisphere knots stop here in late May to feed on the abundant eggs laid by horseshoe crabs in the intertidal regions of the Bay. These calorie-rich morsels allow the Red Knots to tank up for their next long migratory leg.
Phillip Hoose, one of Maine’s own, has recently published a book on Red Knot migration, focusing on one remarkable banded bird. The book is called Moonbird: A Year on the Wind with the Great Survivor B95. Phillip is an acclaimed writer, having won a National Book Award and Newbery Honor for his book on the civil-rights activist, Claudette Colvin. He also authored a book on the conservation of the Ivory-billed Woodpecker. Moonbird continues his vein of excellent, accessible writing.
Red Knots occur on all continents except Antarctica as well as many islands. The subspecies Calidris canutus rufa is the Red Knot we see here in eastern North America. Hoose points out that conservationists are worried about this subspecies. In 1995, there were 150,000. By 2000, the population was plummeting and now fewer than 25,000 remain. Hoose explores some of the possible explanations of this alarming decline.
The star of the book is a male knot of the rufa subspecies, B95, that was banded in 1995. B95 is printed on an orange plastic flag attached to one of his legs. B95 is the only Red Knot with this particular combination of letter and numbers so an ornithologist can identify him by observing the band with binoculars or a spotting scope.
B95 is a survivor, nearly 20 years old. He has been recaptured four times and observed through spotting scopes on many other occasions. Hoose rightly describes B95 as the most celebrated shorebird in the world.
We follow B95 over the course of the years, visiting the stop-over areas of the species. We visit San Antonio Bay along the central Argentinian coast, Lagoa do Peixe National Park in Brazil and of course Delaware Bay. We learn of the threats to Red Knots along the way (falcons, declining horseshoe crab populations, development).
We also meet conservation heroes: Patricia González in Argentina, Brian Harrington and Amanda Dey in New Jersey, Guy Morrison and Ken Ross in the Canadian Arctic).
Hoose describes the various techniques that shorebird biologists use to capture birds so they may be banded. You are there!
B95 proves to be a marvelous tour guide and Phillip Hoose chronicles the tour in lucid, fluid prose. The book is accessible to high schoolers. The wonderful color photographs and figures enhance the joy of reading this book.
[First published on September 16, 2012]
We are all accustomed to misleading or even paradoxical phrases in our conversations that we often use without thinking. Shouldn’t a “near miss” be a glancing collision? Have you ever gotten a “free gift”? Biology is not immune from such phrases and “fall bird migration” is at the top of my list.
Even though none of use wants to admit that the days are growing shorter, we are still in summer. Nonetheless, migration of birds is well underway. The southward movement will continue through autumn and into early winter. “Fall migration” is misleading; I prefer “post-breeding migration”. The majority of our swallows depart by the end of August and therefore never spend an autumn day in Maine.
Shorebirds (particularly sandpipers and plovers) are now migrating through Maine from their more northerly breeding grounds en route to more southerly wintering grounds. Shorebirds are some of our earliest and most remarkable migrants.
The majority of shorebirds that pass through Maine during migration breed at high latitudes on the Arctic tundra. For about six weeks (June to the middle of July), the arctic tundra is great habitat for sandpipers and plovers. Insect life is abundant and the sun never sets.
We describe the reproduction of shorebirds as precocial. The chicks hatch fully feathered, a striking contrast to the altricial development of most songbirds whose young hatch as naked, blind chicks. Within hours of hatching, young shorebirds are walking around, catching their first insect meals. The shorebird parents (sometimes only the mother in some species like the Buff-breasted Sandpiper and sometimes only the father in Red-necked Phalaropes and Red Phalaropes) guard the young but do not typically feed the young. The young do have cryptic coloration, blending into the tundra to avoid the eyes of predators like Arctic foxes and Snowy Owls.
The short breeding season in the arctic means that parents only have time for one brood per season. To increase the chances of surviving another year and reproducing again, sandpipers are under pressure to begin their southbound migration as soon as possible.
Because the nestlings can fend for themselves, the parents usually migrate from the breeding grounds before their young have even learned to fly! As a result, southbound shorebird migration occurs in two pulses. Along the Maine coast, the first adult shorebird migrants begin showing up by the middle of July. The first juveniles show up later; juvenile sandpipers and plovers will arrive mostly in August and September. In some species like the Semipalmated Sandpiper, the adult females leave the breeding grounds before the adult males producing a three-step migratory wave along the migration route: adult females, adult males and then juveniles.
After the parents have departed, the juvenile shorebirds learn to fly on their own and depart for wintering grounds they have never seen. With no adults to guide them, the juveniles must have the migratory route somehow encoded in their brains. We know that juvenile navigation is not as accurate as that of adults. During the southbound migration, shorebirds that occur as rarities are usually juveniles.
By ageing shorebirds, birders can increase their enjoyment and appreciation of the southbound migration. How does one go about determining if a shorebird is a juvenile or adult? Some adult migrants still have some of their breeding plumage. So a Black-bellied Plover or Dunlin with a black belly during southbound migration has to be an adult. However, adults often molt into their winter plumage along the migratory route and may resemble juveniles. A look at the wear of the feathers can usually allow one to distinguish such adults from juveniles. As a general rule, juveniles have brighter and crisper plumages. The feathers of the juvenile are only a few weeks old and the adult feathers are a couple of months older and hence more worn.
[First published on August 5, 2012]
In the fall, I had high hopes for a winter with tons of northern finches. Red spruce, balsam fir, tamarack and eastern hemlock trees were producing bumper crops of cones. These so-called mast years occur sporadically and different tree species do not necessarily have mast years in the same years. This winter was shaping up as a marvelous smorgasbord for the finches that depend on conifer cones for their main food.
As a baseball fan, I can’t help but think of the movie Field of Dreams where Kevin Costner’s character builds a ballfield in his Iowa cornfield because a voice told him ”If you build it, he will come”. Sure enough, the ghost of Shoeless Joe Jackson and other members of the Chicago Black Sox show up to play baseball.
But with our finches, the presence of a bumper crop of food is not enough to guarantee their presence. If trees build large number of cones, the northern finches may or may not come. Obviously, these nomadic birds are finding sufficient food elsewhere. The absence of all of these seed predators is good news for the trees of course because many seeds will find their way to the ground and germinate.
Pine Siskins, Common Redpolls, Pine Grosbeaks, Purple Finches, Evening Grosbeaks, White-winged Crossbills and Red Crossbills were all pretty scarce on Maine Christmas Bird Counts. On a recent trip to the Cobscook Bay area, I found very few northern finches despite the many conifers heavily laden with cones. However, a recent trip to the Flagstaff Lake region yielded sightings of good numbers of both crossbill species. Perhaps we will see a late-winter irruption of some of these species into more southerly parts of our state.
Of the northern finches, the crossbills are perhaps the most erratic in their appearances in Maine. Watching crossbills feed is to marvel at their skill in extracting conifer seeds with their peculiar bills. The hallmark of the crossbills is the odd overlapping arrangement of the upper and lower bill. When observing a crossbill from above or below, one can see that one bill curves left and the other curves right. Sometimes, the upper bill is the left-curving one and in other birds the lower bill curves leftward. Why such a strange bill?
This bill turns out to be an efficient took for extracting conifer seeds. Everyone knows what the cone of a pine, fir, spruce or other conifer looks like. The cone has a number of scales. At the base of each scale, the seeds of the conifer can be found. Most birds have a difficult time negotiating the scales to get to the seeds deep within the cone.
Not crossbills however. A crossbill inserts its bill between two scales of a cone. The crossed points of the bill act as a wedge, increasing the distance between the two scales. One bill pushes to the left and the other to the right. The strong tongue of the crossbill can then be inserted to the base of the scale and the nutritious seed removed and gobbled up.
Although the crossed bill serves these birds marvelously in feeding on cones, the specialization comes at a cost. Crossbills are quite awkward in handling food that other finches readily eat such as thistle or birch seeds. A crossbill beak is not a generalized tool.
Usually crossbills feed on cones that their bills can handle with greatest efficiency. The Red Crossbill has a larger and longer bill than the White-winged Crossbill. Red Crossbills feed on white pine, balsam fir and sometimes hemlock cones. White-winged Crossbills feed on smaller cones like those of larch and red spruce.
Here’s hoping the crossbills will come further south this March so we can all admire the skill with which they extract conifer seeds.
[Originally published on March 18, 2012]
Tags: Migration · Species Accounts
The past couple of weeks have been exciting ones for the fall landbird migration. In today’s column, I will describe a recent field trip to make a couple of points about the nature of the autumnal migration.
October 3 was a rainy, windy day. Realizing the promise of productive coastal birding, Luke Seitz, one of the top birders in the state, decided to check out coastal sites in southern Maine. What a day he had! Along with the more expected warblers and sparrows, here are exceptional sightings made by Luke at Cape Neddick and Fort Foster: 11 Yellow-billed Cuckoos, seven White-eyed Vireos, one Yellow-throated Vireos, a Worm-eating Warbler, two Blue-winged Warblers, four Hooded Warblers, two Connecticut Warblers, one Yellow-breasted Chat, a Summer Tanager and a Blue Grosbeak. You can see some of Luke’s photographs of these birds at 6jvjm5h
The phenomenon that Luke experienced is called a fallout. Inclement weather forces migrating birds to abort their evening’s journey and seek shelter. It’s likely that many of these birds were flying westward off the shore to find cover. Of course, Luke knew that the weather conditions were favorable for a coastal fallout of migrants and his hunch was proven correct. So, windy, rainy days can often be the best times to go birding in the fall.
So why were the birds listed above exceptional sightings for Maine? Yellow-billed Cuckoos are uncommon nesting birds in southern Maine so seeing 11 in one day is remarkable. Yellow-throated Vireos and Yellow-breasted Chats are uncommon breeders in southern Maine. Both of these species are much more abundant south of Maine, all the way down to Florida on the eastern Seaboard. Blue-winged Warblers are uncommon breeders in southern Maine and are most common in the mid-Atlantic states to the mid-West.
The remaining species do not nest in Maine. The northern limit of the breeding range is southern New England for White-eyed Vireos and Worm-eating Warblers. Hooded Warblers reach southeastern New York and New Jersey. Summer Tanagers and Blue Grosbeaks reach their northern breeding limits in southern Pennsylvania.
Furthermore, all of these species winter south of North America. Most migrate to Caribbean Islands or Central America for the winter. Some Summer Tanagers go all the way to South America.
What in the world are these migrants doing in Maine in October? Shouldn’t they be well on their way south to tropical areas?
The explanation is a phenomenon that ornithologists call reverse migration (or reverse orientation). Young geese, swans and other large birds migrate in flocks. Young birds can learn the migratory route from experienced older birds that have made the trip before.
However, perching birds do not migrate in cohesive flocks. These birds have their migratory route genetically programmed. In most cases, a juvenile bird is able to find its way to a wintering area where it has never been. A neat trick!
In some cases, however, birds become disoriented by 180 degrees and hence embark on a north-bound trip in the fall when they should be heading south. Most birds that undertake these reverse migrations are inexperienced, juvenile birds. So for those of us in northern areas, reverse migration in the fall affords us the best chance to see migratory perching birds that are normally found well to our south.
Some extralimital birds seen in Maine may have arrived here by an orientation error different from a 180 degree angle. For instance, a Smith’s Longspur (the second record for Maine) was found in Norridgewock in late September. Smith’s Longspurs breed in the arctic tundra of Alaska and Canada and winter in Oklahoma and adjacent states. The Norridgewock bird had to have taken a southern migration route about 45 degrees to the southeast of its proper route to find itself in Maine.
[Originally published on October 16, 2011]