Measurement artefacts lead to false positives in the study of birdsong in noise

1: Numerous studies over the past decade have reported correlations between elevated levels of anthropogenic noise and a rise in the minimum frequency of acoustic signals of animals living in noisy habitats. This pattern appears to be occurring globally, and higher pitched signals have been hypothesized to be adaptive changes that reduce masking by low-frequency traffic noise. However, the sound analysis methods most often used in these studies are prone to measurement errors that can result in false positives. In addition, the commonly used method of measuring frequencies visually from spectrograms might also lead to observer-expectancy biases that could exacerbate measurement errors. 2: We conducted an experiment to (a) quantify the size and type of errors that result from “eye-balling” frequency measurements with cursors placed manually on spectrograms of signals recorded in noise and no-noise conditions, and (b) to test whether observer expectations lead to significant errors in fr...