The Reverend Thomas Bayes on birding

The very Reverend Thomas Bayes, the son of London Presbyterian minister was thought to have been born in Hertfordshire in about 1702. He studied in my own home town of Edinburgh and amongst other things, attempted to prove that the principal purpose of God was to make us despicable human souls happy. What has the good Reverend got to do with birding I hear you ask? Well, allow me to explain in a rather roundabout manner:

Today I was seawatching from Bass Point. Shortly after securing cracking views of my first ever Great Shearwater from the Lizard, I observed a petrel for about 2 mins as it flew past relatively close.  Detailed description aside, to all intent and purposes, it looked like a Wilson’s Petrel. In fact, based on its appearance, I was about 99% confident it was one. The trouble is, being 99% confident of ID, doesn't mean there’s a 99% probability it actually was one, simply because the obvious confusion species, European Storm Petrel, heavily outnumbers Wilson’s. I don’t know the true figure, but let's say, for the sake of argument, by about 100 to 1. Well, it so happens that the good old Reverend Thomas Bayes came up with a formula for working out this problem, although I don’t think he was thinking about birds when he did. For the mathematically minded, the formula is as follows:

Probability of rare bird = probability of ID x probability of it being one based on numbers / (probability of incorrect ID) x probability of it not being one based on numbers  + probability of correct ID x probability of it being one on numbers).

That entire complicated math leads me to the rather unfortunate conclusion that it there was a 49.7% chance it was one.  Less than 50%! Indeed if I factor in a bit of false expectation due to the fact I’d just seen a Great Shear and the conditions were spot on (say 95% confidence with ID) and we allow European’s to outnumber Wilson’s by 200 to 1, then the figure drops to a woeful 8.7%.

Of course, all of this doesn’t matter one bit if one is certain of identification, but it does get you thinking. I wonder how often the rarities committees take such probabilities into consideration? I suppose they do qualitatively, as mega rare birds increasingly require mega good evidence, but it’s rather worrying that even if one is 99.9% confident with identification of a bird that is outnumbered 1000 to 1 by its potential confusion species, the chances of it actually being one are only 50:50.

SeaWatch SSW

I’ve often wondered whether the Lizard competes with Porthgwarra on a seawatch. I've tried to test the theory a few times, albeit never for much longer than a couple of hours. The few times I’ve tried it, I’ve generally found that, although counts are similar, Porthgwarra fairs slightly better. I think observer coverage may play a part though. After trial and error, consensus seems to be that the best place to seawatch from is sitting just below the coastwatch station at Bass Point. Most seabirds seem take a flight path across the bay from Black Head, passing really close-in at Bass Point, but then continuing in the same direction and thus passing further out when flying past the most southerly point. Oddly, the best viewing is thus looking north across the bay and on the whole, I think birds pass closer than they do at Porthgwarra. This poses a bit of a problem if you’re on your own. You can look out with a scope to catch the more distant stuff, but as the viewpoint is quite high up, a fair amount flies under your field of view.  However, in really poor visibility this can be an advantage as you can just use bins and still see most stuff.

I wonder whether this is why I faired rather better than Porthgwarra this afternoon?  Visibility was dire, at least for the first couple of hours. Their haul for the day (0600 to 1200 and 1400 to 1930) was 22 Balearics, 3 Sooty Shearwaters and 4 Bonxies.  My score in roughly half that time (13:00-19:00) was 16 Balearics, 7 Sooties, 2 Bonxies, 2 Puffins, 5 Stormies and a Cory’s. OK the Cory’s might well have past them while they were on lunch break and the stormies were feeding offshore around a pod of Dolphins and I may have double counted, but on balance, it suggest that the Lizard can outscore West Penwith in the right conditions. Maybe the fact that stuff passes closer is advantageous? Certainly, the guys down there are a lot sharper than me, so if observer skill has anything to do with it, they win hands down.

Anyway ,the real purpose of this post is really just to convince myself that the mythical beast, the Fea’s Petrel is possible off the Lizard and I should put in more effort rather than sacking it off after a couple of hours.