The Turtle Dove is a lovely bird but is declining dramatically in the UK but also in many other parts of Europe.
Although one of the more dramatic problems it faces is being shot by hunters on migration, particularly unsportingly (and illegally) on spring migration, this has never seemed to me to be likely to be ‘the’ cause of the Turtle Dove’s Europe-wide decline. You have to shoot an awful lot of birds to knock a big hole in their population. And actually, there is rarely ‘a’ single cause of a species’ decline.
But when you have knocked that big hole in the population, all else being equal, the survivors ought to have a lovely time of it with copious resources of food and nest sites. And so they should do quite well. And so the population should tend to bounce back.
But in the case of the Turtle Doves the evidence suggests that they are doing worse and worse in terms of breeding success on the breeding grounds – at least in the UK. Removal of critical food supplies have been implicated in these declines (at least in the UK). I discuss some of this in the last chapter of A Message from Martha.
So I was interested to see this paper published recently in the journal Parasitology (by RSPB, Leeds University, Cardiff University and Natural England scientists) which suggests that disease might play some part in this story. All Turtle Doves sampled (n=25) have the protozoan parasite Trichomonas gallinae in their bodies. These infections cause lesions which could make your average Turtle Dove feel rather rotten – or lead to death. Such a factor could be a different explanation for low breeding success – they are just not in great nick!
It remains to be seen, I guess, how important this factor is in the UK decline of Turtle Dove. And, again I guess, whatever the answer it may take some time to check its importance across the whole European population. But it is certainly interesting.
It would be interesting in the case of the Turtle Dove but it is also interesting to me, in what it might mean for the extinction of the Passenger Pigeon too. I am relieved to see that I didn’t dismiss disease as a factor for Passenger Pigeons although there didn’t seem to be that much evidence for it – but then how could there have been?
Trichomonas gallinae is found in pigeon species across much of the world. It would have been present in North America, no doubt (particularly since domestic pigeons were taken across the Atlantic by European colonists).
If, and it is a big ‘if’, and only an ‘if’, but if disease is an important factor in the decline of the Turtle Dove then the question we might want to answer is ‘Why now?’. There is a tantalising, but little more than an anecdotal smidgeon of information referred to in the paper and that is that the same strain (that might not be the right term) of Trichomonas infection is present in Red-legged Partridge too. But whether partridges give it to doves, or doves to partridges, is about as promising a discussion as whether men give women colds (which we don’t) or whether women give colds to men (which they do – often on purpose!), or whether badgers give bTb to cattle (which they do) or whether cattle give it to badgers (which they do).
A morsel of pure (or impure) speculation, that is in A Message from Martha too; maybe the introduction and rapid spread of the House Sparrow into North America (introduced from 1851) was the route by which a disease arrived in North America that drove the Passenger Pigeon to extinction. That would be highly ironic. But it is a big leap from where we are with Turtle Doves to that position. Still, makes one think doesn’t it?
More about Turtle Doves here.