Making it all add up

It is estimated there are around 10,000 species of birds in the world. Some nerdy people keep a list of how many species they have seen. My list contains 1,204 species, 12% of the total. But who decides on how many species there are, and who has the master list? I have chosen the International Ornithologists' Congress (IOC). It was adopted by the British Ornithologists' Union (BOU) as of January 1st 2018.

Looking at the list, a pattern emerges - groups of similar species are concentrated in different parts of the globe. In 1858 the British zoologist Philip Sclater presented to the Linnean Society of London his idea of six terrestrial 'biogeographic realms' (or ecozones) that indicated the general groupings of fauna. This was in the same year, and at the same institute, that Charles Darwin and Alfred Russel Wallace presented their 'theory of evolution by natural selection'. Wallace adopted Sclater's scheme for his book Geographical Distribution of Animals in 1876. Later the Oceania and Antarctic realms were added.

We now know that the realms have evolved as a result of their relative isolation over long periods of time - separated by oceans, mountains, and deserts - shaped by plate tectonics, which redistributed land masses over geological time (a theory not fully accepted until the 1960s!).

The Palearctic realm can be split into two for convenience, so that I find myself in the Western Palearctic - broadly defined by the natural barriers of the Ural mountains to the east (which also mark the boundary of Europe) and deserts to the south. This is the region used by the Collins Bird Guide (The Most Complete Guide to the Birds Of Britain and Europe).

It is the Western Palearctic that I call home - not just my biographical home, but also my cultural home as defined by its philosophies, religions, arts, music and food. I have estimated there are 611 species of birds commonly found in the Western Palearctic, of which I have seen 498 (around 80%), and an additional 1,194 sub-species, of which I have seen 863 (around 70%).

But keeping a list requires constant vigilance. The numbers of species and sub-species on the IOC list keeps changing - but far less from new sightings in the field than from experts 'splitting' and 'lumping' them in laboratory analyses. So, what do all these numbers mean? How do we know when a population is a species or sub-species? The list tells us that species in the Western Palearctic have on average one sub-species - but a lot have none, and the Common Jay has twenty-one (plus another thirteen in other parts of the world). Why? Think on!