For the majority of common birds, seen or heard records from confident recorders are of equal value. This has to apply for much census and survey work, particularly when this is undertaken in woodland or thick scrub. However, as Graham states, much more extra care needs to be applied for scarcer and less familiar species, and regrettably some of those brief flyover records without any additional supporting details have to be disregarded.
Turning to Stuart's comments about species that have become inaudible to him, I am afraid that I possess limited understanding of the auditory qualities of pitch levels and cadence. I can appreciate the fact that high-pitched and relatively soft calls become difficult to hear properly, but I am surprised by the number of people who cannot hear the song of the Chiffchaff which, for the time being, seems loud enough and of a reasonably standard pitch to my own ears. I, too, have noticed over the past few years how the reeling of the Grasshopper Warbler is becoming more difficult to detect over longer distances and while I can still hear Goldcrests reasonably well, perhaps I may be hearing fewer distant birds. Unfortunately, years of sonic assault from chain-saws, hedge-trimmers and the like are now taking their toll. When I was younger, I was blessed with the hearing of a bat, but as with eyesight, field sharpness and stamina, suddenly all start to go into decline.
Until recently, I was not aware that I was beginning to miss the fight-calls of some species. Whilst in the company of young and competent observers, I was dismayed when they called out species such as Brambling and Lapland Bunting passing at moderate range and I could detect no sound whatsoever. It makes one wonder if some of the obvious declines in certain species appear even more pronounced because one cannot hear them.
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and others said photographic evidence was needed to prove what I saw. At the time I didn't have a camera or anything capable of taking videos.