by Paul Lehman
Maps and Illustrations by Virginia Maynard
Successful birding at migration hot-spots such as Cape May requires one to be WEATHER-WISE! A birder needs to know not only where the best places to look for migrants are and at what periods during the season particular species are most apt to occur, but they also need to have a basic understanding of the workings of the weather and how it impacts bird migration—on a daily, weekly, and seasonal basis. This article attempts to give the birder many of those basics, whether they are birding at Cape May or at any migration site in North America.
Understanding and closely following the weather is a crucial component of successful birding. Heading out into the field without first checking a weather forecast, particularly during the spring and fall migration seasons, may hinder your success as much as or more than forgetting your field guide or spotting scope. For the birder, it is important to keep current on what is going on in the atmosphere, for two primary reasons. First, there is the question of personal comfort and safety. You probably do not wish to venture out into the wilds if there is a forecast for 20 inches of snow, or 120-degree temperatures, or 60-mph winds (unless of course you are at a seawatch, or chasing down the remnants of a tropical storm and hoping to find some unfortunate storm-blown avian waif). Second is the much more interesting and engrossing topic of the profound effects of the weather on the movements of birds. A successful vs. unsuccessful day in the field during migration will probably be determined by when and where you decide to go birding and by what types of birds you are looking for—all of which are affected by the weather that day and during the previous days. It is important to remember that the weather usually does not “cause” migration at a basic level; instead, migration is caused by factors such as changes in day length and variation in food supply. However, weather can have major effects on the timing, direction, and magnitude of the specific movements of birds. The calendar might say that it is the peak of the warbler migration, or that it is a great time to expect a major flight of hawks, but if the weather is not conducive for such movements, there simply won’t be very many birds to see. Weather also plays a major role in the occurrence—or at least in the immediate arrival—of vagrant species, those birds that are well outside of their normal range and which cause the birding hot-lines to light up!
Today there are many good and easily accessible sources for up-to-date weather information. For a long time all that was available was the daily weather page in the morning newspaper or a brief forecast given over the radio or on the evening TV news. They were often fairly good, sometimes not so good. (I am referring here to the quality and thoroughness of the forecasts, not to their actual accuracy, which remains somewhat of an imprecise science to this day, no matter what medium is used.) Then came the “weather radio”, easy to purchase at a number of electronics stores, which broadcasts continually the official U.S. NOAA (National Oceanographic and Atmospheric Administration) forecasts, severe weather alerts, tide and offshore waters information, and more. A good number of autos now come equipped with a setting (band) on the radio that receives these continuous NOAA broadcasts.
Recently, a veritable flood of weather information sites have sprung up on the internet (e.g., www.weather.com, www.weatherunderground.com ). Some of these sites are run by for-profit businesses, and many others are established by universities. For those birders contemplating going out to sea on a pelagic trip, there are numerous sites that give wind and swell information from buoys located well offshore. Perhaps it isn’t too late to stay on shore! (Such information on wind can also prove very useful in some regions to birders deciding whether or not there might be a fallout of landbird migrants along a coastline, as knowledge about offshore wind speed and direction or fog conditions may prove crucial.) And last but not least there is The Weather Channel broadcasting on cable television in most markets across the U.S. Regular local forecasts, extended weekly forecasts (not that these are terribly accurate more than a few days out), maps showing the positions and movements of high- and low-pressure systems from coast to coast, areas of precipitation, the upper air (jet stream) wind flow, detailed coverage of tropical storms, etc., are all important pieces of information with which knowledgeable birders should arm themselves.
This article has two objectives. The first objective is to explore the basics of those weather patterns that are most important to the birder: global circulation patterns, upper-level and surface winds, high-pressure and low-pressure systems, warm fronts and cold fronts, and precipitation. The second objective is to take this information and apply it specifically to bird migration, looking at what sorts of atmospheric conditions may or may not produce a major movement or fallout of birds in spring or fall. We will give several examples from Cape May and the Mid-Atlantic region, but will also include other examples from elsewhere in North America—from the Atlantic Seaboard to the Gulf Coast, from the Great Lakes to the Great Plains to the Pacific Coast.
Some Background Basics
Let’s start with a couple of definitions and some background basics. First, there are the terms “weather” and “climate”, which are often used incorrectly. “Weather” refers to what actually happens in the atmosphere from day to day, week to week, or month to month. For example, such statements as “it rained yesterday,” “it sure has been a cool spring this year,” or “we are headed for a drought this summer” all refer to the weather. In contrast, “climate” refers to a more average expected pattern of events over longer periods of time. For example, much of coastal California enjoys a “Mediterranean” climate of mild winters, mild-to-warmish summers, and winter rain and summer drought. (But note that the term “drought” used in this way is incorrect, as that word is usually reserved for when a dry cycle or period occurs irregularly, atypically, and with relatively short warning.) New Jersey, by contrast, has a moist continental climate, with warm summers but cold winters, and an approximately even distribution of precipitation throughout the year.
Next, let’s look at winds and precipitation. Wind is simply the movement of air relative to the earth’s surface. Winds blow typically from areas of higher pressure to areas of lower pressure, thus moving air from areas of “excess” to areas of “deficit,” helping to equalize an existing pressure differential. The stronger this difference in pressures, the stronger the wind. Winds are named for the compass direction from which they come. Thus, a northerly wind is blowing from the north, but the air is moving to the south.
Precipitation results from the uplift of air. This is because as the air is uplifted, it expands and cools, and colder air cannot hold as much moisture as a similar volume of warmer air. If the air is cooled to the point where the air can no longer hold all the moisture it contains, then the “dew point temperature” is reached, and saturation occurs. At this point, condensation—in which water vapor goes from a gaseous state to a liquid state—begins to occur, forming clouds. If sufficient moisture condenses, then precipitation in one or more of its various forms (e.g., rain, snow, sleet, hail) may fall. (Exactly how precipitation forms and then falls through the atmosphere, and in what form, is beyond the scope of this article.)
There are several mechanisms by which large amounts of air may be uplifted.
The three that are most important in North America are orographic (mountain)
effects, convectional processes (see below), and, most important to birds and
birders, cyclonic (frontal) storms. Orographic precipitation results simply from
a moisture-laden air mass coming into contact with a tall mountain range and the
air being forced up and over those mountains (see Fig. 1). The very wet
conditions found on the coastal slopes of many of the mountain ranges in the
Pacific Northwest or on the island of Hawaii result from such uplift. Once over
the summit, the air descends, warming up and drying out. Some of the major arid
regions of this continent—such as the deserts of the Great Basin and the
Southwest, as well as the semi-arid grasslands of the western Great Plains—are
located in the “rain shadows” of the Cascades, the Sierra Nevada, and the Rocky
Mountains, respectively.
Convectional precipitation is best characterized by the late afternoon and evening thunderstorms prevalent in many regions during the late spring and summer. During the heating of the day some surfaces absorb more solar radiation than others. For a variety of reasons, a large city, with its black asphalt, will heat up more during the day than will a green forest. As a result, the air over the asphalt may be warmer than that over the surrounding forest. And if the atmospheric conditions that day are unstable, then a warm air “bubble” may be able to rise and rise until it reaches its dew point temperature, forming puffy, billowing clouds that might turn in to one or more thunderstorms. Such localized storms may have a very small-scale effect on migrating birds by forcing them down, resulting in localized “fallouts.” Severe thunderstorms, with their strong, gusty winds, may also affect birds by destroying nests.
Cyclonic precipitation has the most widespread effects on bird migration because it covers the largest areas. It results from the uplift of air along warm fronts and cold fronts. This topic will be covered in greater detail later in this article.
Finally, let’s look briefly at the circulation around high- and low-pressure
systems and how something called the Coriolis effect plays a role. In a
high-pressure system—also known as an “anticyclone”—surface winds move outward
(Fig. 2); the air is diverging, resulting in subsidence near the center of the
high, which in turn results in fair weather. For a low-pressure system—also
known as a “cyclone”—surface winds move in toward the center of the low (Fig. 2); the air is converging near the center of the low and must rise, which
results in unsettled (cloudy and wet) weather.
This simple system is corrupted by something called the “Coriolis effect.” The Coriolis effect is the result of the earth spinning on its axis. It is strongest nearer the poles and weakest near the equator. In the Northern Hemisphere, it affects flows of air and ocean currents by deflecting motion to the right relative to the direction the flow, regardless of compass direction. In the Southern Hemisphere, in contrast, the Coriolis effect deflects such flows to the left, relative to the direction of movement. As a result, the motion of air about the center of a high-pressure system is clockwise and outward in the Northern Hemisphere, whereas the flow around a low-pressure system is counterclockwise and inward. Coriolis has additional major impacts on global circulation patterns, as we will soon see.
General Global Circulation Patterns
To understand the patterns and movements of weather systems in North America, we must start with a quick look at the general circulation patterns found around the globe. In general, such patterns in the Southern Hemisphere are the mirror image of those in the Northern Hemisphere, and in this article we will restrict our discussion to the latter area.
The general circulation patterns around the Northern Hemisphere are shown in
Fig. 3. As a result of year-round intense solar heating near the equator, there
tends to be a rising motion of the air and resultant low pressure. This air
rises up for several miles and then moves poleward. For reasons we won’t get in
to here, it sinks again toward 30° N (actually anywhere between about 20–35° N).
This subsidence of air from aloft results in the formation of large subtropical
semi-permanent high-pressure systems. These large highs are generally located
over the oceans—as land surfaces are too uneven and conditions too variable to
support them.
These high-pressure belts shift a little north and strengthen in the summer
months, as the sun is higher in the Northern Hemisphere at that time. During the
winter, these Northern Hemisphere sub-tropical highs shift south and weaken. Off
the North American coast, the large high located in summer off the southeastern
U.S. coast is called the “Bermuda High” (Fig. 4).
One of its major impacts is
the flow of very warm and moist air around its back side, up from the Gulf of
Mexico and Caribbean, which bathes much of the eastern half of the continent in
summertime heat and high humidity. Some of this humidity may even reach westward
into the Desert Southwest, contributing to the late summer “monsoon” season well
known to southeast Arizona birders. The location and strength of the Bermuda
High may also affect the track taken by late summer and early autumn hurricanes.
Off the Pacific Coast in summer lies the “Hawaiian” or “Pacific” High (Fig. 4).
Its major impact is to block storms from reaching California and other nearby
areas during the summer. Its east side is dry, characterized by subsidence
aloft, which further hinders the formation of precipitation, and which also
contributes to air stagnation and the formation of smog in places such as Los
Angeles.
Between the subtropical highs and the equatorial lows, a northerly surface wind would be expected to blow, but the Coriolis effect deflects that northerly wind to the right, turning it in to a northeasterly wind (Fig. 3), often referred to as the “northeast trades” by mariners. North of the subtropical highs are found the so-called Mid-latitude Westerlies (Fig. 3), typically several miles above the surface, and located between the latitudes of 35–60° N—where New Jersey and most North American birders live. Here, although winds may blow from a variety of directions, a westerly wind is dominant. To our north exists strong lows during the winter months located in the North Pacific and North Atlantic Oceans, known as the “Aleutian Low” and “Icelandic Low,” respectively. Normally the winds moving between the subtropical highs to our south and these lows to our north would blow from south to north, but the Coriolis effect deflects them to the right, resulting in a westerly wind. These winds—the Mid-latitude Westerlies—are extremely important, as they steer many weather systems across the continent. Our weather generally moves from west to east—or northwest to southeast, or southwest to northeast—not the other way around. Oregon gets its weather from the Pacific. Folks in Omaha want to know what the weather was in Denver. And Cape May weather may be impacted by what happened a day or two earlier around Chicago or Oklahoma City, not vice versa. (These circulation and weather patterns also help to explain why the West Coast has such relatively mild weather year-round, as air masses come in off the relatively temperature-stable Pacific Ocean. In contrast, most of the Eastern Seaboard, although also located on an ocean, receives much of its weather from the continent’s interior, whose large landmass experiences much more variable temperatures—with warmer summers and much colder winters—than an ocean experiences.)
The strongest core of upper-level westerly winds, located a few miles up in the atmosphere and often following the strongest temperature gradient between cold polar air to the north and warm subtropical air to the south, is known as the jet stream. Winds here may blow at speeds of up to 200 miles per hour, but speeds of 100–150 miles per hour are more typical. One often hears about the location of the jet stream during weather reports because of its importance in steering storms and in demarcating the boundary between cooler and warmer air masses.
“Waves” form in the westerly winds, forming anywhere from about four to six “loops” in one trip around the earth. The top of each wave is called a “ridge”, whereas the bottom of a wave is called a “trough”. The circulation around the ridge approximates a clockwise direction. Thus, high pressure is often associated with a ridge. You often hear the weatherman announce that “a ridge of high pressure is building in from the west” and will cover such-and-such an area, resulting in an extended period of fair, dry weather. Storm (low-pressure) systems being steered by the westerlies are forced up and over (to the north of), and weaken, at high-pressure ridges. Conversely, in a trough, the circulation approximates a counterclockwise circulation pattern. Thus, low pressure is often associated with troughs. Storms may form in troughs, and existing storms that come in to a trough may intensify there. In sum, the location of ridges and troughs in the mid-latitude westerly winds, each of which may be hundreds or even a thousand or more miles across, is extremely important in determining short-term, and even some longer-term, weather patterns.
Cyclonic Storms
Mid-latitude cyclonic storms (or “wave cyclones”) are typically what bring us a
day or two of unsettled weather (Fig. 5a, 5b). They may play a critical role in
the timing of migration of birds. Such a low-pressure system develops between
two highs, or at “kinks” in the westerlies. Associated with these lows are
“fronts,” boundaries between two unlike air masses. A “cold front” and a “warm
front” may be drawn in (Fig. 6a), denoting where the cold air is advancing on
the warm, and where the warm air is advancing on the cold, respectively.
Fig.6a
is a plan view (looking from above) that shows the structure of such a “mature”
mid-latitude cyclonic storm. The counterclockwise motion of the air around this
low drags warm moist air up from the south and southwest in what is known as the
“warm sector” of the storm, a very important region for migrants and vagrants.
On the back side of the storm, behind the cold front, colder air is blowing out
of the north or northwest (also an important region for migrant birds).
Precipitation occurs along both the cold and warm fronts because air here is
being forced aloft (Fig. 6b). Cold air is denser than warm air, so along the
steep-sloped co
ld front, the advancing cold air forces the warmer air rapidly
upwards, resulting in a relatively narrow but often intense band of
precipitation. Ahead of the warm front, the surging warm air is forced up and
over the denser cool air. But the slope of the warm front is more gradual than
that of the cold front, so the resultant precipitation tends to be lighter, but
it covers a broader area.
Cold fronts typically move faster than warm fronts, with the result that the cold front may overtake part of the warm front, forming an “occluded front” (Fig. 7). In this area, the warm and moist air is rapidly uplifted, resulting in copious amounts of precipitation. But this process also results in the cutting off of the supply of moisture to the center of the storm. The entire low-pressure system then slowly dies out, thus completing the “life-cycle” of a mid-latitude cyclonic storm.
Weather and Bird Migration
Given the background meteorological information summarized here, we can start to make sense of the wonders of bird migration. And we can ask questions such as the following: What conditions make for a major movement of migrants in the spring and the fall at Cape May, elsewhere along the Atlantic Coast, and along the Great Lakes? What conditions result in one of those fabled spring fallouts along the Gulf Coast? What weather is favored by raptors at various hawk-watching sites in both spring and fall? Is there a “best” weather pattern for many passerines at oases in the western Great Plains and arid Interior West? What sorts of local weather conditions are preferred along the West Coast for seeing good numbers of transients? And in order to improve one’s chances of finding storm-carried seabirds, where should one locate oneself with regard to a hurricane about to make landfall?
A discussion of several of these questions can be found in Figures 8-16 and their captions, which follow:
Figure
1.
Orographic (meaning “associated with mountains”)
precipitation results from the forcing of a moist air flow aloft when it
encounters a major mountain range. Some of the wettest places on earth are found
on the windward side of high mountains found in the path of persistent moist
winds. Examples in North America include the “temperate rainforest” zone located
along the Pacific Coast from southeast Alaska south to northwest Washington, or
the extremely wet areas found at high elevations on the island of Hawaii that
intercept the northeast trade winds. As air rises up the windward side it cools,
and if it cools to its dew point temperature, condensation (cloud formation) and
possibly precipitation will result. As the air descends down the leeward side of
the mountains, it warms and its relative humidity decreases. Many of the major
arid areas of the world are located in such “rainshadow” zones. Arid regions are
found centered around 20-30° N on the east (dry) sides of the semi-permanent
subtropical high-pressure zones, and in the rainshadows of major mountain
ranges. Examples of the latter areas include the semi-arid Columbia Plateau
region located in the rainshadow of the Cascades, the Mojave and Great Basin
deserts found in the rainshadow of the Sierra Nevada, and the semi-arid
short-grass prairies of the western Great Plains located immediately east of the
Rockies.
Figure
2.
Circulation around high-pressure systems in the Northern
Hemisphere is clockwise and outward, whereas that around low-pressure areas is
counterclockwise and inward. The concentric lines drawn around diagrams of highs
and lows found on weather maps are called “isobars,” and are lines connecting
points of equal air pressure. Much as in contour lines on a topographic map, the
closer the lines are together, the steeper the gradient. A stronger pressure
gradient results in stronger winds.
Figure
3.
A schematic diagram showing global circulation patterns
north of about 15° S, assuming no large land-masses are present. These land
areas tend to disrupt such a simple pattern; thus semi-permanent high- and
low-pressure systems are most consistently located over the more stable and
uniform environment of the world’s oceans. Of particular importance to North
America are the semi-permanent subtropical high-pressure areas located over the
Atlantic and Pacific Oceans and centered at approximately 30 degrees N, the
mid-latitude westerly winds to their north, and the location of the polar front.
Figure
4.
The sub-tropical high-pressure cell located in the Atlantic
strengthens and shifts slightly north in our summer. It is known as the Bermuda
High and the circulation around the back side of this high is a major cause of
the very warm and humid summers in the eastern third to half of the United
States and southern Canada. The subtropical high off the West Coast—also
strongest during the summer months—is called the Pacific High. The east side of
that high is characterized by subsiding air aloft. The combination of the high’s
location and subsidence aloft act to suppress the occurrence of precipitation in
the California region between late spring and mid-autumn.
Figures 5A and 5B.
A mid-latitude cyclonic storm (low-pressure system) may
form in a trough located between two high-pressure areas found north and south
of the polar front, a zone in the mid- or high latitudes where cool or cold
polar air comes in contact with warm subtropical air. (A “front” is a boundary
between two unlike air masses.) The polar jet stream is a fast-flowing corridor
of air found within the upper-air westerlies and is often located above this
boundary. Airflow from these two air masses near the surface is converging,
forming an unstable environment (Figure 5A). As a “wave” forms along the front
(Figure 5B), a counterclockwise (cyclonic) flow sets up, bringing warm air north
and cold air invading south. A mid-latitude cyclonic storm is forming.
Figures 6A and 6B.
A mature mid-latitude cyclonic storm (low-pressure system),
shown in plan view ( 6A) and in a side view (6B). In this example the low is
coming ashore along the California-Oregon border. Cool and moist air from the
northwest is advancing on warm and moist air coming up from the southwest. The
boundary between the cool and warm air is termed a “cold front” because the cold
air is advancing on the warm. Colder air is denser than warmer air; thus the
former forces the latter up and over (Figure 6B), the resulting uplift being a
requirement for the formation of precipitation. Rapid uplift along the cold
front results in moderate-to-heavy precipitation. East of the center of the low,
the mild, moist air from the south is over-running the colder, drier air found
to the north and east—resulting in the formation of precipitation ahead of the
warm front. Orographic effects along the windward slope of the Sierra Nevada may
result in additional moisture being wrung out of the atmosphere.
Figure 7.
In this example, the same low-pressure system shown in Figure 6A, some
two or three days later, has been steered to the east by the mid-latitude westerlies (jet stream) and is now centered over Ontario. The air being pulled
in behind the center is cool/cold and dry. Close to the center of the low, the
cold front has overtaken the warm front—forming an “occluded front”—resulting in
massive uplift of a large mass of moist, warm air and copious amounts of
precipitation. The winds behind the cold front are largely from the northwest;
those ahead of the cold front are primarily from the southwest. This latter
zone—located between the cold front and the warm front, and dominated by mild
conditions and a southwesterly or southerly flow—is known as the “warm sector”
of the low. It is a region that—along with the colder air mass behind the cold
front—is very important in bird migration and vagrancy.
Figure 8.
If this simplified weather map denoted the conditions in late April or
May, birders would be chomping at the bit in large parts of the East, from the
central South to the Midwest to the mid-Atlantic and southern New England. A
broad flow from the south and southwest and mild temperatures is conducive for a
major flight of Neotropical and other migrants from the south. In the “warm
sector,” the tailwinds and partly cloudy skies might result in a very strong
nocturnal flight of migrants—as confirmed by radar, moon-watching, or listening
to nocturnal flight-calls. But the weather may, in fact, be “too good,” and
observers in the field the following morning—although logging a number of new
spring arrivals—may or may not see large numbers of birds. Those along the Gulf
Coast would see few transients, as the tailwinds there would allow for most
trans-Gulf migrants to continue farther inland before putting down.
So where would the biggest fallouts of migrants in this example potentially be found? Two likely answers would be Pt. Pelee, Ontario, and the Boston/Cape Cod area, possibly south to Long Island. Areas just south of the warm front, experiencing their first night of southerly wind, and those sites along and just barely ahead of the front, may collect the most birds. The weather near and ahead of the warm front is probably characterized by light rain, drizzle, fog, and/or possibly a less desirable wind direction. Migrants surging north may actually overtake the warm front and enter poor flying conditions, become disoriented, and land where they can find appropriate habitat. In this example, if they find themselves out over the Atlantic Ocean or Lake Erie, unable to properly navigate due to low cloud cover, the woodland oases at Cape Cod and Point Pelee, respectively, could be loaded with birds.
Figure 9.
Major fallouts along the Gulf Coast in spring are often the result of
a late-season cold front entering the Gulf of Mexico. Migrants that departed the
previous early evening from the Yucatan Peninsula or other sites in southern
Mexico, Belize, Cuba, and possibly even points farther south on a tailwind may
encounter heavy rain and a wind shift to the north or northwest as they near and
cross the cold front. In the example here in Figure 9, such poor flying
conditions will be experienced by migrants off the coast of Texas and southwest
Louisiana, whereas those birds over the Gulf farther to the east will continue
to enjoy relatively smooth sailing. For the former group, a journey that
normally might be some 500 miles and take some 15 hours to complete has now
become a battle for life, with wet feathers and a head wind for the last part of
the journey. Under such conditions, many birds may fall into the sea, land on
offshore oil and gas platforms, or, if they make it, pitch in at the first patch
of trees or brush along the immediate coast. Gulf Coastoases from South Padre
Island to High Island to the coastal cheniers of southern Louisiana to Dauphin
Island to the Florida Panhandle, Fort DeSoto, and the Dry Tortugas are
synonymous with superb spring birding—“superb” for the birders, not for the
birds. Fall migration here is not as dramatic, with cold fronts rare until late
fall, and some of the early-season concentrations of southbound migrants along
the immediate Gulf Coast being found after many days of southerly headwinds,
“damming up” these birds.
Figure 10.
Many birders and ornithologists have made the trip during the
late
spring to the islands of western Alaska: Attu, Shemya, Adak, St. Paul, and St.
Lawrence (Gambell). In most cases, what they hope for is the occurrence of
moderate-to-strong low-pressure systems coming out of Asia from the west or
southwest and passing over or just to the north of their location. In such
situations they receive southwesterly or westerly winds and periods of
precipitation. Migrants from Japan heading north to eastern Russia may become
entrained in the warm sector of the storm and be carried offshore, to be
deposited at these Alaskan outposts. If, however, the center of the low passes
well to the south of the island in question, the resultant easterly flow around
the top of the low typically will not be nearly as productive. This same
scenario exists in the autumn, when southbound Asian migrants may be carried
offshore by mid-latitude cyclones; the southwesterly winds ahead of the cold
front also may favor the appearance of reverse (mirror-image) vagrants.
Figure 11.
This is a classic autumn cold front along the Atlantic coast, with
northwest winds following the passage of the front. Large numbers of migrants
will take advantage of the tail wind, falling temperatures, and clearing skies
to move. A good number of these birds may be drifted to the east and end up out
over the Atlantic Ocean. After dawn many will re-orient themselves by turning
around into the wind and attempt to retrace their steps back to shore, landing
at famous birding coastal promontories and islands such as Cape Sable and Seal
islands NS, Monhegan Island ME, Cape Cod MA, Block Island RI, Long Island NY,
Cape May NJ, Cape Charles VA, and Cape Hatteras NC. Those which cannot return to
land may be seen trying to land on boats offshore, although most will die at
sea.
Northwest winds behind the cold front also strike the northeast-southwest trending Appalachian Mountains at approximately a 90-degree angle. The resulting uplift creates a favorable environment for the large numbers of raptors that follow the Appalachians southward.
Figure 12.
The combination of weather and geography in the Great Lakes region
results in some of the most spectacular raptor migration in North America. In
spring, large numbers of hawks will be moving north in the warm sector of a low,
enjoying the tail winds and thermals created by the mild temperatures. Upon
reaching the south shore of one of the broad Great Lakes, however, most of these
birds—especially soaring species—are forced around the lake. These lakes are a
huge water barrier, whose cold temperatures in spring do not support the
creation of strong thermals. Concentrating close to the south shore, these
raptors are censused at such famous spring hawk-watching sites as Derby Hill and
Braddock Bay, New York, and Grimsby, Ontario. In fall, the passage of a cold
front and the resultant northwest winds push southbound birds of prey to the
north shores of these same lakes. Refusing to cross, these birds concentrate
further and further as they head around the lakes to the west, to be enjoyed at
such sites as the west ends of Lake Erie (Holiday Beach, Ontario, and Lake Erie Metropark, Michigan) and Lake Superior (Hawk Ridge, Duluth, Minnesota). Sites
along the eastern shore of Lake Michigan, such as in Berrien County, Michigan,
might have their best flights on a northeast wind, which helps to “pin”
southbound birds to the immediate lakeshore.
Figure 13.
As shown in Figure 8, the southwesterly or southerly flow in the
“warm sector” of a low (which often combines with the similar flow around the
back side of a retreating high-pressure system) is instrumental in the
appearance of large numbers of spring migrants. Such flows also play a very
important role in autumn: in facilitating the occurrence, or at least the timing
of appearance, of a host of vagrant and reverse-migrant species in the eastern
two-thirds of the USA and Canada. The volume of the flow of reverse-migrants is
much less than that of a spring wave, but it may provide birders with some of
their most exciting birding of the year. These flows during late autumn are the
likely catalyst behind the annual November occurrence of multiple Ash-throated
Flycatchers, Cave Swallows, and other western and southwestern strays. Some
individuals of many eastern species actually head north in fall in response to
these tail winds, thus turning up north of their normal range and/or unusually
late in the season. Examples are numerous; a few include the numbers of
newly-arrived Yellow-billed Cuckoos well into October and north of their normal
range, late Chimney Swifts and swallows (particularly Barns) in November, and
the annual appearance of numbers of Pine Warblers and Yellow-breasted Chats in
the Northeast in late fall and early winter.
Figure 14.
Two major reverse-migrant and vagrant events along the East Coast
occurred in November 1997 at Cape May and in October 1998 in southern Nova
Scotia. Both were the result of a weather system that did not follow the usual
script laid out in Figure 13. Instead, a low-pressure area set up shop off
Virginia/North Carolina during the second week of November 1997 and off Cape Cod
and the Gulf of Maine during the second week of October 1998. In 1997,
southwesterly winds in the warm sector were blowing all the way up from the Gulf
of Mexico and Florida, then turned southerly at sea off the Southeastern states.
This flow did not reach any landmass until it had wrapped around the north side
of the low and right into the Cape May region. Over the following week, birders
there discovered North America’s second Brown-chested Martin, five Cave
Swallows, Western Kingbird, Ash-throated Flycatcher, MacGillivray’s Warbler,
Western Tanager, and a host of late (assumed to be reverse) migrants including
multiple Chimney Swifts and Cliff Swallows, over a dozen Barn Swallows, and some
15 species of warblers. In 1998, northwesterly winds behind a cold front along
the Southeastern coast of the U.S. at the start of that year’s “event” probably
carried many birds offshore to the southeast. There they became entrained in the
southerly and southwesterly flow ahead of the cold front and were carried over
the ocean back far to the north, and then back in to shore at the southern tip
of Nova Scotia on strong easterly winds (McLaren et al. 2000). Such a non-stop
flight may have taken the birds almost two days to complete, and many of those
that made it were found in an exhausted condition. The list of “southern” birds
found in a small area of the province was mind-boggling, and included 10
Yellow-billed Cuckoos, 22 White-eyed and 14 Yellow-throated vireos, 5
Blue-winged, 2 Golden-winged, 5 Yellow-throated, 2 Prothonotary, and 14 Hooded
warblers, Summer Tanager, 64 Blue Grosbeaks, and 547 Indigo Buntings (McLaren et
al. 2000).
Figure 15.
When one very broad, strong low forms over the North Atlantic—or when
two lows are found, as shown here—there may be a moderate-to-strong easterly
flow all the way from northwest Europe to the Atlantic Provinces. Such a set-up
in spring has resulted in the occurrence of a number of much sought-after
European vagrants, particularly in eastern Newfoundland. Flocks of European
Golden-Plovers and numbers of Northern Wheaters are probably the most visible
examples of this phenomenon, but other species found there at this time of year
include Pink-footed Goose, Garganey, Eurasian Oystercatcher, Common Redshank,
and Common Chaffinch.
In fall, a fast-moving low crossing the North Atlantic may carry with it—in its warm sector—numbers of North American passerines all the way to the Azores, Ireland, England, and northwest France. A plethora of Nearctic species has been found there, whereas we in eastern North America struggle to find a mere handful of Palearctic passerines over many decades. The prevailing winds are against us! Strong-flyers with good fat reserves are the most likely to survive the eastward crossing to Europe, and include such species as Gray-cheeked Thrush and Blackpoll Warbler. Many hundreds of British birders collect every year at such vagrant traps as the Isles of Scilly off southwest England during late September and October in hopes that such cyclonic storms come their way. Recently, farther south, the Azores have been shown to be a particularly rich hot-spot for North American vagrants.
Figure 16.
Passerine migration along the West Coast is less influenced by the
passage of warm and cold fronts than it is in areas east of the Rockies. Such
fronts are fairly rare in California during much of the late spring and
early-to-mid fall migration periods. One weather phenomenon that does play a
major role is the presence of coastal fog (“the marine layer”), which acts to
disorient nocturnal migrants by blocking out their visual, celestial cues.
Morning fog also helps to insure that many landbirds returning to shore remain
at the small patches of trees along the immediate rather than continuing farther
inland. Birders here search for both regular, western migrants and for a
selection of eastern vagrants, as well as a few wanderers from the Southwest and
Alaska/Asia. This marine layer forms as the result of mild, moist air flowing
from the Pacific to relatively lower pressure inland that forms due to
increasing land temperatures between mid-spring and late fall. (This “thermal
low” is not a mid-latitude cyclonic storm with precipitation, but rather it is a
long-lasting area of low pressure that results from warm air rising.) Before
reaching the coast, this moist air is chilled from below by the cold California
Current found just offshore. If the air cools to its dew point, fog is formed.
Low, dense fog typically is not as productive for migrants/vagrants as is a
higher ceiling of this marine layer, in part because under such conditions
either the birds can’t find the trees or the birders have a hard time seeing the
birds. The number of migrant birds along the coast may also be greater if the
dominant northwesterly winds offshore slacken for a time.
Cited References and Recommended Reading
Able, K.P., ed. 1999. Gatherings of Angels: Migrating Birds and Their Ecology. Cornell University Press, Ithaca.
Duncan, R.A. 1994. Bird Migration, Weather, and Fallout: Including the Migrant Traps of Alabama and Northwest Florida. Published by the author, Gulf Breeze.
Kerlinger, P. 1995. How Birds Migrate. Stackpole Books, Mechanicsburg.
Lehr, P.E., R.W. Burnette, and H.S. Zim. 1987. A Golden Guide: Weather—Air Masses, Clouds, Rainfall, Storms, Weather Maps, Climate, revised edition. Golden Press, New York.
Ludlum, D.M. 1995. National Audubon Society Field Guide to North American Weather. Alfred A. Knopf, New York.
McLaren, I., B. Maybank, K. Keddy, P.D. Taylor, and T. Fitzgerald. 2000. A notable autumn arrival of reverse-migrants in southern Nova Scotia. North American Birds 54:4–10.
Moss, S. 1995. Birds and Weather: A Birdwatcher’s Guide. Hamlyn, London.
Schaefer, V.J., and J.A. Day. 1981. A Field Guide to the Atmosphere. Houghton Mifflin Company, Boston.
Shrader, J., and K. Shrader. 1995. To go ... Weather or not? Weather resources for birders. Birding 27:292–297.
Note that almost any college-level introductory physical geography textbook will have one or more chapters on basic meteorology.
An earlier version of this article appeared in the November/December 2003 issue of BIRDING magazine.