BirdForum is the net's largest birding community dedicated to wild birds and birding, and is absolutely FREE!

Register for an account to take part in lively discussions in the forum, post your pictures in the gallery and more.

Reply to thread

Message: <blockquote data-quote="Bluetail" data-source="post: 166181" data-attributes="member: 4248">Apologies to refugees from uk.rec.birdwatching, who may have seen the following before since I posted it there about 18 months ago. In March 97 BB published an article by Pete Fraser called "How many rarities are we missing?". Fraser analysed the BBRC statistics for 1958-92 and found that, despite steadily increasing observer coverage over the years, there wasn't much variation in the percentage of rarities found at weekends (44% in 1958, reducing to 40% in 1992). Sites well-watched by weekend visitors from the big connurbations showed the most weekend bias; those covered more exclusively by local observers showed the least.He then devised a model that ran thus: Assuming that no site is covered with 100% daily efficiency, the rarities found on any given day will consist of (1) a percentage of that day's arrivals, (2) a percentage of the previous day's arrivals which were not discovered then and (3) a (small) number which arrived even earlier, also previously undetected.  At well-watched sites he reckoned that 90% of all rarities were discovered, of which 40% were recorded only on one day. At less well-watched sites 70% were recorded on one day only, probably because with less coverage it is easier to lose track of the birds and because fewer are found on the day they arrive. He ended up with an estimate that 60% of rarities remain beyond one day; of these 70% are still present the next day, 70% of those the day after and so on. Then it was a question of first finding the best fit between the model and the statistics. In the Isles of Scilly the reduction in numbers of birds found on Monday pointed to an efficiency of 92% on Sundays and less effort on Saturdays (76%) than on weekdays (78%) - which seems logical since Saturday is the big change-over day there.In 1990-94 the average number of rarities discovered each year was 755, while, at a conservative estimate about 800 were missed. The rate of discovery for passerines, near-passerines and waders was significantly less than for larger birds (unsurprisingly). For passerines, the percentage missed in different areas were:Fair Isle   11%Scilly/Cape Clear   11%Orkney/Shetland   45%South Coast   53%East Coast   57%Elsewhere   60%What Fraser didn't cover was whether there are any differences between the different seasons - a pity since, on average, autumn birds stay longer than spring ones and winter ones longer still and I would have thought this would be reflected in variations in the finding efficiencies.Anyway, it seems that we're finding less than 50% of the rarities. Get outthere, folks!</blockquote>

Verification