Tag Archives: bird

Bird recording in the field with a GPS and audio recorder

After a long break, I looked at getting back to using Birdtrack. When I used it before, I used an audio recorder and a GPS, taking a waypoint with the GPS and identifying the bird and waypoint in the audio note. The advantage of this is I could automate taking in the position data, and just transcribe the bird and count.The obvious way to do that now is to use something like the Birdtrack app. Trouble is this wouldn’t run on my device. That wasn’t the main problem, to be honest, it’s easy enough to solve by chasing the upgrade cycle. There’s a deeper problem – it’s not what I want to get out into the field to do, to spend more time peering at an illuminated screen. Not only would Eckhart Tolle disapprove, but you also have to take your eye off the birds. Don’t get me wrong, the BTO have probably done a great job with the app, and if that’s how you want to list birds that’s great, each to their own. It’s just doesn’t do that much for me. But it’s a continuum – after all, what is the real point of recording what you see anyway, particularly as I’m not particularly good with ID so have to pass on a lot?easygpsSurveying an area repeatedly does mean I get to build up a spatial map of territories and get to know some of the birds by the foibles of their song. Now this is made easier I will see if a picture of territories does build up over time in the breeding season. GPS audio mapping isn’t a new idea, I came across it from the OpenMap community, who had largely solved this problem with audio mapping. They simply leave the recorder on capturing a stream of consciousness, because they want to capture data about the whole environment, that’s what a map is. It makes the synchornisation problem easier, as they set a datum at the start of the run, doing the same job as the clapperboard in shooting a movie. The GPS track is a log of where you go, with the sections individually timestamped, which I hadn’t realised. So start the recording at a datum point, and you know that the audio runs in sychronism with the GPS. Modern digital field recorders are crystal controlled to about 100ppm – about 4 seconds off in 10 hours.Now I don’t really want to spend as much time writing up a trip as doing it, so I took a leaf from their book and used a Perl script to parse the GPX data tracklog, find the nearest section to the audio clip timestamp and create a waypoint there, with a link to the audio file in the same directory. Rather than making one massive recording, I can simply make short audio notes when I see the bird, and the Perl script will tally the record time and sort out where I am. It’s easier to find a record and there is much less audio data to process.

Due to the vagaries of real-life devices you have to be able to put in an offset to take out summertime and any slight differences, but an error of 1 minute at walking speed isn’t so bad for bird logging. At a steady 3mph it’s about 80m uncertainty, but in practice you tend to slow down when observing birds, improving the accuracy. File timestamps do go down to the second, but keeping the clocks within a minute is a big ask. The GPS timestamp is always very accurate because it is set to an atomic clock, so it’s down to setting the recorder properly. Or identifying the skew properly at the combining stage.

As an added bonus, this can work on my field recordings too, I can just save the GPX file and I know where they were made too. Using something like GPS Visualiser I can see where everything is on a Google map.

For bird logs, I need to open the audio and edit the waypoint names to the BTO two letter short codes of the sightings, comma separated if there are several at one point. A second run through another script expands these into the long names and creates an Excel file suitable for uploading to Birdtrack.