Fall migration is an exciting time for birds. With migrants on the move your local birding site can be transformed from the static to the dynamic overnight, with a suite of new species to identify, an abundance of individuals, and a feeling that anything is possible. We realize some of our best birding days center on being at the right place at the right time. For many birders this is mere happenstance; perhaps a long-planned weekend trip to Cape May results in a great encounter with bird migration. But these fantastic interactions with bird migration can be reliably predicted with a basic understanding of birds and weather. The savvy fall birder plans carefully both when and where to look for birds. In this article we’ll discuss the basics of fall bird migration as it relates to weather, and put you on the right track to find more birds.
Fall migration starts earlier than most people realize, with many shorebirds on the move by late June, and the first landbirds heading south soon thereafter. August through October are peak months, but migration continues into December for some species, especially shorter-distance landbirds (e.g., sparrows, blackbirds), raptors, waterfowl, and seabirds. Birds are often on the move every day during this entire period, but there are reasons that some days are better than others for observing them. The volume of migration depends on the weather, with many birds waiting until conditions are favorable before initiating migratory flights.
Bird migration is spectacular, complex, and difficult to generalize — even within closely related species the pathways and strategies may differ. Birds use varying migratory strategies depending on, among other attributes, mode of flight (e.g., soaring vs. flapping), migration distance, and the species’ natural history. For this discussion, however, we can break birds down into two basic groups: diurnal and nocturnal migrants. Soaring birds that rely on thermals to generate lift migrate during the day (e.g., raptors, American White Pelican), whereas most long-distance migratory landbirds (e.g., warblers, sparrows, thrushes) migrate at night — or at least they initiate their flights under the cover of darkness! Relatively few species do both, unless faced with some geographic barrier (e.g., the Gulf of Mexico) or meteorological phenomenon (e.g., hurricane displacement). For some species, migration is a nightly process, with birds ‘dropping-out’ in the morning to spend the day refueling for the next flight; but for others migration is essentially non-stop once initiated, and certain species travel from the Canadian Maritimes all the way to South America in a single non-stop flight! For some species, migration is almost by the clock, departing breeding and non-breeding grounds based on dates representative of past ecological, evolutionary, and climatological histories, whereas, for others, migration occurs when conditions necessitate (i.e. essentially an obligate vs. facultative dichotomy, respectively).
Diurnal migration can be spectacular to watch, and birders traditionally gather at locations where weather and geography conspire to concentrate migrants, most notably along coastlines and ridges, and especially peninsulas with southerly orientation. Nocturnal migration is more challenging to understand and experience, but our understanding of that phenomenon is rapidly advancing. There is an increasing number of birders interested in nocturnal migration, and many spend nights listening to birds passing overhead in complete darkness, with many species having a distinctive ‘flight call’. At a few sites (e.g., Higbee Beach, Cape May, NJ) birders can see tens, hundreds, or thousands of nocturnal migrants flying by at dawn as they head back inland after finding themselves over open water. At sites like Higbee, amazing numbers of migrant passerines are counted during migration, including species such as Connecticut Warbler that are otherwise very unlikely to be seen in flight. Most birders encounter these nocturnal migrants the morning after landfall, when they are conspicuously searching for suitable foraging and refueling habitats in preparation for their next flight. Depending on weather conditions and refueling performance, birds will remain in the area for a few days to a week before departing on another migratory flight.
During the peak of fall migration, birds are on the move every day, particularly as the North temperate zone days become increasingly short. But the volume of birds is greatly affected by local and regional scale weather patterns. It is perhaps unsurprising that birds are affected so greatly by weather—humans don’t necessarily like to be out when its raining either! But in addition to precipitation, wind direction plays a major role in creating favorable migration conditions. Think about how it feels when you go on a bike ride and the wind is at your back—easy sailing right? But returning against the wind requires a lot more of your energy to cover the same distance, and you get tired quickly. Moreover, if it is too windy, riding becomes challenging no matter which way you try to maneuver. The same is true of migrating birds, and many species await favorable tail winds – tail winds that are not too strong – before undertaking long migratory flights. When weather systems create these winds, birds move en masse.
Most birders with a basic understanding of bird migration know that the passage of strong cold fronts, associated with movements of low pressure centers, produce big flights of migratory birds in fall. The conditions associated with these weather systems often produce what are known as ‘fall-outs’, when thousands of birds concentrate in a relatively small geographic area. In some cases these fall-outs will be local, and in others they may extend across entire regions (e.g., the Mid-Atlantic Coast). Indeed, “birds and weather” junkies often spend more time watching the Weather Channel trying to predict the movements of these systems and their associated bird fall-outs than they do actually looking for birds! But these are the same people that miraculously appear on every great migration day, as if they had some insider information that tipped them off about the flight. In reality they have been watching the weather, waiting for the right day to use their vacation time!
Figure 1. A synoptic weather map, 11:45 PM EDT, 8:45 PDT, 14 September 2011, showing the geographic position of low and high pressure centers, the boundaries between air masses associated with these pressure centers (blue lines and triangles represent a cold front; red lines and half circles represent a warm front; a mixture of the two represents a stationary front; and the purple triangle and half circle represents an occluded front). Overlaid on this map are data from the US network of weather surveillance radars (WSR-88D), with colors ranging primarily from blues to greens in this image with some spotty areas of yellows and reds (represent different densities of targets in the atmosphere). In this image we see many birds (and insects and bats!) detected by radar, in active nocturnal migration, in relation to a cold front stretching from the eastern Great Lakes southwest to the southern Great Plains. Note the blue and green circles behind (to the west and north) the cold front, over the Dakotas, Great Plains, and Mississippi River valley —these represent high numbers (probably up to 1000-1500 birds per cubic kilometer) of migrating landbirds. Additionally, we see the characteristic red and yellow blocky patterns of precipitation and thunderstorms associated with the boundaries between air masses – for example, just west of the Appalachians from Tennessee north to the Great Lakes and just east of the Rockies in New Mexico and the Texas panhandle). Ahead of the cold front there is much less bird movement apparent, with what is likely only light migration occurring across the southeastern US, where conditions are marginal but not terrible for birds to migrate nocturnally.
Fronts are simply graphical representations on a weather map of what meteorologists consider the boundaries between air masses. In the US, cold fronts precede centers of high-pressure, with clockwise circulation of cool and dry air, moving south and east across the landscape (represented by blue lines on weather maps). As the cool air advances south and east, it lifts the hotter more humid air high into the atmosphere, where its moisture condenses into rain. When the temperature gradient is strong, lines of powerful thunderstorms and violent winds can occur, all advancing in a generally eastward direction across the continent. Ahead of these systems conditions are unfavorable for fall migration. Strong south winds and unstable air cause many migrants to wait it out for days and sometimes weeks in the case of extremely slow-moving systems. This bottling-up of migrants for extended periods can result in spectacular flights when the dam breaks, and the cold front finally passes.
Behind a typical fall cold front the air is dry and cool, and the wind direction generally has a northerly component, creating favorable conditions for migration. These conditions can last for days, but usually the migration is most intense on the day immediately following the passage of a cold front. This is the day you’re looking for–the day to get out birding! Early fall cold fronts generally result in the pleasant fall weather we all love, but late fall cold fronts can bring arctic blasts across the country, sending the last northern migrants on their way south.
Since cold fronts bring winds with a westerly component, birders on the West Coast of the United States use somewhat different strategies for finding large numbers of migrant passerines (strong cold fronts are very good for witnessing seabird movements from shore on the West Coast). While winds with a northerly component often trigger bird movement in the West as well, it is the rare weather events with easterly winds that produce the most impressive migration events in the West. In southern California, occasional warm “Santa Ana” winds can result in good fall-outs of migrants along the West Coast ,as birds are pushed westward and offshore, and then reorient back towards the coast. The same general rule holds on other large water bodies (and maybe even deserts): when westerly winds occur, get to the west shore of the lake for passerines or east shore for waterbirds; when easterly winds occur, the eastern shore may be better for passerines and the western shore for waterbirds.
One other general rule during migration (both spring and fall), is that when a good migratory movement of birds encounters rain, low overcast, or foggy conditions, spectacular fallouts can occur. Along an abrupt line where the rain or fog starts, birds may be quite literally dropping out of the sky along that frontal boundary. But when this fog occurs along the coastline, a higher percentage of birds may accidentally fly offshore and thus the return flight to the coast will likely include more birds and make those birds more likely to land right along the coast, since they are probably stressed from disorientation in the fog or weakened from flying through rain. These conditions can be dangerous for birds, but they can make for spectacular birding with large numbers of birds often providing excellent views.
Most of you will already know the great migration points in your area, and all of these are represented in eBird by various ‘Hotspots’. But finding your own local migration hotspot can sometimes be more rewarding, and surveying these more poorly birded sites can help us learn a lot more when it comes to understanding the magnitude and composition of bird movements in relation to weather. There are a few things to consider when trying to find a good local patch for migrant birds.
General geography–Migrant birds often follow coastlines and other ‘leading lines’ during migration. Good sites are often situated on peninsulas with a north-south orientation, effectively ‘funneling’ migrants toward the tip as they move south. Other good sites can abut high mountains or extensive deserts.
Isolation–In areas of expansive agriculture or generally unfavorable habitat (e.g., desert, large metropolitan centers), migrants will pile into the few remaining isolated patches of favorable habitat. These could be lakes with isolated stands of willows, ranchsteads with surrounding tall trees, city parks, or any other similar ‘oases’ for tired, hungry migrants.
Water–A fresh water source is critical for attracting and holding migrant birds, and can be as small as a backyard birder’s drip, to as large as a lake or pond.
Open views of the sky–although this is a fundamentally different consideration than the preceding three, it is often useful for those interested in seeing birds engage in morning flight (e.g. continued migration or redirected migration) to find locations to sit from just prior to sunrise for several hours. Finding a place associated with the preceding three considerations to watch the sky for several hours, you may be treated to a wonderful spectacle of passing migrants in flight in the morning sun searching for their next stopover location.
Fall is indeed an exciting time for birds and birding. By watching the weather you can increase your chances of encountering migrant birds at a regional migration hotspot, or in your local patch. The important thing is to get out birding, and make sure to submit every bird checklist to eBird, not just those on which you had birding highlights. By submitting to eBird every time you look for birds, we can learn about what birds you find, but also importantly about what birds you didn’t find—the latter equally important from an analysis perspective, though admittedly less interesting from a birder’s perspective! Moving forward we’ll be creating tools that model bird migration in relation to weather, and your observations will be a critical component in gaining a better understanding of these complex processes.
Brian Sullivan, Marshall Iliff, Andy Farnsworth, and Chris Wood
Thanks to Dave Slager for helpful comments pertaining to stopover duration.