Mission Improbable, or, some half-baked tips on practical bird identification
A few weeks ago I received a message from a family friend with a photo of a bird of prey, taken in Scotland, accompanied with the question “could this be a Rough-legged Buzzard?” It was not a great photo and it was impossible to identify the subject bird with absolute certainty. My carefully worded response was that the literal answer to the question was that, well, yes, it *could* be a Rough-legged Buzzard, but only insofar as I couldn’t see the bird well enough to definitively rule that species out. And yet — as I tried to carefully explain — the chances of any given Buzzard sp in Scotland, this one included, being a Rough-legged rather than Common Buzzard are vanishingly small. Now, do I know with total certainty that the bird was *not* a Rough-legged Buzzard? No. But do I think we can say with reasonable certainty it was a Common rather than Rough-legged Buzzard? In my view, yes: it was in an area where Common Buzzards are common, and where Rough-legged Buzzard has never been recorded, to my knowledge at least.
This little episode got me thinking about the role of probability in bird identification. Contrary to what we might assume, bird identification is often an exercise in judging probability, rather than an exact science. Let me give an example. Imagine one sees a silent Treecreeper in a local wood. The chances are you will simply assume it’s a Eurasian Treecreeper. I doubt anyone routinely scrutinises Treecreepers on the off-chance they might find a Short-toed. And yet, surely at some point a genuine Short-toed Treecreeper in the UK has been passed off as a Eurasian. As such, a large percentage of Eurasian Treecreeper records are not, strictly speaking, 100% certain (unless you’ve heard it call).
That’s an extreme example, but the principle applies more widely. If I get a bad view of an unstreaked acrocephalus warbler on my local patch in the West Midlands, I will put it down as a Reed Warbler: I might not be able to definitively rule out Marsh or Blyth’s Reed, but the chances of it being either of these species are so low that it’s still a pretty safe ID. Likewise, can I say for certain that every female Kestrel I’ve seen was a Common rather than a Lesser? No. Can I say for sure that every silent Chiffchaff I’ve seen in the UK was a Common rather than an Iberian? Also no. The female Sparrow I can see outside my window right now could, for all I know, be a Spanish Sparrow! But no-one would seriously quibble a House Sparrow record because Spanish (or even Italian!) Sparrow hadn’t been ruled out. Why? Because defaulting to commoner species, even if we can’t rule out rarer species with total certainty, is standard birding practice. Sometimes, inevitably, this results in errors being made (the obvious case in point would be the Yorkshire Amur Falcon), but it’s something we habitually do all the time.
I suspect I’m not the only one to feel that it was only rather late in my birding career that I came to realise the probabilistic rather than absolute nature of much bird identification. And I think this might be because the birding community feels uncomfortable admitting that bird identification is not always as precise as we might wish. But I fear we run the risk of giving new or less experienced birders a misleading impression of the practicalities of bird identification. For to be a good birder is not only, or even primarily, about having the raw knowledge necessary to identify a bird. It is also about the capacity to accurately judge the relative probability of certain species in particular contexts.
Take Goshawk for example. In recent weeks, I’ve seen several pics online captioned “is this a Sparrowhawk or a Goshawk?” Even before seeing the pic, we know that the odds are massively skewed towards Sparrowhawk. There are about 70 times as many breeding Sparrowhawks as Goshawks in the UK. Factor in differences in behaviour and habitat preference, and the chances of any given UK accipiter being a Goshawk must be less than 1% (and I’ve certainly seen more than 100 times as many Sparrowhawks as Goshawks). As such, any attempt at tackling accipiter identification must take account of this disparity. Put more simply: assume Sparrowhawk unless there’s a very clear reason not to. A similar example occurred recently where someone posted a comment to the effect of “I have a woodpecker in my garden, is it Great or Lesser?” Suffice to say, UK woodpeckers are easy to identify, but even absent any knowledge of what the bird in question looks like, we know for sure that the chances of it being a LSW rather than GSW are miniscule. I see GSW a minimum of twice a week, which works out at around 1000 GSW encounters in the past ten years. In the same period, I’ve seen precisely 2 Lesser Spotted Woodpeckers in the UK. I suspect my experience is not atypical, so extrapolating from this would mean that the chances of any given “pied” woodpecker being a Lesser would be about 0.2%.
As such, awareness of likelihood and probability are essential to the bird identification process. Indeed, misidentification often arises not just from misperceiving a bird’s features, but from confusion created by over-estimating the likelihood of rarer species (such as Goshawk or Rough-legged Buzzard). It may sound obvious, but learning that rare species are precisely that — rare! — and incorporating that knowledge into one’s birding, is a harder skill to learn than many realise, but is absolutely essential if one aspires to become a better birder.
