Tag Archives: spring migration

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!