I was birding last week and thinking about the 'Birds in Scandinavia' thread , when it dawned on me that my spark bird was a drake Mallard! It was about the 23rd October 1973 and I was 8; one moment I was whacking a reed head against the ground at Mai Po and the next moment 10 Mallards went up - 4 drakes and 6 female - heads shining brilliantly in the sunlight. This was my moment of conversion.
I have always been interested in spark birds, since I came across the concept. For many there is no precise moment but I guess around 50% of birders have 'a moment'.