Entries Tagged as 'Migration'
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]
The fall landbird migration is peaking now. Most warblers have already passed south. Thrushes and sparrows will dominate the October migration.
The fall migration is more leisurely than the spring migration for most birds. There is an urgency about the spring migration as birds are driven to arrive on the breeding grounds as early as possible to stake out good territories and find a mate.
In some ways, the fall migration is even more impressive. This migration involves the spring migrants that survived the breeding season as well as all the young born in the summer. Ornithologists estimate that five billion birds migrate in North America alone each fall!
Some species of birds migrate during the day. These include pelicans, hawks, falcons, hummingbirds, swifts, and swallows. All of these birds are strong fliers. The larger birds can take advantage of thermals that develop during the day. Swifts and swallows can feed on the wing during the day as they migrate.
The majority of land birds migrate at night. These include cuckoos, flycatchers, warblers, vireos, thrushes, orioles and sparrows. Most of these birds are denizens of woods and other sheltered habitat. These birds are not extremely agile fliers so need dense habitat to avoid bird predators.
Migration at night has at least three advantages. Birds do not have to worry about falcon or hawk attacks. Second, the air in the atmosphere is usually less turbulent than during the day. Lastly, the air is cooler at night. A migrating bird produces a huge amount of excess heat that needs to be released. Most of the heat is lost from the unfeathered legs. The colder the air temperature, the more quickly that heat can be dumped.
Migrating birds wait for favorable winds before starting a migratory leg. Keep in mind that high pressure systems rotate in a clockwise fashion and lows are counterclockwise. So, the leading edge of a high or the trailing edge of a low have the southerly winds that favor a strong migratory flight that night.
The evidence of a strong migration can be a fallout of many birds the following morning. But there are more direct ways to experience nocturnal migration.
First, you can set up a spotting scope or use your binoculars and watch the face of the moon. It’s very cool to watch migrating birds wing across the lighted surface. Most nocturnal migrants start shortly after dusk and peak around midnight.
Migrating birds can also be seen on radar. In the early days of radar, echoes of many, small targets were seen but poorly understood. These echoes were called “angels”. Now, we know that the angels are actually birds and sometimes bats.
Radar is now used as a powerful tool to study nocturnal migration. Sid Gauthreaux of Clemson University is one of the pioneers of this field. Check out the radar image showing many angels at his website: http://virtual.clemson.edu/groups/birdrad/COM4A.HTM
Yet one more way to appreciate nocturnal migration is to use your ears. Nocturnal migrants are noisy, regularly emitting short flight notes. In some cases, the flight notes are similar to the calls the birds give while they are on the ground. In many cases, however, the flight notes are only given during a nocturnal flight.
Bill Evans has been a pioneer in the study of nocturnal flight calls. Visit his website at http://www.oldbird.org/
On his website, Evans has sonagrams for a number of warblers and sparrows. He also sells a CD with recordings of the nocturnal flight calls of 211 species. It’s a good investment.
On a night that is not too windy, you can hear the flight notes above. However, a microphone will capture many more of those vocalizations. Evans provides directions on how to build a microphone system using cheap materials like a plastic flowerpot, saran wrap, a dinner plate and an inexpensive microphone.
[Originally published on October 2, 2011]
September 15th, 2011 · No Comments
The fall bird migration is a protracted spectacle extends well into November or beyond. The presence of numbers of sandpipers and plovers on the mudflats and the scarcity of Tree Swallows and Barn Swallows tells us that the fall migration is already well underway.
In contemplating migration, I continue to be amazed by the ability of birds to fly such long distances. Sure, bats and insects can fly but none can hold a candle to birds when it comes to feats of flying.
We all know about many of the features of birds that allow them to master the air: hollow bones, light yet strong feathers for producing lift and thrust, and streamlined bodies. However, other adaptations for flight are subtler and perhaps unexpected. The various organ systems for birds all contribute to make a bird a consummate flying machine.
Let’s start with the urinary or excretory system. The function of any excretory system is to rid the body of nitrogen-containing wastes from the breakdown of proteins. The most common waste product is ammonia, a toxic material.
For fish and invertebrates that live in freshwater or the sea, it’s pretty easy to get rid of the ammonia by producing a large quantity of dilute urine. Water is not a problem for an aquatic organism.
Humans and other mammals can’t use this same mechanism. We would have to essentially spend our lives drinking water and urinating to flush the toxic ammonia from our bodies.
To solve this problem, we convert ammonia to a substance called urea. Urea is toxic only in very high concentrations and can be dissolved in water. So, the problem is solved for mammals. By converting ammonia to urea, our kidneys can concentrate the urea and get rid of it with a moderate amount of water.
This method of removal of nitrogen waste does not work for birds. Many of the avian adaptations for flight involve making the body as light as possible. If birds produced urea, they would have to carry around an unacceptably heavy load of water to flush the urea from their body.
Instead, birds convert their ammonia wastes to a compound called uric acid. It takes more energy to convert ammonia to uric acid than to urea. However, the cost is worth it for birds because uric acid is non-toxic and also does not dissolve in water.
Birds therefore get rid of their nitrogen waste by using only enough water to push the paste-like uric acid down the excretory system. The white center in bird guano is uric acid.
Let’s consider the digestive system. Living birds do not have teeth. Rather the grinding of food is accomplished by the gizzard, the second of two stomachs of a bird. How can this arrangement contribute to flight?
First of all, teeth are heavy. Particularly for long-distance migrants, a few tenths of a gram can make all the difference. Secondly, teeth would make it difficult for a bird to keep its head in the proper position during flight. A bird’s head needs to maintain a particular position to be aerodynamically efficient.
By having its “teeth” in its gizzard, a bird can lower its center of gravity. The position and weight of the gizzard enable the bird to maintain an efficient posture during flight.
The demands of flight can be seen in the reproductive system. During the reproductive season, the male reproductive organs or testes are quite large. However, once the breeding season is over and migration begins, the gonads of the males shrink to less than 1% of their breeding season weight. That is some weight savings! The difference between the weight of the gonads of House Sparrows between the breeding and non-breeding season is 500 times.
Most female birds have only a single ovary. Exceptions are most hawks and some pigeons and gulls that have a pair of ovaries. The ovary swells during the breeding season and then regresses dramatically during the non-breeding season to reduce the weight of a flying bird.
Finally, we can consider one aspect of the skeleton in birds: the shape of the sternum or breastbone. In flying birds, the ventral side of the sternum has a large sail or carina. This carina serves as the attachment point for the two muscles that raise and lower each wing during flight. Most of the power generated during flight occurs during the downstroke and this muscle is the larger one.
The breastbone of ostriches and other non-flying birds is flat like our breastbones, strongly suggesting the carina is a structure that evolved to facilitate flight.
[Originally published on August 21, 2011]
Tags: Migration · Physiology
The spectacle of migration is well underway. Warblers, vireos and thrushes are streaming south in good numbers now through Maine. The sparrow migration will pick up speed in October.
Migration, even for short distances, is an arduous task that demands a high expenditure of energy. Two other events in a bird’s life demand equally high energy costs: reproduction and molting. The costs of each activity are so high that no bird can do two of them at once.
There are about 10,000 species of birds in the world so exceptions always arise when one tries to make universally true comments about some aspects of birds. That is certainly true of molting; one size does not fit all. Nevertheless, we can make some general observations on molting and then look at some of the interesting variations on the theme.
Generally, birds undergo one complete molt every year. During that molt, every contour feather on their body is replaced. For a bird like a Song Sparrow, that entails replacing about 2,000 feathers. A Tundra Swan will have to replace over 25,000 feathers. No wonder molting is so expensive.
The usual pattern is for the complete molt after nesting has been completed but before any migration is begun. The bird then enters its basic plumage (sometimes called winter plumage or non-breeding plumage).
This sequence of nesting to molting to migration has obvious advantages. After the breeding season, food is generally abundant enough to allow a post-breeding bird to find enough energy to fuel its molt. Then, it is ready to migrate on fresh, efficient flight feathers.
A typical bird will have a second, partial molt near the end of the winter. Some of the body feathers will be replaced, transforming for example a drab, greenish male American Goldfinch into a stunning yellow bird. However, the flight feathers on the wing and tail are generally not replaced. So, the northward migration must be done on worn feathers.
Some birds do undergo two complete molts a year. Extremely long distance migrants and species that live in abrasive habits (thorn scrub or coarse grass) replace all their feathers twice a year. In Europe, the Short-toed Lark only has one complete molt per year in the summer but an Asian race of this species that lives in sand-blown deserts has a second molt in the spring.
Molting can improve a bird’s physiological condition. The Salt Marsh Sparrow undergoes two complete molts each year. Seaside Sparrows, nesting in the same marshes, have a complete molt in the fall and a partial molt in the spring. Seaside Sparrows have more bird lice than co-occurring Saltmarsh Sparrows.
It’s easy to see evidence of molting in the flight feathers of a flying. The flight feathers are usually replaced in a sequence so that only a few feathers are missing at any time. The innermost primary feathers and the outermost secondary feathers are molted first. In a molting bird in flight, you can see gaps or shorter feathers showing the current stage of wing molt.
Geese, swans and ducks as well as loons opt for the fast track during their flight feather molt. All of the primary feathers are shed at once. Until the feathers regrow, these birds are flightless. The birds find sheltered wetlands with enough food to allow them to hide and feed as their primaries grow.
To get to such a favorable habitat, many waterfowl stage a molt migration. After the breeding season, lakes with lots of vegetation may be populated with thousands of flightless waterfowl that flew there for the express purpose of molting. Canvasbacks that nest in the northern United States stage a molt migration north(!) to the Prairie Provinces of Canada after breeding. After their molt, the Canvasbacks then migrate south for the winter.
Anna’s Hummingbirds have a molt migration as well. These birds nest in the chaparral of coastal California in the spring, move to the summer in the high mountains to take advantage of the abundant nectar where they molt, and then migrate to Arizona or Mexico for the winter.
In 1983, the AOU Check-list Committee lumped the Baltimore Oriole and Bullock’s Oriole into a single species, the Northern Oriole because these two forms hybridize where they overlap in the Great Plains. These forms have now been re-split into two species. Part of this decision stems from the finding that Bullock’s Orioles stage a fall molt migration to the southwestern United States and northern Mexico before molting and then continuing their fall migration. Baltimore Oriole’s, even in the same habitat, molt before beginning their fall migration.
[First published on September 25, 2010]
Tags: Migration · Physiology