Tag Archives: birding by radar

Throwback Tuesday: Old And New Perspectives On Migration

Spring Migration: The Early Birders’ View

William Brewster, the famous 19th-century ornithologist and Cambridge resident, imagined that spring migrants preferred the rural countryside west of Boston to the woodlands near the city. But when he moved to Concord in 1892, he was surprised to find fewer migrants than he had become used to seeing in Cambridge.

Brewster’s student, Ludlow Griscom, hypothesized that this was the result of birds’ migratory routes. Brewster’s data, collected over decades, seemed to show that migrants did not move evenly across the state, but rather took routes based on the shape of the landscape.

Paraphrased, Griscom’s theory went like this: a big stream of birds passes up the mid-Atlantic coast, and two major contingents form in New York. One, with many inland migrants, would hit the Hudson river valley and follow it north, and the other would travel along the Connecticut coast. A small contingent of birds would then split off and follow the Housatonic River, and a major one would follow the Connecticut River Valley. The rest turn northeast just ahead of Narraganset Bay to avoid the pine barrens of southeastern Massachusetts and Cape Cod. These birds turn north near Boston Harbor, passing through Canton, Milton, Brookline and Cambridge, before continuing north into Essex County and along the New Hampshire coast.

Griscom and Brewster’s theory of migratory routes is roughly illustrated in this map:

Almost, But Not Quite

This particular set of routes has not been borne out by modern data gleaned from the radar. While some studies show that certain areas are regularly “birdier” than others during migration, (including sites along major river valleys), wind and weather patterns ultimately have more sway over bids’ trajectories than the topography of the landscape. Some expert birders still swear that migrating birds following “sight lines” or topographic features, but these observations remains anecdotal.

Even if migratory routes are not as fixed or as specific as Griscom imagined, radar does often show higher concentrations migrants in some areas than others. For example, in early May 2018, birds seemed to avoid southeastern Massachusetts and Boston, staying northwest of I-495. This is demonstrated on the radar maps below (note that the radar station is the white cross in the center of the circle, and that radar can detect birds equally in all directions- check out our blog series on reading radar images if you haven’t yet!)  Bear in mind that these are by no means typical nights—birds take very different migratory paths through Massachusetts every night, mostly depending on wind direction and time of year.

While the radar doesn’t show nocturnal migrants grouping together in narrow ribbons in the air, compiling images like these helps scientists observe patterns in bird migration. For example, this map by Kyle Horton of the Cornell Lab of Ornithology shows the direction in which birds are heading. The length of each bar shows how many birds are flying in that direction over the course of a season. The key takeaway: many birds fly over the ocean from Massachusetts, preferring to take the direct route over the Gulf of Maine rather than follow the coastline.


This map was recently featured in an awesome video by Jackson Childs, a local birder and friend of Mass Audubon. Check out Jackson’s video for more cool information about bird migration, including dawn flight, and some close-up footage of colorful warblers.

If you’ve found surprising patterns in spring migration, let us know in the comments!












Predicting Spring Migration: Part 3

(This is the final installment in a series on birding by radar. Read the first and second post first so this one makes sense!)

On May 20, 2017, Bay-breasted Warblers seemed to drip from every tree at Mass Audubon’s Marblehead Neck Wildlife Sanctuary. Birders tallied dozens of this normally scarce migrant practically on arrival, alongside equally impressive numbers of Canada Warblers, Blackburnian Warblers, and other migrants. The air filled with high-pitched warbler songs so much that it was difficult to distinguish one from the next. Plum Island was equally loaded, with some observers tallying 123 species for the day. Was this a fallout, or just an excellent day for migration?

Fallout is one of the most exciting spectacles a birder can hope to experience in migration. Serious birders mistakenly use this term all the time to mean “a lot of migrants in one area,” but fallout refers to a very specific phenomenon: birds that cut short their migratory journey due to severe weather or exhaustion.

Birds will fall out along the coast if they are blown far off course over the ocean; they return to land hungry and tired, and large numbers feed at ground-level in coastal vegetation.  Fast-moving fronts of severe weather can also cause fallouts when they interrupt bands of migrating birds, and stationary fronts can stall migrants that land when they encounter it and build up along its edge.

On May 20th, 2017, birders who read the radar saw that northeastern Massachusetts experienced a borderline fallout; a storm had blown birds against the coast and over the ocean, but the weather cleared early enough that many grounded birds continued migrating afterwards. Regardless, the superb birding that day was undeniably predictable.

Reading the Radar on May 20, 2017

The radar for this night showed moderate migration, with a front of severe weather pushing birds south and east. The dense (green and red) precipitation is pictured up against a group of birds, represented by the blue line between the edge of the storm and the mass of birds in the center of the frame.

As the front moved east (see below), the density of migrants increased just to its south. The birds at the edge of the storm, pictured in blue above, appear to have been pushed into the main mass of birds, where they show up as a streak of green (higher-density) in the image below.

The velocity map below paints a slightly different picture. The black areas between the storm and the birds show that the storm is grounding birds. But the birds just away from the edge—that red spur in New Hampshire, for example—are not getting pushed south by it.

The red color (that is, increased relative velocity reading) of that patch of birds shows that they are either 1) continuing to fly east but increasing their speed or 2) flying north instead of east, as if to go around the storm, and maintaining their speed.  In either case, the fact that these birds are being detected further away from the station than the rest of the cluster indicates that they increased their flying altitude (recall that the further away birds are from the station, the higher they need to be to show up on the radar). It’s anybody’s guess why they would be doing this; the storm exists at a higher altitude than the birds, so flying up into it seems counterintuitive.

What Was Missing

Since the front passed fairly early in the evening, many migrants had a chance to pick themselves up and move along after the storm passed. It is not a reach to imagine that the birds that built up along the edge of the storm took off again after the storm passed, and moved northeast again, landing in similar areas along the Maine coast.

What Looked Promising

Storm or no storm, a forecast of west winds turning northwest at dawn is always a good sign for coastal sites. West winds blow inland migrants against the coast, where many prefer to land instead of flying over the water. Other birds overshoot the coast in strong winds, and when winds turn northwest at dawn, these ambitious flyers drop back in at coastal sites like Plum Island and Marblehead Neck.

The Results

A small but significant stream of birds poured off the ocean and onto the coast in the morning. Some experts say that this was strictly because they were pushed east by the storm, but some hold that these birds would have overshot the coast with the west wind anyway.  In either case, velocity readings from early (4:30-5:30) the next morning show many birds over the ocean colored in yellows, olives, and some blue: birds that are not moving directly away from or directly towards the radar station. In some areas, this means they were moving towards the coast.

Arrows on this map indicating bird direction were determined by drawing a line from the radar station (circled) out to a point with birds, and then drawing an arrow slightly over 90 degrees to this line for birds moving slightly away from the station (yellow).

Likewise, the arrow would be at exactly 90 degrees to the line for birds moving neither towards nor away from the station, slightly under 90 degrees to the line for birds moving slightly towards the station (light blues and greys) and in the direction (or close to it) of the line for birds moving strongly towards or away from the station (colored red or deep blue). If you didn’t follow this, don’t worry: the key is that birds over the water at dawn often means coastal fallout.

To sum it up, there were three elements of that evening’s radar that practically screamed “Go birding on the coast tomorrow”:

  1. Radar showing many birds moving more east than north, and some shooting over the coast at high speed
  2. A strong storm that could force migrants against the coast even more vigorously than the winds could, and might even ground many of them.
  3. Most importantly, birds coming in off the ocean early in the morning (4:30-5:30).

Lo and behold, it was an incredible day on the coast the following morning, even though arguments over how much the early-evening storm had to do with it remain unresolved.

This is just one example of how reading the radar can lead to better birding.  Try it for yourself this spring and see if you strike spring migrant gold!

Predicting Spring Migration: Part 2

Last week, we posted an article on predicting bird movements with radar.  Here’s what we went over:

—How birds show up on Doppler radar as solid, expanding circles of radar interference around radar stations, and why this happens

—How to tell these signals apart from precipitation or normal weather patterns

—How larger circles don’t necessarily mean more birds

This week, winds over Massachusetts are shifting. Steady southwest winds may bring a major influx of migrants as early as Tuesday night. So, here’s the rest of what you’ll need to know about watching birds on Doppler radar!

A Need For (Wind) Speed

it’s possible to see airborne objects’ speed relative to the ground using Doppler radar. Birds fly at about 10-15 knots, and know where they want to go. So, they’ll either be moving 10-15 knots faster than the wind if they’re flying in the same direction as the wind, or they’ll be moving in a different direction entirely. Other airborne objects, like insects or dust particles, will always move with the wind.

Here’s how to see the velocity for radar-detected objects online:

Go to the national website for radar data.

  1. In the top left, click on “0.5° Velocity”. (Selecting “reflectivity” will show you the density of the signal, but not the speed.  Velocity, on the other hand, won’t show you how thick the air is with birds—it will only show their speed).
  2. In the top right, go to the drop-down menu for “end time” to select the end time for the series of radar images you want to view. It’s in Universal Time, which is 4-5 hours ahead of Eastern Standard Time. Selecting “0500” is best if you want to look at last night’s migration, and selecting “most recent” is best if it’s the early nighttime and you want to see what birds are currently passing overhead.
  3. Go to the drop-down menu for “loop duration” and select 5 hours (or however long you want the series of radar images to be).
  4. Click on the letters “BOS” over Eastern Massachusetts to view radar images!

Image modified from the National Center for Atmospheric Research

In radar images, velocity is always measured relative to the radar station.  So, parts of a cluster of birds with “negative” velocity are moving towards the radar station, and parts of a cluster with positive velocity are moving away from it. The red areas in the image show movement away from the radar station (the dark dot drawn in the center of the image), from southwest to northeast. Blue areas show movement towards the radar station—also from southwest to northeast. Birds in the yellow, gray, and green areas are slower-moving relative to the station: as they pass by it, they are neither moving towards it nor away from it.

The wind was about 20kts (knots) from the southwest when this image was taken.  Since this radar signal shows objects moving NE around 35kts, it’s clear they aren’t just drifting with the wind.  These are bona fide birds!

To check wind speed and direction in your area (to compare to clusters of airborne objects on radar maps), try using Wind Map. Just click to zoom, and hover your cursor over Massachusetts to see the wind speed.

Where Will Airborne Migrants Land?

Velocity data can also tell us where birds are going to end up.  Birds usually migrate for between 5-7.5 hours a night, so multiplying their airspeed by around 6 gives us a very rough approximation of how far they’ll travel in one night. On most nights, this works out to be around 150-200 miles. This means that you can often get a general sense of how good the birding will be in Eastern MA based on early-evening images from Southern CT and New York.

For example, when there’s a big early-evening movement of birds over OKX (the radar station for Southern Connecticut and Long Island) and the radar velocity data show birds moving to the east or northeast, chances are that birding will be good in Eastern MA the following morning. If migrants are moving steadily due north, however, that can be a good sign to head further inland the next day.  Of course, it’s always best to check early-morning radar images as well—on some nights with north winds, migrants will pour in from the Atlantic Ocean right before dawn, making for great coastal birding.

In our next post, we’ll discuss radar images from the night before the best day of spring migration in 2017. It should serve as a case study in how watching the weather and radar can lead to encounters with incredible concentrations of migratory songbirds. Stay tuned!

Predicting Spring Migration: Part 1

If you ask birders what their favorite holiday is, a few will always smile and reply, “spring migration.” Protracted over several weeks, spring migration can indeed feel like a holiday, or at least an annual ritual: time off from work (to go birding), reconnecting with community (other birders), and seasonal gifts that nature drops off in our yards (in the form of colorful warblers).

For beginning birders, mornings in spring can feel as unpredictable as waking up on Christmas as a child to see what Santa brought. Migratory birds appear (or don’t) as if by magic, and the species differ from day to day. There’s no telling what a morning in May can bring. But birds are creatures of habit; with a few tricks, it’s easy to take the guesswork out of birding during spring migration.

Warblers Follow The Wind

West and Southwest winds bring us the greatest numbers of migrants, which would ordinarily move straight North from their Mid-Atlantic stopovers. In Westerly winds, warblers sometimes build up against the coast as they try to avoid being blown out over the water. When the wind is from the South, the Berkshires and Pioneer Valley hold more migrants.

Read the Radar

Doppler radar, mostly used by forecasters use to track weather patterns, also readily picks up signals from migrating birds. Radar works by emitting radio waves that are reflected back to the transmitting antenna by any objects in the way. It’s sensitive enough to detect droplets of water in the air, so it was no shock when ornithologists in the 1950s realized radar could pick up birds as well. Now, advanced birders as well as scientists rely on radar to understand birds’ mass movements at night.

A Brief Radar Primer

There are many ways of accessing radar data online, but the most beginner-friendly is Paul Hurtado’s bird radar website.  Just click on the date you want to see radar maps for and you’re good to go!

The most important part of reading the radar is distinguishing birds from weather patterns.  Rain or hail shows up as denser interference (represented on this map by green and yellow colors) in irregular, ragged shapes. Birds, on the other hand, appear on the map as distinct circles, and show up as lower-density (pictured on this map as blue and light blue). Hard to believe as it may be, most of what the radar is picking up in the image below are birds.

Visualization by Paul Hurtado; modified from www.pauljhurtado.com.

Why do these birds show up as clusters or circles around radar stations? Radar antennae point their beams up at an angle, creating a funnel-shaped zone of detection. Groups of migrating birds fly relatively low compared to rainclouds and so only pass through a circle-shaped cross-section of radar beams. Think of a cone-shaped searchlight- the lower down in the beam you go, the smaller its area is.

So, larger circles don’t mean more birds—they just mean birds flying higher up, where the radar beam is broader. (You can see in the image below that some of the blue circles have started to overlap). But more birds filling the sky will mean that more of the radar beam is reflected, creating a higher-density signal. Dense areas of migrating birds are represented by lighter blues and greens (see Texas and Maine on this map).

Stay Tuned For More

This is just the tip of the iceberg! There’s much more to learn. In a few days, we’ll follow up with a post on how radar can show birds’ speed and direction, and where they land after a long night migrating.  Subscribe to our blog to get part 2 in your inbox!