Hearing tests for bird survey workers?

Fifty years ago, in the spring of 1962, I took part in the first year of the BTO Common Birds Census, my patch being in the grounds and adjacent woodland of the National College of Food Technology at Weybridge, in Surrey. One of the commonest birds was the Willow Warbler Phylloscopus trochilus and most were located by ear. If I repeated the census now, and assuming there had been no population...