Colin
Axeman (Retired)
I think it is OK to use a field guide in the field but as Ashley says, make notes first. While you root out your guide and start thumbing through it, the bird may well have flown and then questions like 'What colour were its legs?' start springing up and there is no answer cos you were looking through the pages for something like the bird you had only taken a cursory glance at (cursory, in as much as you saw enough to not be able to id it). So, the field guide has its place in the field but not as the first resort.
Colin
Colin