understanding bird migration, with cornell lab of o’s frank la sorte

RideWaveBIGI WILL MISS THEM, after the last hummingbird passes through the garden sometime in September, continuing its long trip southward, with me as just one pit stop on the way. They’re the most obvious sign this time of year at my place of the biological phenomenon of avian migration—a subject that though I’ve witnessed for decades, I admit I don’t really know much about.

In January, I read about a newly published paper about bird migration by scientists at Cornell Lab of Ornithology. It was brought to life with an exciting, almost hypnotizing animated map they’d created, illustrating both the spring and fall routes of 118 bird species in the Western Hemisphere.

Frank La Sorte, a research associate and ecologist at the Lab of O, was a key part of that research, and I was excited at the chance to learn more about migration from him. Read along as you listen to the Sept. 5, 2016 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).

Learn what cues get birds moving north in spring and south in fall—which perhaps surprisingly isn’t necessarily along the same path each time (as the green and orange loops in the illustration up top roughly indicate). Find out how by recording your bird sightings in eBird, the online citizen-science database, you can contribute essential information for more such dramatic studies. (How to participate is detailed at the bottom of this page.) And learn why studying bird migration matters: how it can help with conservation in a changing world.

my bird-migration q&a with cornell’s frank la sorte



Q. It’s that time of year, isn’t it, Frank?

A. It certainly is.

Q. They’re moving; they’re moving.

A. I see some outside my window right now.

Q. Who’s moving by?

A. Goldfinches.

Q. I guess first: can we at least briefly define what migration is, since various species of birds move different distances, or not at all, or very little. What’s migration?

A. It’s actually a very diverse phenomenon. You have a broad range of strategies that birds use from very short migration to extremely long migration.

You have species whose population all migrate, and species which have populations that some migrate and some don’t—partial migration.

It’s actually a very diverse, complex phenomenon, with lots of different variation among species.

Q. People think, “Oh, I don’t see the bluebirds in the winter.” But in fact, they’re not in the Tropics. Would they be a partial migrant?

A. Yes, some species just move to regions where they can still find food. Others are committed to long-distance movement to the Tropics, where they have very specific wintering grounds.

Q. Why do birds migrate, anyway? It seems like a great risk, of literally life and limb? Is it for food?

A. It’s for food. You have this pulse of resources in the temperate region that occurs in summertime. The thing is, the birds who live here year round can’t necessarily take advantage of all those resources. So you have these species that have evolved this capacity to move their populations north into these regions where there are abundant insects, and few competitors, and they can raise young fairly effectively. And when they’re done, they just go back south.

Many of these species just spend a few weeks on the breeding grounds. They’re really just visiting this area, to take advantage of the resources, raise their young, and then take off back south to their wintering grounds where they spend most of the year.

Q. How do they know when to go—and it’s probably not a universal answer, and different species probably have different cues. But how do they know when to go? Is it the food’s running out? Daylength? Temperature? What are the triggers?

A. For birds that winter in the Tropics, there really aren’t many triggers.

Q. [Laughter.]

A. The environment’s really consistent across the year.  So in that case they have to rely on internal cues often. Those internal cues tend to be fairly rigid, so they leave at very specific times, and they make their way north and arrive at very specific times. There’s little flexibility for those guys.

When they come back south, there’s changes in daylight, changes in temperatures that occur at higher latitudes that are very obvious, and provide strong cues—especially as insect populations start to decline.

Q. So there are multiple cues.

A. Some of the species are very flexible in their ability to use resources, so some species will eat insects in the summertime, but will switch to seeds and fruit in the autumn to a certain extent. They’ll stay a little bit to build up their body fat—to get enough energy to make the southward journey.

Q. Give us a couple of examples, as an expert in this biological phenomenon, that really impress you or stand out—like who are some of the longest-distance migrants?

By Cephas from WikimediaA. From the ones we study in the Western Hemisphere, one that has always been very interesting is the blackpoll warbler [above]. They breed up through the boreal forest of Canada. What they do, in the autumn, is they migrate east to the Atlantic coastline, in New England, and then they leave the coastline and cross the Atlantic Ocean straight down to South America.

It’s one of the longest migratory flights for the warbler species in the Western Hemisphere. Only recently have we gotten enough evidence to really establish that they’re flying out over the Atlantic. Before, it was thought that they would follow along the coastline and down to Florida, but in fact they’re largely going over the ocean.

Q. Again, it seems like risky business. But they must be doing something right.

A. In the autumn, it’s a more direct route, strictly south over the Atlantic to South America. If you look at the globe, you see it saves you a lot of time. [Illustration below.]

Q. So the blackpoll warbler is impressive. What about some birds that don’t go as far that are migrants—some that just go to Florida, or northern Central America?

A. Many do that, and just go to the Caribbean and Central America. Those present very interesting case studies, because they are going from very large breeding areas, encompassing large portions of North America, and then wintering in very confined areas on islands or within Central America.

Q. Two drastically different environments, in different parts of their lives. Huh.

A. They occur often at high densities in the wintering grounds, so that’s one reason we’re concerned about habitat loss in those areas. If you have high densities of a high numbers of species and you have habitat loss, it will impact more individuals and more species.

Q. When you began work on the paper I read about in January, what got you asking the questions that became this study in the first place? What were you proposing to find out?

A. It’s very challenging documenting migration for birds, because they move such long distances, and this often occurs at night, at high altitudes. To observe this phenomenon has been something we’ve been able to do effectively only recently. Usually this involves banding the birds with some kind of tracking device.

But here we’re talking about taking advantage of a new data source: eBird, which is an online, citizen-science database, where bird watchers enter their observations. We’ve compiled enough observations for a number of species that we can document where they are across the entire year.

Piecing together millions of observations across the Western Hemisphere, we can say the population is likely here, during this day, then here, and here—and we can piece it all together into a giant map that shows where they occur across the year.

One interesting thing that was the focus of that study, was that the birds don’t necessarily follow the same general path in the spring and in the autumn. Many species tend to follow looped paths, and these loops can be quite large, especially for species in the eastern portion of North America.

What happens is that in the spring they come up usually over the Gulf of Mexico, and then up through the continent. Then in the autumn they tend to fly off to the east, and off over the Atlantic Ocean back south. The blackpoll warbler is a good example.

In the spring you have strong winds blowing over the Gulf of Mexico. They likely latch onto those winds and get pushed north. In the autumn, those winds are gone, but still the winds are primarily from the south. And it’s much more efficient to go directly south, over the Atlantic.


[Fall 2015 illustration, above, by Misaki Ouchida drew on then-unpublished research by Frank La Sorte, eBird data and more. Read more about the risky strategy of Atlantic crossings.]

Q. So the paths are often loops, but they’re not the same loops in spring and fall. It’s not like they take the same route—like if I’m going to my sister’s house, I take the same route all the time. It’s not like that. [Laughter.]

A. No, and there is a lot of diversity, too, in the shape and width of those loops. It’s likely related to wind. Wind plays a big role in the amount of energy they have to consume. If they can get a tailwind, that can boost the migration speed tremendously.

A lot of these large-scale migration patterns are defined by seasonal patterns of wind, and how they change across large regions of North America and South America.

Q. When birds are going to start the journey north, to the breeding grounds for that short time they’ll be in the north, do they stoke up in the same way they seem to before the journey south?

A. There is increasing evidence that these birds have stopover sites in South America, where they’ll stop for a week and feed intensively, then make the long jump over the Gulf of Mexico, up into the breeding grounds.

But the research in South America has been more sparse. Most of the research has been in the United States and Europe; research within the Tropics has been less intensive. A lot of these questions are still not well addressed.

Q. Is one direction, north or south, always easier or harder?

A. Yes, it’s generally harder going south, because of the prevailing winds.

Q. I guess I should have backtracked and asked: We have a number of “flyways,” as I always heard of them talked about. But is that what I am seeing on those maps of birds moving down or up the continent—general areas birds use to travel (once referred to as flyways)? 

A. We’ve looked at flyways as well—that’s an old concept.

Q. That’s what I thought.

A. It was originally designed for waterfowl. Waterfowl were relatively easy to track 100 years ago.

Q. Ah: big birds!

A. Yes, so the big birds migrate during the day, at low altitudes.

Q. …honking, quacking…[laughter].

A. So the original concept was designed for waterfowl. Recently we’re trying to shift those flyways to consider terrestrial birds—warblers, thrushes. Those do show up in some of our research. They tend to be broader than waterfowl. Waterfowl of course have to follow the Mississippi River or other features, where terrestrial birds can migrate across broad fronts. The flyways for terrestrial species tend to be much larger.

Q. OK. I knew it was a word I had in my head, from being interested in birds for many decades as a hobbyist. But I realize it didn’t really say that in the new research; it’s an older concept.

A. We’re actually trying to remold it to consider the new information. It’s probably still a useful concept, but has to be placed within its modern context.

Q. On these animated maps—there are two maps, one with dots, and one with numbers, each number representing a different one of the 118 species. Can you help us paint a visual picture of the maps? How did you even get that idea to make that map? [Laughter.]

A. It was a couple of years in the making. With bird there’s so much data, and it’s easy to get lost in it. The goal here is to try to summarize all that information in a conceptually simple framework for interpreting patterns.

Eventually we just settled in that we’d do a single dot for each day. They’ll summarize the overall pattern, and when you put the dots together you’ll get the overall trajectory or pathway where the population is moving. We tried it out, and it was very effective. It was easy to interpret, and allowed us to look at these very board-scale patterns, which really hadn’t been looked at before.

Up until now, you video-tagged a handful of birds, and you followed where those birds go. Often there will be mortality events and you’ll lose data, or other things will happen, and you’ll have very limited information. Here we have really comprehensive information about where the population is every day of the year.

Cornell migration maps[Above, two screenshots from the animated fall migration map. See the map in action at this link.]

Q. Because you’re using eBird. So for instance for years, I’ve put my observations—minimal as they are—into eBird. When I was driving to the studio today to talk to you, I saw probably 100 swallows on the wires by a farm field just down the road from my house, and the last few weeks I’ve seen hundreds of swallows at a time. If I had been recording those, and saying what species they were, and recording the location—that’s the kind of thing, the kind of movements, that you would be using.

A. All that information, thousands of observations, go into generating the summaries. So we don’t throw any eBird observations out; each observation is important, and fits a much larger puzzle. It helps me piece together these very general summaries.

Q. At the beginning we were talking about birds that do and don’t move, and move different distances—some that are not migrants to the Tropics every year. Was this study strictly about longer-distance migrants?

A. For this study we focused on species that migrated at least 2,000 kilometers. We wanted to encompass the full hemisphere, and the more extreme-distance migrants.

Q. Do the routes have wiggle room, from year to year? Can the birds decide to go “off course” a little bit?

A. We did another study where we looked at the question. It’s a very important question because how much flexibility will these birds have when they migrate, in terms of their behavior? As the environment changes, are they going to be able to adapt during migration?

Q. There is a tropical storm coming, or upgraded to a hurricane, so, “Uh-oh, uh-oh, what do I do now? I’m migrating.”

A. What we’re finding is that species that migrate longer distances tend to be more flexible when they’re migrating, which makes a lot of sense. If you’re migrating a long distance, it makes sense when you are on that journey to look for conditions that are more favorable as you move north, and take advantage of those conditions as best you can.

With the short-distance migrant, there really isn’t the pressure to move quickly, and you tend to be less selective and just move south in a general pattern.

Q. I think I read a study where you collaborated with a colleague at Stony Brook, that cited hummingbird migration and talked about wiggle room along the routes.

A. Hummingbirds are a unique case.

Q. In all ways, aren’t they? [Laughter.]

A. In migration, they are difficult to observe. The basic natural history of their migration is not understood. We assume they migrate at lower altitudes, possibly just above the tree line. When you see them fly, they can fly very quickly. But you know they need a lot of resources to maintain that energy level.

How they did that was not well understood. But we looked at it using eBird data, and we documented some very broad-scale patterns of movement. We also see the loop trajectories as well.

Q. In terms of one that stand out that I asked you about, like the blackpoll warbler—are hummingbirds another sort of migration hero? I don’t know how you get a tiny little body so far. [New research shows ruby-throateds travel 2,000 km at a clip.]

A. It’s something that we’re actively studying, and a key question is exactly how do they time their migration to be able to find resources to keep up their energy levels. Exactly how they are doing that is not really clear. Many of the species we studied are in Western North America, where we have mountains and a lot of heterogeneity in the environments.

Some of our work indicates that they are actually taking advantage of these elevational gradients. So as plants flower that will move up in elevation, and they’re likely tracking that gradient as it develops in the spring into the summer, so they’re able to get resources all year round, just by following the gradient of flowering plants.

[Video above from published research on hummingbird routes, using citizen-science data. More on that.]

Q. Also in terms of heroics: How far do species go at a clip during migration, without resting?

A. I don’t study the individual birds, so I can’t say for a particular species, but at the level we look at it—which is the population level—there are several species that migrate very quickly.

There is evidence that often in the spring when they cross the Gulf, if they still have time they just keep going north, and will go up into the Appalachians before they actually stop. So they build up large energy reserves, and I think they are willing to use up all that energy to make a very dramatic movement to cross some of these very large barriers.

Migration speeds we’re talking about at the population level are very dramatic. At the individual level, which we don’t study, if the birds can get a good tailwind, the speeds can be very dramatic as well.

Q. Why does it matter that we humans understand migration better? You don’t just do this work academically, for the pure knowledge, obviously. What can we do with a deeper insight into bird migration?

A. The big theme of a lot of our work is that we want to be able to look at bird migration across the full year. Often when you talk about birds and some of the challenges they’re facing, you just focus on the breeding season, because that’s where we have most of the information—on the breeding grounds in North America.

But the reality is they only spend a few weeks here, and they spend much more time on the wintering grounds, and they spend time in migration, but what’s actually happening in those areas is less well understood.

Once we can get a full annual-cycle picture of what’s going on, we can pinpoint the areas where the pressure is likely to be greatest, and we can focus our limited conservation resources at those locations to help maintain their populations.

Q. So it would help us with focusing our conservation efforts. You said they spend most of their time on the wintering grounds, and a minority on the breeding grounds—and some time migrating. How long is a trip?

A. Species that migrate the longest tend to travel very quickly. So species that winter in southern South America will travel to their breeding grounds in North American very quickly, over a period of a couple of weeks. If the breeding grounds are up near the Arctic Circle, they will just stay there for a very brief time, then start heading back south.

There is a lot of variation on that pattern. Each species has its own unique resources they need to time their migration to coincide with, and they have their own unique patterns of courtship and breeding that they have to fit into that window.

It‘s a remarkable diversity of strategies, and the more we study it the more we see that there really isn’t a lot of similarity. We can make some generalizations in terms of some species are doing this in this region, but over all there’s a lot of variation. It’s remarkable that they have evolved to capture all of these unique strategies that help them to maintain their populations from one year to the next.

more from cornell on migration

do you eBird? sign up now

ebird logoTHE ONLINE bird-counting application called eBird, a project of Cornell Lab of Ornithology, celebrated its 11th year in 2016. Unlike well-known counts such as Great Backyard Bird Count and Project Feederwatch (more on them below), which have specific start and end dates each year, eBird is a lifetime tool, for use anytime, and whether at home or elsewhere.

I like eBird’s new-in-2014 “Location Explorer,” a dashboard allowing you to zoom in on who’s seen what in a particular area of interest—as useful for seeing what other species are around where you live as it is for planning a birding trip to some exotic distant place.

Within it I can drill down to the “hotspots map,” then zoom in and see how many species have been recorded in my vicinity. For instance, in the public land surrounding my place (the Taconic State Park—Copake Falls Area; Columbia County, New York), there are 101 species recorded from 17 checklists (at the time of this writing). What’s exciting: About two-dozen of those species are ones I have yet to see, and my own little life list in the garden here includes a bunch that aren’t among the 101. In the adjacent county across the Massachusetts border, which has many more people (and eBirders, specifically), there are already 295 species recorded.

It’s important that we all participate, and share our data, so a fuller picture can be had by researchers than they could gather on their own. As in: Do you see what I see? Probably not. Off I go to transfer the numbers on my pad of paper into my eBird account.

  • eBird: Sign up here and use the online checklist anytime (which helps by suggesting possible birds in your area).
  • GBBC: Sign up for the next Great Backyard Bird Count, in February (a joint project of Cornell, Audubon and Bird Studies Canada, and powered by the eBird technology).
  • Project Feederwatch: Join Project Feederwatch (from Cornell and Birds Studies Canada), which runs November into April and as it sounds is about recording birds in your backyard in feeder season, at the feeders themselves or in your garden plantings or using its water features—counting birds using something you provided.

prefer the podcast version of the show?

MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its seventh year in March 2016. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the Sept. 5, 2016 show right here. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).


(Photo of blackpoll warbler by Cephas, from Wikimedia under Creative Commons license. Charts and maps from Cornell Lab of Ornithology.)

  1. Pat Howell says:

    I live in Middle Tennessee, our male hummingbirds left a few days ago and females usually stay until mid-October. Goldfinches have been here for 2 weeks and I have noticed a sudden influx of chickadees, Carolina wrens, titmice and blue jays; the season is turning.
    Excellent article, thank you.

  2. Diane S. says:

    We miss the Tree Swallows that had their young in our gourd,the whole process was two months,can’t wait till next spring.These birds are special.

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