Disease implicated as a cause of Turtle Dove decline – a bit.

3)Turtle Dove : By Miguel González Novo from Melilla, Spain. (Paloma bastarda) [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
3) Turtle Dove : By Miguel González Novo from Melilla, Spain. (Paloma bastarda) [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
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.

MarthanewcoverBut 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.

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8 Replies to “Disease implicated as a cause of Turtle Dove decline – a bit.”

  1. Trich is a real "problem" it seems and there are many "bird lovers" (people who love to feed the birds in their garden) who still seem to not know of it.
    These people often take great delight in having constantly-topped-up seed feeders, but never (or very rarely) clean them.
    In the mid noughties, great flocks of greenfinches would squabble over this concentrated (unnatural?) food source - and drink from filthy, unwashed bird baths.
    Since 2006 the population of our greenfinches (and to a lesser degree, chaffinches) has crashed - this is acknowledged to be a result of Trich, primarily.
    And yet still, there are many "bird lovers" in the UK who will never (or very rarely) clean their bird feeders, let alone their bird baths, coo with delight when they see a "tame", fluffed-up greenfinch on the ground below the feeders (aaahh how sweet) and often criticise cat owners or pigeon shooters in farms etc for "not caring about " (or "loving") birds.
    Of course "Frounce" (raptor Trich) happens too - when bird-eating raptors eat enough prey with infectious Trich protozoa. And yet the "caring" bird-lovers don't appreciate this either.
    Bizarre.
    Of the three houses we've lived in over the past ten years, we were not the only ones who fed the birds on our street, but it did seem like we were the only ones in our immediate area that were regularly cleaning (properly cleaning) our feeders and baths.
    But if no-one else does, Trich can get a foothold (it dessicates and dies easily - so can be controlled), if bird feeders and baths especially are cleaned regularly and thoroughly).

    I'm not that surprised to hear that Canker (pigeon/dove Trich) has been possibly implicated in the Turtle Dove's decline. I'm not sure if scientists have discovered what turns an avirulent Trich protozoan (present in almost all adult doves/pigeons as you say Mark) into a highly virulent strain. Perhaps they know now - I'm not sure.

    What is relatively surprising to me is that collared doves and woodpigeons (especially) don't seem to have been dramatically affected by Trich. Their UK population explosions have been massive over the years, and yet they all carry Trich to a degree.
    I note in the latest BBS all (bar the turtle dove) of our pigeons/doves population expansions have levelled off in the last couple of years (or dropped slightly).
    I doubt very much this is because of Trich though.

    We keep free range hens in our garden these days, feed our "wild birds" far less and have taken to fencing off the bird feeder area so that our hens cannot get under the feeders - which they certainly would do if they could.
    Our garden is now in "zones" - I do not want wild birds giving their Trich to our girls (or vice versa)!
    We still (of course) disinfect the hen's food hopper every week and drinkers every day and ensure that when we are feeding the wild birds (not yet in this year) we do the same with their "stuff".

    Feeding birds responsibly is great fun - but it is expensive and quite dirty, hard work - IF you do it responsibly. Many don't. Or WON'T.

    And worse still, they just won't have it that they may be doing more harm than good, if they when creating a very unnatural food-source, do so in a filthy, disease-promoting fashion.

    Please can I take this opportunity to promote the excellent Garden Wildlife Health website, which will help readers understand more about animal diseases (and report them) and therefore also help scientists understand more about them too.
    It's a SUPERB site.
    http://www.gardenwildlifehealth.org/

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    1. We do our bit for garden birds by providing trees, shrubs and flowers that provide a rich source of food and cover for both nesting species and visitors. I have always suspected that enticing large numbers of birds to feed in a limited area must provide ideal conditions for the spread of disease. Same principle must apply when vast numbers of pheasants and partridges are released into limited areas!

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  2. Sorry. Forgot to say.
    "Why now" you say? A very pertinent question. Kerreist knows?
    Similarly, why the mid noughties (in particular) for the explosion in finch Trich.
    Was it just then that it was first noticed and identified...?

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  3. Doug MackD
    Good points. You mention the area under the feeder. This is where the chaffinches love to feed on the split grain. This gets sour in winter with droppigs so the feeder needs moving every week at least!
    This is also where the pheasants sit out and wood pigeons. So I put a cage over it and the small birds can access it. This season the voles have taken over this job. I have never seen so many voles round the garden. Is it a vole year? I hear barn owls at night but never see them in the evening so I assume that together with the dry weather and plenty of voles they get enough food in the night and do not have to fly long hours.
    So why are the kestrels not doing well.

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  4. Maybe just an illusion on my part but do we know why this disease would seemingly not have the same affect on Collared Doves which frequent gardens and feeders where TDs don't?

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    1. Rob - no idea! Trich is present in most pigeons - and often birds of prey - but why it has more of an impact in some species than others is beyond me.

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      1. My guess (and I've not yet read the paper) is that disease is one proximate, but not the ultimate, cause of the turtle dove's problems. Turtle doves are presumably more susceptible to this disease because the females are unable to recover condition when they return to their breeding grounds in spring. If they're in poor condition, they could suffer diseases which they might otherwise live with. They rely on a ready supply of weed seeds to put on weight and start producing eggs, but this ready seed supply has largely vanished from the British countryside due to the switch from spring to intensive autumn sown cereals, meaning we see far less weedy stubble in spring. So disease susceptibility is a symptom, but not the cause. As for collard doves and wood pigeons, they are both largely non-migratory, generalist (i.e. adaptable) species, able to maintain body condition through the winter, so less likely to be adversely affected by this disease. I may well be wrong, but I think that, if we can only persuade UK farmers to include a bit of weedy, spring stubble in their holdings, a major problem for turtle doves (and skylarks, yellowhammers, linnets etc etc) will be sorted.

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