I once saw a flock of small, streaky birds land in a birch tree outside my house, I was able to get very close so I could see that some of them had a smudge of red on their foreheads – giving them away as Redpolls, a tiny finch that loves to feed on birch and alder seed as these ones were. More recently I came across a sizable flock of Lesser Redpolls gorging on larch cones in a local forest, they repeatedly flew up into the air on mass and circled the clearing I was in, twittering in high squeaky notes before landing again in one of the tall pines.
Lesser Redpolls (Acanthis cabaret) are local breeders in the UK, preferring young woodland with lots of birch and conifers for food and nesting, they are secretive during the summer but are more widespread and sociable in the colder months. This is our smallest finch and can be distinguished from the similar-sized Siskin by its heavily streaked, mostly brown plumage and the red patch on the face which in males can extend down onto the breast. Populations are added to in winter by influxes from the continent and resident birds often move around the country, often southwards, to find food and avoid the worst of the weather – in bad years they may fly over to France or Germany.
Unfortunately the Lesser Redpoll has declined by around 80% since the 70’s and is now of high conservation concern. This species does have something of a boom-and-bust breeding cycle depending on seed availability but this long-term decline is possibly linked to a loss of suitable breeding habitat. Interestingly the Lesser Redpoll was only given full species status by the BOU relatively recently, before this century it was regarded as conspecific with the Common Redpoll Acanthis flammea which is a larger, paler bird found in Scandinavia.
Now here comes the tricky part; most authorities currently state that there are three Redpoll species – the lesser, the common and the arctic – however some believe there are actually five, by upgrading two of the subspecies to full species. Then there are also those who think that in actual fact there is only one Redpoll species and that the plumage variations we see are no more than variations.
This brings up the question of what actually defines a species? The thing is no-one can really agree on a set check list of what an organism must have to be a full, separate species in its own right. You would think that if an organism doesn’t hybridise with other, similar organisms then it is a species – but in nature many plant and animal species frequently interbreed willy-nilly, seeming to flout our attempts to categorise them. DNA should be a simple way of separating species but some very similar creatures can have wildly different DNA and very different looking ones can be genetically identical. Of course you also have to decide how much of a difference in DNA is enough to make something distinct – 5%? 10%? 25%?
Perhaps our attempts really are futile; nature after all is in constant flux as organisms evolve constantly to adapt to their changing environments and exploit new habitat niches. So does the very idea of a ‘defined species’ even make sense in this world? Are the organisms we see just snapshots of a very long process that begun eons ago and could potentially never end? Yet we humans have a need to categorise, to make order from the complexity of the universe, and our lifetimes are short so we work with what we can see. Having defined species is also rather useful and important when it comes to conservation, knowing exactly what plant or animal people are talking about is rather handy for environmental organisations and the general public.
Back to the Redpolls and the question is are we looking at one species in the early stages of evolving into three (or five) new ones or are we looking at three already formed species evolving into five? Or are there just two? Recent DNA analysis shows that the plumage differences we see in Redpolls are not based in changes to the genome but just an alteration in gene expression (in essence they all have the same genes but each ‘species’ uses different ones). But it doesn’t quite explain why Lesser Redpolls in Sweden do not interbreed with Common Redpolls which share the same habitat, and seeing as Lesser-reds have a (mostly) distinct breeding range from the others, differentiation would be expected.
More DNA molecular analysis is clearly needed to make the picture 100% clear, if it ever will be, and the high degree of variation in each species’ plumage needs to be taken into account – some individuals are impossible to separate to species level in the field. Whether you are pro-lumping or pro-splitting we can all agree that Redpolls are a confusing bunch of similar-looking twincy red-headed finches. I personally like to think that we are seeing these birds at a point in their evolution where they are at an in-between stage of not-quite full species but a little bit more than sub-species. Would I tick an Arctic Redpoll on my list though? You bet I would.