Monday, 28 March 2011
Declines in House Sparrows
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.
How birds and brains become mutually exclusive