You never know when this will come in handy. The other day a local birder (hardcore patcher!) got dreadful views of a flighty pipit. Luckily it called a few times. https://twitter.com/sconebirding/status/1313373997216796673">https://twitter.com/sconebird...
Also luckily, the observer had a recorder handy, and even more luckily, some spare batteries. Even with just a recorder (i.e. without a directional mic) the recording was good enough for a conclusive ID.
It was suspected to be an Olive-backed pipit in the field and the recording confirms this. There is some maths you can do in these things to be super sure https://www.google.co.uk/amp/s/avesrares.wordpress.com/2013/09/27/identification-of-olive-backed-and-tree-pipit-by-call/amp/">https://www.google.co.uk/amp/s/ave... but even eyeballing the sonogram allows you to make a good case.
The peak frequencies consistently being around 8.5 khz also supports OBP. Tree can call over 8 khz but it is not all that frequent.
So - a pockets worth of kit has saved this from being one that got away. It’s now possible to make a very strong case for this bird to the relevant committee.