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Monday, August 30, 2010

Listening for Flight Calls

During migration season, I often wake up early in the morning to listen for flight calls.  Flight calls are short vocalizations given by birds as they fly over duting their nocturnal migration.  These calls are often unique to each species (although some are very similar), so identification is often possible.

Flight calls are an under-appreciated part of birding.  First of all, few people are willing to spend several hours outside in the pitch dark hoping to hear a peep that only lasts 50 milliseconds.  Also, many flight calls can be difficult to learn.  Warblers in particular are known for their difficult flight calls; the calls are short, high-pitched, and are often very similar to other species.  However, the flight calls of thrushes, cuckoos, and other similar species are fairly easy to learn.  The calls of these larger birds are often longer, lower-pitched, and more distinctive.  Although it may turn many away, the challenge of flight calls is what lures birders who are dedicated and curious.  Here are some common questions I have been asked regarding flight calls:

What is a nocturnal flight call?

Basically, a nocturnal flight call is a short (often between 50-200 milliseconds) vocalization given by nocturnally migrating birds.  It is believed that these calls are used as contact calls to keep a flock of birds together.

When is the best time during the night to listen for nocturnal flight calls?

Migrating birds will call throughout the night, but the highest frequency of calling occurs in the hour before sunrise.  This may be because the birds are changing altitude and are making sure they stay together, or it may be that when the birds are higher up it is more difficult for humans to hear the calls.

What time of year is best for listening?

Since birds make these calls as they are migrating, the best time to listen is during the migration seasons.  For songbirds, the best times are April-May and late August-mid October.  Other birds, such as ducks and geese often make flight calls as well.  For these species March and November have been the most productive for me.

Are some days better than others?

Yes.  Bird migration is very dependent on weather patterns.  The best nights for listening to migrants is the night after a cold front has passed.  Sometimes huge numbers of birds migrate on nights like this, making listening very worthwhile.  Before heading out to listen, I often check the NEXRAD radar.  Migrating birds are visible on this weather radar, so I can determine whether there is a good flight or not.  A good site for checking this radar is the NCAR radar site.  For more information on using radar to view bird migration check out woodcreeper.com.  Finally, windy nights are not the best for listening.

Here is an example of a good migration night.  From rap.ucar.edu/weather/radar

This is what the radar might look like on a night with few birds in migration. From rap.ucar.edu/weather/radar

Where should I go to listen for flight calls?

Since nothing stops birds from spreading out once in the sky, so in theory, flight calls could be heard from anywhere on land.  However, certain factors make some locations better than others.  Human-created distractions are often the biggest problem.  Busy roads can be a pain while listening.  Cars passing by will limit the number of birds you hear.  It is best to pick a spot set back or away from any roads that have a lot of early morning or late night traffic.  It is also good to be set back from any buildings.  Walls can create annoying echos which can be confusing.

It is also good to stay away from forests.  As well as muffling the calls, trees can bring unwanted annoyances (tree crickets, tree frogs, katydids) which can make listening close to impossible at times.  A large field or other open area is great as it eliminates many of this unwanted noises.  Finally, positioning yourself on the top of a hill, ridge, or mountain can be very helpful.  The higher up you are, the closer you are to the calling birds.  Also, these high areas are often the first places birds land when coming down from the sky.  Once the sun comes up, ridgetops are sometimes hopping with migrants!

Here is my Big Sit spot.  The Big Sit is a birding competition where birders stay at one spot for a day and find as many birds as they can.  Nocturnal flight calls are an important part of the Big Sit.  As you can see, my spot is in an open field away from any roads and trees.  Although not visible from this photo, my spot was at the top of a ridge.  There is a larger mountain in the distance, but since there are no good clearings there, I decided to pick the next best ridge.  Photo ©David Husic

Ok, so I know where to go and when.  What do I do once I get to my listening spot?

Listen and enjoy.

I have no clue what I am hearing and I want to identify the calls.  What should I do?

The easiest way to help identify the calls is by recording them.  I record calls using a Zoom H4 recorder.  While out listening I record the calls in case I hear something unfamiliar.  I also record the calls for part of the Lehigh Gap Nature Center's Sound Field Guide.  Once the calls are recorded, I download them to my computer.  Using the program Audacity, I cut out calls and amplify them so that they are easier to hear.  Then I use Raven Lite to create and adjust the spectrogram of the call.  Both Audacity and Raven Lite are free software.  Then, to identify the calls, I use a CD from Oldbird.org.  This valuable resource has recordings and spectrograms of almost every bird in eastern north america that makes a nocturnal flight call.  It is available from the Oldbird website.

 Here is an example of a sonogram opened in Raven Lite.  The program allows you to adjust the image so that it is easier to see the call .  There are several calls in this image.

I would love to record flight calls, but do not want to get up every morning at 4:30am.  What should I do?

The best way to get around this problem is by installing the microphone unit as designed by the Oldbird team.  Detailed instructions on how to make a relatively inexpensive microphone can be found on the Oldbird website.

If you have any other questions, feel free to leave a comment.

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