Today I was catching up on Charlie Moores' Podcast series Talking Naturally when I came across the 2 part podcast on Sparrowhawk predation as a cause of House Sparrow decline. Normally I switch off at the mention of mundanities, only recently have my bird watching urges been supplemented by conservation related thoughts, but the author being interviewed was an extremely interesting character.
Before I plough on may I mention that I have not read the thread on birdforum and have no intention to do so. Dr Chris Bell comes across as a maverick, someone for whom the idea of convention is anathema but in his bid to prick the pomposity of presumed wisdom manages to unveil a very large ego. In short this is a man who is sure of himself. Despite this, likeability is not the foremost pre-requisite of being a good scientist. Being good at science is. Having listened to the podcasts I have heard the layman's account of the science and the opinion aired by the aforementioned. I thought it wise to seek out the paper where the science is entombed to make my own judgement on its methods and conclusions.
I have had a brief skim through to acquaint myself with methods and discussion. There seems little argument can be had with the survey methodology. Bird census techniques are well established and these seem reasonable. Ish. Weekly counts of mean numbers of sparrows? Presence of Sparrowhawks indicated by seeing them at the feeding station more than once? On the first count - integrating ringing schemes to see how counts relate to actual numbers of sparrows. Also - counts of other species at feeding stations would be useful as these would show what proportion of the bird table population is made up by sparrows and as such whether they are preferentially predated. Liklihood would suggest that other bird table species of similar size and diet would be impacted equally by the introduction of a new predator and yet Chaffinch, Greenfinch and Goldfinch numbers all increased in the survey period (although Greenfinches are now suffering due to Trichomoniasis). Counting presence or absence of Sparrowhawks by their attendence of bird tables is a huge bias - what if Sparrowhawks are present but dont attend bird tables? How would you count them and include them in the data - you can't.
The modelling of populations is reasonable. There is a direct negative correlation in Sparrow numbers with increasing Sparrowhawk numbers but this doesn't equal cause and effect. Other 'accepted' reasons for decline are debunked by short shrift rather than by exploring the evidence base. An effort to seek out papers poorly illustrating decreasing invertebrate populations and a lack of correlation with farming intensification must be made to reinforce the point. Just by saying something doesn't exist does not make it true. It is up to the author to disprove preconceived wisdoms. The argument counter to this is that the paper only explores relationships between sparrows and sparrowhawks. The key word in that is relationship and it indicates a desire to go deeper than simple correlation and yet the paper fails to do this. At no stage in the paper is there a mention of any other factors such as the increase in other predators either avian or mammalian. Where is the interpretation of the impact of corvids or pet cats or grey squirrels? Also if you replaced the work Sparrowhawk in the paper with Buzzard or Peregrine then it would still be broadly correct except neither of these predate garden birds.
So if it was accepted that the modelling and surveying was correct - what of the findings? That Sparrowhawks are THE cause of House Sparrow declines. Maybe that would be fair and I am not suggesting it couldn't be the case. This paper merely puts it forward as a potential case. I have heard it suggested that after many years of no predation, urban sparrows have lost some of their more discrete behaviour and subjective observation suggests that sparrows now are increasingly wary. Perhaps the arrival of Sparrowhawks is a selection event disposing of the fat, chirrupy sparrows sat upon your roof to the benefit of wiry, wary sparrows in your hedge? There may be a 'bounce' back. An analysis of sparrowhawk diet may well indicate preferred food (I am sure there will be research on this but the post is more stream of concious than proper essay) and thus looking at how other prey species are faring in relation to sparrows may be profitable.
At the moment Sparrow numbers continue to decline and Sparrowhawk numbers are plateauing and perhaps in a slight decline after reaching near saturation levels. Whether the limiting factor is persecution, prey resource or habitat I am unsure (a brief scan of research suggests recruitment of juveniles is reduced when prey levels decline but the paper I have seen was Goshawk).
So Sparrows eh? They used to occur in plague like proportions and yet they didn't when Sparrowhawks first came back. A lack of solid evidence leads to this being refuted but anecdotes suggest that Sparrows were more plentiful in London in 1900 than 1970 and yet this decline cannot be attributed to Sparrowhawks. This again leads us to look at the relationship as correlation not anything more? Or perhaps the Sparrowhawk predation is the straw that broke the camels back with those birds best able to persist under the resident pressures at increased likelihood of predation?
Thats the science-y stuff I guess. In no means comprehensive and please realise that it was written as it occurred to me without draft or structure (I work and have a child...need free time). Now for the moral, human and conservation-y stuff.
1) Is there a reason why tonnes of sparrows are good?
2) Will Sparrowhawks cause the extinction of Sparrows?
3) Is having more Sparrowhawks and less Sparrows good/desirable?
4) Do fringe populations of sparrows have any relevance. Rural and urban sparrows were sampled but not those in rurban areas where birds are likely to have more nesting sites, feeding sites & access to invertebrates & water? Are these the mythical sink population?
5) Does Chris Bells opinion that its all Sparrowhawks cloud his objectivity and undermine his science?
6) Why does the RSPB/BTO refuse to acknowledge the work despite it being published in an established journal?
7) Is Chris Bell a puppet for songbird survival and anti-raptor pressure groups?
My personal opinion on the matter is that I don't want Sparrowhawks to be the culprit. This colours my judgement and thus you should be aware of it. I also believe that Sparrowhawks are likely to be a predator of Sparrows but suggest that cats are the pre-eminent cause of Sparrow predation. Pure numbers must win out with 150,000ish Sparrowhawks vs several million cats. Not all Sparrowhawks are in contact with Sparrows but almost all cats will share their territory with sparrows or former haunts of sparrows. Nest predation by corvids/parakeets/woodpeckers/grey squirrels (god I havent mentioned burgeoning numbers of parakeets in london - they must have exploded as sprawks arrived in london!) could well be a factor as all of these have increased since 1970. I believe in predator prey models and whilst sparrowhawks may switch targets when sparrows run short they will strike a balance and more resilient sparrows with a more balanced ecology will return in the long run. I dont believe that in a healthy biodiverse community masses of sparrows are a good thing. They indicate a depleted ecosystem the same as poluted estuaries are home to massive monocultures of tolerant organisms such as some ragworm species. I think those populations of house sparrows that continue to blossom should be looked at the presence absence and relationship with sparrowhawks should be investigated. They do exist - I get tonnes of spadgers in my garden. Chris Bell is probably not a puppet for songbird survival et al but he has some sympathies for them and in terms of his research he definitely has a degree of anthropomorphism going on with the 'poor little sparrows'. The BTO & RSPB have made blaming raptors for this sort of decline almost impossible and in order to remain relevant and credible they need to address this research ASAP fully and with respect as it is credible work despite its weaknesses.
Any comments are welcome. I may make some more if ideas occur...
They might not.
9 comments:
Thouroughly enjoyed that - well done, mate! Have some more meat for the bones....
Whatever happened to the natural balance of predators and their prey species? The more garden feeding stations there are the more you will see Sprawks in your garden if you have them in the area - most places do. What with Sprawks and Magpies allegedly decimating the small bird population what will they eat when they are all gone? We know it won't happen as a balance will be found - prey declines, predators will decline. I very much doubt that any bird of prey will go out with a single thought to catch a certain species of bird - 'must have another one of those Sparrows, YUM!' When these studies are carried out how much time is given over to check the breeding habitat, the changes on the habitat and how long it actually takes for those changes to have an effect on the breeding birds?
I could sit here all morning and rant away but I won't. I'm off out to see if I can see any Sprawks decimating the local bird population!!
Excellent post, James. I must confess not reading the report in full yet, but have read Chris Bell's posts on BirdForum and found his attitude to be a bit off. The biggest flaw for me from what I've read is it seems to take Sparrows and Sparrowhawks in isolation, without taking into account other predators or prey. Are Sparrows even the favoured prey of Sparrowhawks, and are Sparrowhawks even the biggest predators of Sparrows?
Also, again as you say, perhaps the boom in Sparrows was partly due to the fact we'd persecuted all their avian predators for so long, and a reduced population is a more "natural" level?
But I also think politically it would be very difficult for the RSPB et al to accept raptors as an influence on songbird declines, as it would be hard for them to work out which "side" they were on (and especially with captive release schemes of raptors going on all over the place). So sometimes I wonder if they bury their heads over these studies a bit, although I'm not sure (from what I've seen) Chris Bell's report is too persuasive at all, and Songbird Survival should be treated as the rabid nutters they are. But science should be about what the evidence shows, not what's politically convenient.
I'm in the camp that thinks changes in the wider countryside, to do with invertebrates and nest sites, are what's doing for sparrows, and if raptors (or any other predators) play any part at all it's adding extra pressure to already dwindling populations.
I'll read the article by Dr Bell in detail and ramble any more thoughts I have...
What about unleaded petrol? I thought it had been scientifically proved that unleaded petrol had an adverse effect on Sparrow numbers. Doesn't his research indicate that there may be factors other than predation to blame?
I have just re-read the discussion and it pretty much derides all other explanations and promotes sparrowhawks. I remember vaguely the idea that unleaded petrol caused decline but cant remember the science. It is unlikely to be the driving factor as it only came in to being in c1990 yet sparrows have declined since the 70s.
James, at a recent talk by Prof Ian Newton on the Sparrowhawk he confirmed that Sparrowhawk are in fact now in decline after a period of plateau after an initial bonanza post poison etc.
He believes that the Sparrowhawk decline is due to the decline of House Sparrows and other declining passerine prey (as well as Goshawk predation) and that due to the large decline in numbers of such a range of species the Sparrowhawk cannot be the sole reason for the decline in House Sparrows (and other passerines).
Essentially, the decline of the passerines from habitat loss/agricultural change etc (includes House Sparrow) is forcing the Sparrowhwaks decline, i.e. it is not the Sparrowhawk that is soley responsible for the Sparrows decline, in fact it is the other way round...
Very enjoyable commentary, to which I've responded on my Youtube channel www.youtube.com/CultoftheAmateur: see http://www.youtube.com/watch?v=npaz7w22T5I&feature=channel_video_title
Good post, but I can find quite a bit wrong with CP Bell's methodology. One thing is that he uses very small sample sizes and a heavily convoluted spatial pattern that the data quality doesn't really support (i.e. his samples get knocked down into low single figures for some categories). It also relies on the somewhat flawed data of the Garden Birdwatch - this survey has just shown us that small birds seemed to do well last winter, although they suffered terribly in the previous one that was less severe, and also that Blue Tits have undergone a long-term decline over recent decades. All of which is patently untrue according to other/better surveys, but that's what the skewed recording of that survey shows (e.g. it can't detect that cold weather pushes more birds into gardens, but more might still die overall, because it is not recording in the wider area to see what's really going on). So it puts a big question mark over the garden sparrow data too, because it just records birds *in gardens* and ignores anything going on immediately outside of gardens (e.g. cold weather movements into towns, cat avoidance, disease epidemics etc). Add that variation to the small sample sizes (e.g. 4 gardens in one category) and it's a bit ropey.
CP Bell's paper also assumes that urban and rural sparrows are 'equal' whereas in reality one may be acting as a source for another. This would explain CP Bell's and others' findings that sparrows in rural areas declined first, followed by urban ones. So if something nasty is happening in just one area (e.g. rural or ruban) it can turn off the tap of recruits going to the other place, which will then show a decline completely unrelated to where they are and what is going on there. So presuming that sparrow mortality and Sparrowhawk predation is uniform among urban and rural areas is a huge assumption that should have been addressed - because it is very likely untrue and would have big implications for Bell's interpretation of his results.
At the end of the day, CP Bell has shown a correlation, not a mechanism. One could probably contruct a model that found a correlation between colonisation of Buzzards and sparrow decline, especially if you broke the data down into very small sample sizes - do that and you'll get all kinds of potentially spurious results.
Getting a paper into Auk isn't easy, but I suspect that it got an easier ride from reviewers than it would have done in Ibis, because American editors would have been less aware of the intricacies of the sparrow decline and proposed factors (very poorly addressed in CP Bell's introduction and discussion). I note that the co-author/s seem to have distanced themselves from it (refusing to collaborate further), and CP Bell has been publicly scathing about the scientific rigour of one of his own co-authors (which kind of undermines his own paper!), and also that of the world authority on Sparrowhawks (Newton). But CP Bell doesn't seem to have any publishing or research record with either sparrows or sparrowhawks, so has no special expertise of interpretation here. So, what we have instead is a methods paper. Not an ornithological paper. It's interesting more for the method, rather than the result - the sparrowhawk/sparrows bit is incidental because it is not comprehensive enough to nail his case, and the method isn't perfect either (see above). So it is far from the Last Word that CP Bell hopes it is. Until I see better proof, using better data and a clear proven mechanism, it's a potential red herring. Bell suggests that drawing up a correltion with arable change and sparrow declines is misleading because the BTO/RSPB can't show what the mechanism is, but his paper is doing exactly the same thing.
Its probably not a good thing to have HOSP in epidemic proportions but in britain that time has long past.Chris Bell has made a valid correlation between sparrows and hawks, its common sense to me that the relatively rapid appearance of a deadly winged predator in towns where there were none before would have a strong negative impact on sparrow numbers. As sparrows for a proportion of their lives rely on healthy insect populations to sustain their chicks, it stands to reason that sparrows need a healthy diverse habbitat like other birds. I am sure you agree that a varied invertibrate count usually indicates good habitat quality, especialy when those invertibrates are related to plant material. In the days of the horse and cart and coal fires HOSP had virtually no natural predators, and horse dung and feed as well as poor sanitation catered for the sparrows needs. Those circumstances have long since past.The main worry is that sedintry HOSP colonies will become too isolated and will no longer be viable (The Alee Effect).
I would agree that sparrows in epidemic proportions are not a good thing in terms of biodiversity, however in briton that time is long since past. I disagree with your assertion that large numbers of sparrows could indicate a poor environment. As you are aware sparrows rely on a healthy and varied insect population, I am sure that you agree that a high and varied number of native plant related invertibrate would indicate a healthy environment. It is true that in the days of horse drawn transport and coal fires there were more sparrows, this was down to spilt grain, insects form horse dung and probably poor hygene, but this has more to do with Sparrow habbit and its relationship with man and his animals and does not realy have much to do with the quality of the environment at the time. We are now in a different era, if sparrows are around it usualy means there is a good habitat structure and a healthy insect population. Its nice to see more bird variety in gardens but this may well have been caused by pressures in the countryside.
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