Sparrowhawks responsible for House Sparrow decline says scientist (1 Viewer)

[spencer f]

You make a good point,
the lack of winter field stubble and the disappearance of hedgerows has had an effect on farmland birds. Experiments in bird food crops and wider field boumdries have met with success. There are simularities in the urban landscape but on a smaller scale.

 

[John Morgan]

Where's the evidence for this? Would sparrows become so 'ecologically naive' in such a short space of time?

Being a sociable species, they may have avoidance tactics that are learned rather than innate.

 

[George Hogg]

Magpies?

Also, isn't it a little dangerous to suggest that sparrow declines can be solely attributed to sparrowhawk increases?

My own question would be whether Magpie/Sparrow fortunes have been compared.
Magpies seem to have urbanised alongside sparrowhawks and my own observations of them systematically pulling out House Sparrow nests suggests a similar Magpie/House Sparrow data comparison would show very similar results...George

 

[spencer f]

yes magpies are one of many factors contributing to the spaarow decline but I believe that lack of nest sites, dense shrub layer and invertebrates are the major factors in the decline.

 

[ColonelBlimp]

Being a sociable species, they may have avoidance tactics that are learned rather than innate.

Indeed they might- but without evidence for it, it remains pure speculation.

 

[spencer f]

I don't doubt your central proposition that sparrow numbers may be moving to a new equilibrium under conditions of a reversion to vaguely 'normal' predation levels, or that issues of ecological naivete may play a part.

Are they going to share our implicit understanding that this rebalancing process is positive and 'natural'? .

If Sparrowhawks were the only factor then this might have been accurate.

However with cats and habbitat loss in the picture there is never going to be a 'New Equilibrium' or a 'Posotive and Natural Rebalancing'.

 

[CPBell]

Capercaillie71 (#120) – To be clear, I haven’t said that I rule out agricultural intensification as the most important factor in most bird population declines, just that critical evidence is lacking, and counter-intuitive though this might be, the fact that in some cases bird numbers have increased where the effects of agricultural intensification have been reversed does not amount to critical evidence.

Let me illustrate the principle with an example. Let’s say you and I share a flat that has a mouse problem. I decide to do something about this, but I know you are against cruelty to animals, so I resolve to act without your knowledge. I therefore visit the local hardware store, and buy a large wooden mallet. Each day when you are out, I wait for a mouse to emerge, and I clobber it with the mallet. The resulting increase in the mortality rate causes the mouse population to decline. You notice this, and remark that you are mystified as to why the mice are disappearing, and I shrug innocently in response. We are then joined by a third flatmate, say ColonelBlimp, who happens to be a bit careless with leftovers, and is in the habit of leaving half-eaten pizzas around the flat, which the surviving mice tuck into. The extra food enables the mice to breed more quickly, and although I am still at work with the mallet, the mice start to increase. You notice this and berate ColonelBlimp for encouraging vermin, meanwhile remarking to me that the mouse population is clearly related to food availability, so this must have previously declined for the mouse population to have gone down – a seemingly reasonable deduction from the information at your disposal, but wrong nevertheless.

I should also point out that in general, measures to reverse the putative effects of agricultural intensification on birds haven’t worked, and given that £½ billion or so is spent every year implementing these measures, I think it’s important to challenge the assumptions and evidence on which they are based.

 

[Mike Shurmer]

To be clear, I haven’t said that I rule out agricultural intensification as the most important factor in most bird population declines, just that critical evidence is lacking, and counter-intuitive though this might be, the fact that in some cases bird numbers have increased where the effects of agricultural intensification have been reversed does not amount to critical evidence.

As far as I am aware, there are a whole host of studies that suggest that agricultural changes have caused declines of farmland specialists. I assume you don't agree with any of them?

I should also point out that in general, measures to reverse the putative effects of agricultural intensification on birds haven’t worked, and given that £½ billion or so is spent every year implementing these measures, I think it’s important to challenge the assumptions and evidence on which they are based.

I agree with you that we need to challenge evidence and ensure where we are spending money appropriately. However, agri-environment schemes have certainly been proven to work where they are highly targeted, particularly in the case of species like stone-curlew and cirl bunting. Where they have been less effective is where they have been implemented in a less targeted way.

So, I also agree with you that there is some way to go with the schemes in that we have not seen huge increases in all farmland bird populations, despite there being stewardship schemes of one form or another for several years. But I think you are being over simplistic by suggesting that this means that the evidence base for these measures is flawed. You need to look at how the schemes have been implemented, how different options have been taken up (or not, as the case may be) across the country and the quality of the delivery of the options. The £half billion figure you give is a very misleading one, as this is total spend on all the schemes including options to protect the historic environment, resource protection, landscapes etc... The actual spend on the good farmland bird options is much lower than this.

There are plenty of examples where, on certain sites, there have been increases in bird populations where scheme options have been implemented at the right scale. Evidence from the RSPB's Hope Farm shows that on an intensive arable farm good deployment of options can lead to increases in bird populations, for example. The trick is to get this replicated on a population scale, which is not happening at the moment as the scheme options are not being taken up at anything like a large enough scale to get recovery of farmland bird populations.

 

[spencer f]

Here is a simple calculation:

Less diversity of habbitat = less feeding and nesting oppertunities= less birds

More predation + more competition = less birds

The answer:

encourage diversity, create and promote more reserves, reward good practice and log, monitor and communicate the results.

 

[Madonbirds]

Here is a simple calculation:

Less diversity of habbitat = less feeding and nesting oppertunities= less birds

More predation + more competition = less birds

The answer:

encourage diversity, create and promote more reserves, reward good practice and log, monitor and communicate the results.

I think that is it in a nutshell, tbh!

I think the public as a whole, should be more responsible for our wildlife than they are nowadays.

I'm a keen gardener and get really annoyed at people and their view of 'pests' in general.

How often to you see on forums/MB's this type of question!

Hi, just found this insect on my plant, what can I use to kill it? Nine times out of ten, the answer people give is Provado and if they only knew what was in it, they might think twice! Trouble is, nine times out of ten, if you just left it, it's predator (small birds included) would come in to deal with it (in time) and so up the chain to the Sparrowhawk!

How often do you hear of people wanting fences, 'as they are less work than a hedge'? Those cats as well, less work than a dog!

We as a nation, have to look at the whole picture! Plant for nature and it will re pay you, with pest control.

'My' Sparrows here, devastate my ornamental grasses (esp a Stipa) and basket liners for nesting material, even though I put stuff out for them, but if it means it allows them to breed and produce more Sparrows, so be it! If some of those broods get taken by predators, so be it. It's life after all, but maybe, we aren't helping the little guys out as much as we should, to give them a fighting chance in the big scheme of things.

When you go to flower shows that have show gardens, how many are wildlife friendly nowadays? Not many I expect. I went to Gardener's World Live two years ago, with friends who did a wildlife garden and got a Silver Gilt, but people were more interested in the big blousy ones, than theirs. They would have had gold, if the Silver Birch they had, had been bigger!!! They weren't on as big a budget as others were! These garden design programmes are just as bad, sometimes. Using weed suppressing membrane in between plants and on top, I've even seen broken up CD's used as a mulch! How many times do you see mirrors in these gardens, not very wildlife friendly, are they?

Sorry rant over!

 

[spencer f]

I strongly suspect that much of the change to our garden habitat in recent decades is irreversable and will probably get much worse. What little green space that is left will need to be of good quality in terms of biodiversity, no more green desserts and endless expanses of concrete and decking.

Our environment is facing huge challenges with climate change, increasing overpopulation and dwindleing natural resources. This means that good quality green space will become even more vital to all living things.

Governments will only usually act on popular opinion so it will be up to us as individuals to make an informed decision on how we use our living space . Sparrow decline may be a warning sign that our urban environments are becoming biologically degraded.

Improving the quality of our gardens will certainly help all of our familiar species, but for species like Wheatear, Skylak and lapwing the answer must lie in how we manage our countryside. Unfortunately for this, large financial insentives will probably be needed.

As for the Sparrowhawk study that started this thread I believe there are more opinions and studies that go against this theory than for it. So on this occasion I must go with the majority.

 

[spencer f]

Several posters have mentioned that Sparrows remain common in their neighbourhood despite hunting Sparrowhawks. We found this was often the case when analysing the data we used for the study - the BTO's Garden Bird Feeding Survey. However, when averaged over all sites the effect of Sparrowhawk presence is quite striking. This may not show up in individual sites, each of which has its own special characteristics which affect the way in which the predator and prey interact.

I suspect that the 'Special Characteristics' of individual sites are the amount of available cover and thick vegetation available as well as siting of feeders.

Sparrows have always had many predators including humans, I can not see sparrows being that coplacent when it comes to predation.

 

[CPBell]

StonedCurlew – (#128) I can only speak for the studies that I’ve read personally, but I find that in general the inferences they draw about the role of agriculture in bird declines are not supported. The context in which these studies arise also provides grounds for caution, since the assumption that agriculture is responsible was behind the government’s decision to adopt the policy objective of reversing declines in farmland birds, thereby releasing a substantial stream of public money for agri-environment research. Researchers who benefit from this have precious little incentive to seek or publish results that undermine this assumption, and very many researchers and research organisations have benefited greatly. It’s no coincidence that the BTO have held no less than three conferences on conservation of lowland farmland birds in the last decade.

The RSPB are quick to wheel out Corncrake, Cirl Bunting, Stone Curlew and Hope Farm whenever the basis of agri-environmental management is questioned, but as you say, these are intensive, targeted projects aimed at the recovery of tiny populations, and their outcome cannot be generalised to widespread species in the wider landscape. Apart from anything else, the costs of scaling up the measures these projects employ would be colossal.

The implementation of wider agri-environment management, exemplified by the Entry-level Scheme, is based on the inferences drawn from the host of studies to which you refer. This provides a continued veneer of credibility despite the stubborn failure of farmland birds to recover, since the policy can be presented as ‘science-based’ and just in need of more time to work, or better implementation, or inevitably, even more research. However it would be more accurate to describe the policy as ‘natural history-based’ since the knowledge base does not, on the whole, meet the standard of scientific enquiry.

Incidentally, if you have the breakdown of agri-environment funding by objective, i.e. habitat enhancment, restoration, historic landscapes, genetic resources etc., I would be interested to see it.

http://www.cpbell.co.uk

 

[lewis20126]

Capercaillie71 (#120) –
Let me illustrate the principle with an example. Let’s say you and I share a flat that has a mouse problem. I decide to do something about this, but I know you are against cruelty to animals, so I resolve to act without your knowledge. I therefore visit the local hardware store, and buy a large wooden mallet. Each day when you are out, I wait for a mouse to emerge, and I clobber it with the mallet. The resulting increase in the mortality rate causes the mouse population to decline. You notice this, and remark that you are mystified as to why the mice are disappearing, and I shrug innocently in response. We are then joined by a third flatmate, say ColonelBlimp, who happens to be a bit careless with leftovers, and is in the habit of leaving half-eaten pizzas around the flat, which the surviving mice tuck into. The extra food enables the mice to breed more quickly, and although I am still at work with the mallet, the mice start to increase. You notice this and berate ColonelBlimp for encouraging vermin, meanwhile remarking to me that the mouse population is clearly related to food availability, so this must have previously declined for the mouse population to have gone down – a seemingly reasonable deduction from the information at your disposal, but wrong nevertheless.

But isn't that what you have done? The correlations may be striking but you have failed to demonstrate causality - and as Jos points out your evidence in respect of predation levels on House Sparrows is depauperate to say the least.

cheers, alan

 

[spencer f]

CPbell
do you have any alternative solutions to the farmbird problem or do prehaps believe there is another cause. I ask this because you seem to have a habbit of going against the grain. Sincere question?

 

[CPBell]

Lewis20126 (#134) – No, what I’ve done is to carry out a survey of 264 flatshares and then demonstrate a correlation between the date of mallet purchase and the beginning of mouse decline. Whether you infer causality from this is a matter of personal judgment, since causality can’t be demonstrated, only inferred.

Spencer f (#135) – Well, first of all, I would attempt to diagnose the problem using a scientific approach, which means erecting a falsifiable hypothesis. I would then derive predictions from the hypothesis that are unlikely to occur by chance and which differ from those of alternative hypotheses, and then take the necessary steps to determine whether the predictions are fulfilled.

I’ve already done this for the House Sparrow, in that I’ve erected the hypothesis that Sparrow decline is caused by Sparrowhawk predation. I’ve then deduced that if this is so, variation in Sparrow population trends by region and habitat will be predicted by corresponding variation in Sparrowhawk population trends, and that site by site variation in the timing of Sparrow decline will be predicted by variation in the timing of Sparrowhawk establishment, neither of which is predicted by alternative hypotheses or likely to occur by chance. Both of the predictions turn out to be corroborated by the evidence, from which I draw the provisional inference that Sparrowhawk predation has caused House Sparrow decline.

The alternative hypothesis is that Sparrow decline is caused by agricultural intensification. This is not a variable, however, but a ‘panchestron,’ i.e. a concept that is so ill-defined that it can explain every conceivable observation, and is therefore incapable of generating a falsifiable hypothesis.

If a result similar to the one we obtained for the House Sparrow were to be obtained for the other farmland species that have declined, it would be reasonable to infer that Sparrowhawk predation has been generally responsible, and this would allow a judgment to be made on an appropriate course of action. We could decide to do nothing, on the grounds that the predator has reached a natural balance with its prey, or we could attempt to induce a recovery among farmland birds by some form of general or local control of Sparrowhawk populations.

Several of the contributors to this thread (including yourself Spencer) have expressed a belief in the natural balance of nature. I would observe that if the avifauna of the British Isles were fully intact, the larger raptors would be much more common than they are, including the Goshawk, which is an avid predator of Sparrowhawks where the two species overlap. The current high density of Sparrowhawks might therefore be attributed to the absence of its natural predator, a phenomenon known as ‘mesopredator release’. However, if we regard this as a possible solution, we might end up like the old woman who swallowed a fly.

http://www.cpbell.co.uk

 

[Jos Stratford]

...these are intensive, targeted projects aimed at the recovery of tiny populations, and their outcome cannot be generalised to widespread species in the wider landscape.

Come on, you can hardly take this line - you generalise rates of predation based on observations of one single pair of birds!

 

[dantheman]

I’ve already done this for the House Sparrow, in that I’ve erected the hypothesis that Sparrow decline is caused by Sparrowhawk predation. I’ve then deduced that if this is so, variation in Sparrow population trends by region and habitat will be predicted by corresponding variation in Sparrowhawk population trends, and that site by site variation in the timing of Sparrow decline will be predicted by variation in the timing of Sparrowhawk establishment, neither of which is predicted by alternative hypotheses or likely to occur by chance. Both of the predictions turn out to be corroborated by the evidence, from which I draw the provisional inference that Sparrowhawk predation has caused House Sparrow decline.

The alternative hypothesis is that Sparrow decline is caused by agricultural intensification. This is not a variable, however, but a ‘panchestron,’ i.e. a concept that is so ill-defined that it can explain every conceivable observation, and is therefore incapable of generating a falsifiable hypothesis.

[/URL]

Or the factors that have been mentioned above (in this thread) in terms of the change in 'habitat' of suburban/urban sparrow populations? ...

... which could be tested for, although how, not so sure how though ... (eg by comparing date of building of new diy stores in vicinity?)

And is it not the case that multiple positive correlations can often be found if you look for them?

 

[spencer f]

CPbell,
Interesting comments worth pondering, thank you.

 

[CAU]

CAU – Your help in interpreting this material really is invaluable, as I must admit there are few things more liable to give me a headache than looking for too long at anything written in Finnish. Too many K’s!

The early-mid-late comparison would indeed seem to rule out winter mortality as a factor in the decline. However I can now see that the Sparrow decline is not so uniform after all. According to the landuse map in my school atlas, Finland is a land of continuous forest except for a sliver of arable landuse along the south coast. If this is remotely accurate it suggests that the severity of the Sparrow decline shows rather a neat correlation with the proportion of arable land in the different regions, which is exactly what has happened in Britain. The orthodox interpretation is that agricultural intensification is even more intense in arable areas than elsewhere, but this is confounded with the effect of organochlorines on ‘Varpushaukka’, which were wiped out in arable areas and then flooded back in when the effects of organochlorines wore off, just at the time when ‘Varpunen’ were starting to decline.

You mentioned earlier that Sparrowhawk brood size had increased, probably in the late 70s early 80s, which presumably also reflects the recovery from organochlorine contamination. Unfortunately, the raptor report does not include a regional breakdown in population trend for Sparrowhawk as it does for some other species, but is it possible that the arable area has seen an increase in Sparrowhawk density? The area is so small in relation to the country as a whole that it might not show up in the national figures.

http://www.cpbell.co.uk

I have a figure that shows the the number of found nests as a function of year on a regional level (there are about 20 regions), but it's not online. In most regions (including the more agricultural in SW Finland) the population seems to be pretty stable, without any large fluctuations (some other regions may show increases or decreases, but it's possible that this is mostly just noise created by changes in the activities of the ringers). I can't say whether Sparrowhawk has increased in arable land in Finland, but I haven't heard anyone suggesting it. However, it has been suggested that a slight increase in the overall population is possible, based on the increased number of migrants observed in the autumn (but an increase by a factor of four like apparently in Britain should be out of the question).

Anyway, Sparrowhawk is mostly a forest-dwelling species in Finland, and they are actually quite difficult to see during the breeding time (as they keep to the forests). Here's the abstract of a paper about the diet of Finnish Sparrowhawks:
http://md1.csa.com/partners/viewrec...birds+in+southern&uid=789414143&setcookie=yes
Three of the five species shown to be especially vulnerable to Sparrowhawk predation are increasing (Greenfinch and Blue Tit are increasing strongly, Great Tit slightly, Pied Flycatcher is relatively stable and Song Thrush is decreasing slightly).