What is a species? In most cases, it’s pretty clear cut. Blue Tits are different from Great Tits, and Coal Tits are different again. However, there are plenty of cases where the lines between different kinds of birds are blurred, and somebody, somewhere, needs to decide where those lines are drawn.
How the lines have been drawn has changed over the years. Early taxonomists (taxonomy is the ‘science’ of putting things into related groups – i.e. species and subspecies) were led by appearances, and particularly structure. More recently, what could breed with what has informed taxonomic decision making, and in the present day, most splits (i.e. decisions that two groups of birds are different species) or lumps (i.e. decisions that two groups of birds are the same species) are based on genetic data. However, the earlier techniques are still relevant and, ultimately, a decision on whether a bird is a species or not should rely on phenotypic (i.e. what the bird looks and sounds like) as well as genotypic (i.e. a bird’s genetic makeup) information.
Even when we look at things at a molecular level, it may be surprising to you that we still don’t have a single, failsafe criteria for defining whether something is a species or not. Historically, we would define species by whether they were reproductively isolated. Organisms that could effectively reproduce with one another were the same species. This still holds true in many cases. Lots of ducks can hybridise with each other, but by and large, any offspring that survive are not able to reproduce with either parent species, or anything else for that matter – they are a reproductive dead end. However, there are an increasing number of groups of birds that are known to produce fertile hybrids – so that notion as a sole defining criteria for speciation has had to be parked.
The more modern genetic and molecular methods can tell us a lot about the relationships between different birds. They can tell us roughly how long ago two different groups began to evolve their differences. They can tell us the amounts of genetic difference between two groups of birds, and they can tell us that, for example, species X is more closely related to species Y than it is to species Z. However, as yet, they cannot definitively say what is or isn’t a species.
With all that said, it’s not too unusual for birds to ‘become species’. Flicking through an old fieldguide next to a new one will give many examples of where this has happened; Rock Pipits are now Rock and Water Pipits, species such as Yellow-legged and Caspian gulls have sprung from Herring Gulls, and Subalpine Warblers have become three separate species. Even further back, we have the splitting of the ‘Willow Wren’ into what we now know as Willow Warbler, Chiffchaff and Wood Warbler. These decisions seem obviously correct these days, but of course they were cutting edge at their time.
Where things are less obvious, birds can flit in and out of ‘specieshood’. A great example of this is the Red Grouse. For a long time, the Red Grouse was considered Britain’s only endemic species of bird. It is closely related to the Willow Grouse (or Willow Ptarmigan, as it is sometimes known) of Europe and North America but differs in its moult strategy – having dark, not white wings – and in not having a distinct white winter plumage. In years gone by, this was enough to call it a species. However, about 50 years ago, doubts began to creep in, largely due to a small population of island- based grouse off western Norway that appeared to be intermediate, plumage wise, between the British and Norwegian birds. This diminished the idea of a clean break between the UK and European populations, so our Red Grouse was reduced to being a subspecies of the widespread Willow Grouse.
It has remained in that position ever since but recent studies suggest that this could all be about to change. Molecular work has revealed that the British Red Grouse is genetically quite different from other Willow Grouse – including those strange Norwegian island dwellers. With taxonomists looking across the suite of differences (plumage, moult strategy and genetics), it has been recommended to treat Red Grouse as a species in its own right again, with Irish birds to be treated as a subspecies of Red Grouse. This is significant, as with doubt permanently circling over the currently accepted crossbill taxonomy, we may have found ourselves in a position of having a UK avifauna with no endemic species. This might not have any particular relevance from a conservation standpoint, but I think most UK-based birders would be comfortable with this iconic upland bird, whisky seller and eagle dinner being recognized as a species in its own right.
So what might the taxonomists have in store for us next? It’s no big secret that we may lose some species of redpoll soon, with no genetic differences between the groups previously recognized as species – and as I’ve already alluded to here, maybe crossbills will go the same way. Persuasive arguments for considering variations within these groups as ‘ecotypes’ rather than species were published recently - whatever an ecotype is, of course! We may also see Bean Geese treated as a single species in future but on the flip side, those of us who have birded in Spain or south eastern Europe may stand to get a couple of extra species of Dunnocks on their lists, after recent genetic work suggests that there could be three species of Dunnock in Europe! These decisions are not made yet and as you can imagine, they are hot topics within the birding world. Birders, and in fact taxonomic authorities, don’t always agree on what should be considered as species, and until that silver bullet of speciation is found, the debate will rumble on. Birders mitigate against taxonomic changes by focusing on ‘identifiable forms’ (i.e. something that can be identified in the field, regardless of taxonomy); subspecies such as Siberian Chiffchaff and birds like Kumlien’s Gull – a hybrid swarm of Iceland and Thayer’s Gulls – have value to birders. Ultimately, however, species is valued over ‘lesser ranks’ and many birders will be looking forward to welcoming the Red Grouse back onto their lists. Long may it stay there!
SOC Birding & Science Officer
Red Grouse photos (adults and chick) © Rosie Filipiak