Skull and bones are different. Appearance is rather different. Size is different. Yet they are the same birds?
That's bird taxonomy for you. On the other hand you have Long-billed Pipit and Kimberley Pipit that are virtually indistinguishable yet they are not the same birds!
There are different criteria that taxonomists use to determine species. The most obvious and oldest criteria where of course morphology. If it looks like a duck and it quacks like a duck, it's a duck. Conversely, if it doesn't it's not.
As better observations where made and better tools became available, the criteria also shifted a little. Taxonomists started looking at the birds' behaviour and specifically their breeding behaviour. So at this point, two birds might look very different and would, according to the older criteria have been considered separate species (think of the different colour morphs of eagles or the two forms of Brown-throated Martin or Olive Bush-Shrike), but now it transpires that they actually interbreed, leading taxonomists to the conclusion that they are indeed different looking members of one species.
But then the tools got better still and birds where observed on molecular level. It was discovered that some birds that, according to the above criteria are seperate species (ie. they look different, they act different, they don't interbreed etc.) have near identical DNA. What now? Obviously they must be one species then
But back to the gulls, and if you want to know about a real nightmare for taxonomists, consider the Lesser Black-backed Gull complex. We are remotely affected by this as the Lesser Black-backed Gull and Heuglin's Gull do show up in SA from time to time (and the Herring Gull even less often) but DuQues, you should be more familiar with these birds as they are fairly common in your neck of the woods (at least some of them).
So, there is the Herring Gull (Larus argentatus
) in Europe and the American Herring Gull (Larus smithsonianus
). They look the same (but not quite)
, they don't occur in the same regions (but sometimes they do)
, they can interbreed (but they usually don't)
and genetically they are very similar (but not exactly)
. You get the idea, for the most part they appear to be one species.
But now, let's forget the Herring Gull from Europe for the moment and consider the Vega Gull (Larus vegae
), occurring just across the Bering strait from the American Herring Gull's turf. It looks almost like the American Herring Gull and they can interbreed, but it doesn't resemble the Herring Gull all that much anymore and they don't occur anywhere close to each other to even try to interbreed.
Moving further west along the arctic circle, we now encounter the Birula's Gull (Larus birulai
) which again, looks a lot like the Vega Gull and they can interbreed but it is starting to look conspicuously different from American Herring Gull and even more so from Herring Gull.
But the trend continues because further along the arctic circle we get the Heuglin's Gull (Larus heuglini
) and yes, you guessed it, it looks like the East Siberian Gull and they can interbreed but it doesn't quite look like the Vega Gull, less like the American Herring Gull and not anything like the Herring Gull.
In the western parts of Siberia we get the Siberian Lesser Black-backed Gull (I don't even know the scientific name
) which continues the trend and finally, back in north-western Europe, we get the Lesser Black-backed Gull, similar in many respects to the Siberian Lesser Black-backed Gull but nothing remotely similar to the Herring Gull with which it co-exists as this picture taken from Wikipedia illustrates.
So we have a ring of birds, circling the arctic. At any one point along the ring you can't quite tell the difference between the birds (and not even can they themselves as they do interbreed) but where the extremes meet there can be no doubt that the birds are of seperate species. So the million dollar question is, where should taxonommists draw the line? How many species are there along this circle? Looking at the extreme ends of the ring there certainly are at least two species but how many are there in between.
Speaking of taxonomic issues that will probably never be resolved, here is a prime candidate.
And this is all put quite simply, the matter is much worse than just this because on some points along this ring it touches with other rings that branch off in other directions (the Vega Gull for instance doesn't only have unclear relationships to the Lesser Black-backed Gull complex but also
to the Caspian Gull / Mongolian Gull complex. Many of the birds mentioned above have different subspecies (like Heuglin's Gull having at least three subspecies) and some of them are still not considered species in their own right (like Siberian Lesser Black-backed Gull or Birula's Gull). It's really one big mess.
And I don't think birders and taxonomists always have the same agenda