Ovenbird by davidcreebirder

Using recorded birdsongs, scientists have developed a new method of accurately measuring bird populations.

Existing techniques usually meant that an observer had to actually see the bird. This is time consuming and can be very difficult. Even experienced birdwatchers can have a tough time finding a bird in the wild. Other methods required the birds to actually be captured in nets, which can be very stressful for them.

This new technique doesn’t require the bird to actually be seen, only heard. It was the result of research by Deanna Dawson of the US Geological Survey and Murray Efford of the University of Otago, New Zealand. They developed it by recording a North American warbler called the Ovenbird in a forest at the Patuxent Research Refuge near Laurel, Maryland. The population of this particular bird is known in that area because it had been the focus of various studies for the past five years. This allowed the research team to check their data. Also, Ovenbird males sing in the lower levels of the forest and have a distinctive song which makes them easier to record.

The research team placed 4 omnidirectional microphones in a square with 21 meter long sides. Over 5 days they moved the microphones to 75 different locations. At each location they made a 5 minute recording.

The recordings from each microphone for the particular area measured were combined and fed into a computer program. Since each bird has its own unique voice, the number of birds in that area can be counted. Knowing how many birds are in a given area yields the bird density for that area.

To improve density estimates in the area measured, researchers can also look at the intensities of the sounds recorded. If a bird is closer to a microphone, its song will be louder or more intense. Researchers can see this on the spectrogram generated by the computer. This can help them determine approximately where in the given area the singing bird is.

The basic principles of this technique can be used to measure the populations of other animals that are often heard but not seen such as whales and dolphins.