Understanding birds & weather: Fall birding basics

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 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.

1. 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.
2. 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.
3. 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.
4. 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.

Good birding,

Brian Sullivan, Marshall Iliff, Andy Farnsworth, and Chris WoodAcknowledgements
Thanks to Dave Slager for helpful comments pertaining to stopover duration.