Friday, 26 October 2012

Redwing Migration

I have been following the growing interest in monitoring birds migrating at night by making long running sound recordings and analysing the resulting sound files to see what species called while flying overhead. "Vis-mig" is fascinating and this has been a great week for it with ideal weather setting up perfect "fall" conditions on the east coast.  Huge numbers of thrushes and finches have been on the move with over 21,100 Redwing, 9345 Fieldfare and 2675 Brambling recorded at Spurn on Monday 22nd alone!  I had heard some Redwings flying over the house earlier in the week and thought it would be a good time to experiment to see what I could record.

I left a portable recorder with a built in microphone and a 4gb memory card outside in the garden for the evening.  The batteries went flat after about 4 hours but not before a 2gb sound file had been recorded. The file covered the period between 17:20 - 21:00 and using a freely available software program called Audacity I reviewed the file.  Audacity can display the spectrogram of the file which shows a picture of the sound by graphing sound frequency against time.

The picture below shows the call of a migrating Redwing flying over the house.  The red pattern at the bottom is low frequency background noise.  The Redwing call is the down-sloping red line between 8 and 6 khz. (I used another freely available program called Raven Lite available from Cornell Labs to print the spectorgram)
Redwing spectrogram recorded 25th October, Mellor, Lancashire
There were 25 Redwing calls recorded in the file over a three hour period as well as a Tawny Owl and some as yet unidentified sounds, some of which I think are high-pitched mammal sounds - such as mice and shrews - but some are other birds.

This is my first ever attempt at recording night migrating birds and my equipment and techniques will need some tweaking to improve the quality of the recordings, but i am more than happy with the first night's results.  Ultimately I'd like some way of automatically processing the large files to pinpoint the high-frequency sounds for further investigation.  Some programs have been developed to automatically identify certain American birds, so it would be interesting to see what's available for European migrants.

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