Although strictly this is not a sighting, it is sightings related. Simon Barnes' book is entertaining and informative. Here is a bit I like:
He talks about jizz recognition (this is something I can do with herons) - 'But looking and absorbing the shapes and sizes and movements of a bird is how you begin to get the hang of them. From the chaos of biodiversity comes a pattern and from the pattern comes the beginning of an understanding. You acquire the skill of jizz recognition simply by looking at birds you have already identified; because you see, identification is the beginning not the end of the process, and that is why birdwatching, good and bad, is the exact opposite of trainspotting'