"learn their calls" to me as an advice is similar to when people tell obese ones "just don't eat so much" or depressed ones "try to be more positive". It's perfectly logical with the slight handicap of being largely impossible. To me it all sounds the same in separation. I can identify a longer melody, I can also tell if you play me two sounds in a succession which was higher. But a single short piip doesn't hold any practical information for me, no matter how much I try. I should probably just start recording everything, but to be honest, it's a bit of a chore for a hobby. So even knowing the drawbacks, I still prefer to take pictures and try to learn how to ID from them, it just sometimes runs into problems such as difficult light and not very clearly marked individuals.