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

[John B (not the sloop)]

Well it's in the Daily Wail so automatically suspect but scientists were involved allegedly


http://www.dailymail.co.uk/sciencetech/article-1269580/Soaring-sparrowhawk-population-leads-shock-decline-humble-sparrow.html?ITO=1490

 

[John Cantelo]

The article claims the source is research from the BTO and Cambridge University. Given the spin that the press applies to research results it would be wise to track down the original report and see what it actually says. Even if the Sparrowhawk population has quadrupled since its pesticide induced nadir (as claimed), I'd guess it's still no greater than in the 1940/50s so why weren't sparrow populations similarly depressed? The fact that both sparrows and sparrowhawks were common at that time needs explaining. No, I'd suggest sloppy science or sloppier reporting is at the root of this claim,

 

[dantheman]

Interesting the report was published in an American journal ....

Of course there are going to be a whole range of factors which are involved in determining the abundance of any species. You could maybe blame the Sparrowhawk for living in more afluent parts of town, but then I wonder if those areas were ever the strongholds of the House Sparrow, or its preferred habitat. It's not as if House Sparrows are a typical woodland species ...

 

[John Cantelo]

Interesting the report was published in an American journal ....QUOTE]

Yes, I wondered about that!

I certainly don't live in a wealthy part of town (my house is in a typical Victorian terrace), but rarely House Sparrows in my garden (they used to appear regularly). Sparrowhawks I see tolerably often, but I see them more frequently en route to work ..... where I also tend to see sparrows,

 

[Adam W]

Cant help feeling there's a touch of double standards going on here, how many times have we seen the opposite argument on here critising those against raptors for ignoring scientists findings but now they say want you dont want to hear the first reaction is to assume the scientists are wrong.

I'll point out again that i'm not one of those against raptors quite the opposite,its always a special experience seeing birds of prey but its just not fair to have one set of rules for those who happen to agree and another for those that dont.

 

[John B (not the sloop)]

Cant help feeling there's a touch of double standards going on here, how many times have we seen the opposite argument on here critising those against raptors for ignoring scientists findings but now they say want you dont want to hear the first reaction is to assume the scientists are wrong.

I'll point out again that i'm not one of those against raptors quite the opposite,its always a special experience seeing birds of prey but its just not fair to have one set of rules for those who happen to agree and another for those that dont.

In fairness, nobody has suggested that the research is flawed. Some suggestions have been volunteered as to why it could be, and I hinted that the Daily Wail couldn't necessarily be relied upon as an accurate information source. Once people have tracked down and digested the research report they will develop more informed views and, quite possibly, air them around here somewhere. I look forward to that.

 

[John Cantelo]

Cant help feeling there's a touch of double standards going on here, how many times have we seen the opposite argument on here critising those against raptors for ignoring scientists findings but now they say want you dont want to hear the first reaction is to assume the scientists are wrong.

I take the point (and was conscious of it as I posted), but I suggest that there's a world of difference between querying a single report that apparently runs against the scientific consensus and actually denying that consensus on entirely unscientific (not to mention logically inconsistent) grounds,

 

[earleybird]

scientists were involved allegedly

tsk scientists eh !..... tsk tsk :-C scientists couldn't agree on anything if their lives depended on it.

How many sparrows did they interview eh? eh? was it a representative cross section ?

 

[Adam W]

Fair points John and John, i'm not for one minute saying I agree with the research i honestly dont know if its correct or not and it may well be right to challenge it but i dont think there can be any denying that the same sceptisism and challenges wouldnt have been made if the research had simply said what you wanted it to say infact i'm sure that would have quickly been used against those who are not so keen on raptors.

 

[ChrisKten]

All these reports that are commissioned and paid for; all they had to do was ask me. They could have monitored my garden for a few years and maybe observed Nature in action, so to speak.

70+ Starlings, 30+ House Sparrows; each having more than one brood per season. Almost daily attacks by Sparrowhawks; Starlings being the preferred prey of both sexes. And Starling and Sparrow numbers are, if anything, increasing.

Of course, I could kill a few Sparrowhawks, then watch as more of the Sparrows and Starlings died of starvation (removing the Sparrowhawks won't increase available food). I could also watch the sick and weak take a bit longer to die. Oh, and I suppose I could watch as the extra Sparrows and Starlings struggle to find nest sites.

Much more to say, but I think I'll wait for the next Report/Study; I'm sure there'll be another soon.

 

[abi107]

Official summary of the article in the journal:

The Role of the Eurasian Sparrowhawk (Accipiter nisus) in the Decline of the House Sparrow (Passer domesticus) in Britain

Christopher P. Bell, Sam W. Baker, Nigel G. Parkes, M. De L. Brooke, and Dan E. Chamberlain
The auk, volume 127, pg(s) 411–420

We compared the pronounced geographic pattern in the recolonization of Britain by the Eurasian Sparrowhawk (Accipiter nisus) from 1970 onward with the spatiotemporal pattern among House Sparrow (Passer domesticus) populations over the same period, using data on the occurrence of both species at garden feeding stations. Using a simulation of House Sparrow population trends based on a logistic model that incorporated a predation index derived from Eurasian Sparrowhawk incidence functions, we generated a close approximation to the unique trajectories among House Sparrow populations in rural and urban sites in different regions of Britain. We carried out further comparisons using two contrasting methods that focused solely on temporal patterns. We used estimates of the varying date of Eurasian Sparrowhawk recolonization at different sites to derive time variables in relation to recolonization date. One such relative time variable proved to be a better predictor of House Sparrow numbers than chronological time; it indicated that House Sparrow numbers were generally stable or increasing prior to recolonization by Eurasian Sparrowhawks but declined continuously afterward. We also detected a significantly greater decrease in House Sparrow numbers when Eurasian Sparrowhawks were present using a method that compared annual changes in the abundance of prey species in the presence or absence of a predator. On the basis of these results, we argue that predation by Eurasian Sparrowhawks may be a sufficient explanation for the decline in House Sparrows in Britain. We also argue that urban House Sparrow populations' long-term release from predator pressure made them especially vulnerable when urban habitats were colonized by Eurasian Sparrowhawks.

 

[Stoggler]

I found the abstract earlier, was going to post it but you just beat me to it abi

Without looking at the full study itself and check out the methodology, I can't help feeling that its a bit of correlation not necessarily proving causation. I can't believe that Sparrowhawks are the sole reason for sparrow population declines (when is anything in nature that straightforward?).

It would be nice to look at the full study (but would have to pay - boo!) and compare it with other studies - as all scientific studies should, its methodology needs to be scrutinised, and its results checked. And could some form of meta-analysis be done on the available data?

My biggest gripe however is always with our press reporting science stories - they will report the findings of one study without putting into perspective by considering already-existing research/evidence.

 

[Stoggler]

Interesting the report was published in an American journal ....

Browsing through the different abstracts while looking for this particular study, I couldn't help but notice that there are studies from all over the world in this journal. Made me wonder if this is the premier ornithological journal and is the obvious place for such studies.

 

[ColonelBlimp]

I have access to this paper: would PMing people the PDF come under fair use?

 

[CPBell]

I'm the lead author of the study, and I would be happy to discuss the findings here with anyone who is interested. A pdf of the paper can be downloaded from my website: http://www.cpbell.co.uk/home/pubs.

Just to respond to a couple of points already posted. John Cantelo asks why Sparrows were not depleted by Sparrowhawks when they were common in the 1940s. As far as I can tell, Sparrowhawks were never common in cities prior to the late 1980s. Remember that cities were very polluted prior to the Clean Air Acts, and Sparrowhawks would also have been shy of humans since they were persecuted in the countryside. In the country, Sparrows would formerly have been used to Sparrowhawk attacks. However, with Sparrowhawks absent for several decades after the 1950s Sparrows may have lost their predator averse behaviour, making them easy pickings when the predator returned.

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.

 

[Stoggler]

Many thanks for the link to your site - shall read the article with interest.

Regards

 

[ColonelBlimp]

However, with Sparrowhawks absent for several decades after the 1950s Sparrows may have lost their predator averse behaviour, making them easy pickings when the predator returned.

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

 

[ChrisKten]

One thing that always confuses me about this, and other, Reports - why is it accepted that Sparrowhawks eat many Sparrows?

I don't remember ever seeing a female Sparrowhawk take a Sparrow. Even the males take more Starlings/Robins/Dunnocks/Blackbirds/etc than Sparrows. So where did the idea that Sparrowhawks kill mostly Sparrows come from?

Or is this just happening in my garden, and when you "average it out" (as this report seems to have done) it's mostly Sparrows being selected as prey.

 

[ColonelBlimp]

Also, isn't it a little dangerous to suggest that sparrow declines can be solely attributed to sparrowhawk increases? The analysis seems to center pretty squarely on matching sparrow/sparrowhawk population changes and then *hey presto* assigning causation when they match?

Even if sparrowhawk number increases are in part driving a movement in sparrow numbers to a new equilibrium, which I don't dispute is a clear possibility, not considering and quantifying other potential influences seems a little risky...

Incidentally I've seen lots of sparrowhawks and no sparrows at the Downing site recently...

 

[John B (not the sloop)]

I'm the lead author of the study, and I would be happy to discuss the findings here with anyone who is interested. A pdf of the paper can be downloaded from my website: http://www.cpbell.co.uk/home/pubs.

Just to respond to a couple of points already posted. John Cantelo asks why Sparrows were not depleted by Sparrowhawks when they were common in the 1940s. As far as I can tell, Sparrowhawks were never common in cities prior to the late 1980s. Remember that cities were very polluted prior to the Clean Air Acts, and Sparrowhawks would also have been shy of humans since they were persecuted in the countryside. In the country, Sparrows would formerly have been used to Sparrowhawk attacks. However, with Sparrowhawks absent for several decades after the 1950s Sparrows may have lost their predator averse behaviour, making them easy pickings when the predator returned.

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.

Thanks for providing some additional clarification here.

So if we subscribe to the not unreasonable notion that Sparrows lost their avian predator awareness during the post war period of depressed Sparrowhawk numbers, are we seeing a readjustment to "normal" Sparrow population levels now that a significant predator is back in town? If this is the case may we expect Sparrows to recover their survival skills over time and population levels to stabilise (at a possibly lower level than in the forties to eighties)?