estimating chaffinch numbers

I thought I might need to explain how I came up with the outlandish figure of 10000 migrating through Llanon the other day when I was with Arfon. As we walked across the fields we would stop regularly. The birds were coming on a front that stretched from the beach to the low ridge next to the coast road. Scanning through 180 degrees from left to right we could get a sense of how many were passing. Sometimes we could see about a hundred at a given moment. Say, 100/minute. At other times there were brief lulls and pauses. Say, 100/5 minutes. Assuming migration occurred between 7am-10am then the numbers passing would lay between 3600-18000. 10000 is just a nice round number half way between the two extremes. It was as rough and ready as that really. However, if you watched Autumnwatch last night you might have heard them mention that 9000 were over Bardsey that day. In fact, Liz gave me a link to the observatory earlier in the week.Thanks Liz. Apparently, 9349 migrated overhead. An astonishingly precise figure. They obviously weren't chatting like we were! So maybe 10000 is closer than I would have dared imagine. I understand they also named every single Chaffinch as it passed overhead, a bit like they do with hurricanes and typhoons.