BirdCapeMay.org

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 cold 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.