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

[redeyedvideo]

Has any correlation been drawn on the dramatic decline in Tree Sparrow numbers in the U.K? My local reserve had a healthy population with guaranteed sightings of double figures in most months prior to 2009. None bred at all last year and they were only present in small numbers.

Sparrowhawks are a regular sight with Snipe, Starling & Collared Dove being noted as prey species. I'm not saying they don't take Tree Sparrows but as they are opportunistic hunters any tit or bunting around the feeders often fall prey and their numbers still abound.

 

[Jos Stratford]

I would therefore hazard a guess that the Finnish decline is connected with something that transcends geography and habitat, and climate is the obvious candidate.

Why would you assume climate the obvious candidate?

Do you know what the trend looks like in Sweden or the Baltic states?

Tree Sparrows largely replace House Sparrows in much of this area, including urban. Would you presume your model also should lead to decline in this species?

...we assume that Sparrowhawks take 20-30% of Sparrows in any given locality.

Is there evidence to show this is generally the case?

 

[ColonelBlimp]

This means that predator averse behaviour would be lost, either through cultural learning or by selection for inherited traits. If the latter it may have been difficult for Sparrows to adjust quickly when Sparrowhawks suddenly appeared. There is an interesting paper on the topic freely available here.

The paper refers to behaivoural modification due to reduced or removed predator pressure in island ecologies. It cannot be used as a broad cipher to exemplify sparrow naivete without clarification on whether the overall predator pressure has in fact diminished, or, whether decline in pressure due to sparrowhawk removal can be behaviourally compensated for by increasing pressure from other sources, which have you tackled.

The real elephant in the room here is the political side of the argument. Like it or not, this paper has resulted, and always was going to result, in a lot of inflammatory headlines. 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.

However, are the general public or the media realistically going to understand this issue in terms other than "oh no nasty sparrowhawks!!!!1!!" Are they going to share our implicit understanding that this rebalancing process is positive and 'natural'? If not, releasing this paper without clarification over the implications of the issues raised, and least of all potentially in to hands of types like the Daily Mail and Songbird Survival, seems a little irresponsible.

 

[CAU]

What is the general opinion over there about why your Sparrows are declining? For me the striking thing about the pattern is its uniformity – it looks more or less the same in different parts of the country and in rural and urban habitats, which is in marked contrast to the situation in Britain. I would therefore hazard a guess that the Finnish decline is connected with something that transcends geography and habitat, and climate is the obvious candidate.

Here are some speculated reasons that I've heard of:

1. Changes in the agricultural practices: The numbers of cattle and horses have decreased and the agricultural habitat is more uniform than previously, which has presumably caused a decrease in the number of insects that Sparrows feed to their nestlings (insect food is crucial for young sparrow nestlings). Counterargument: Why would also urban Sparrows decrease?

2. Climatic change: The spring arrives earlier than previously, but the May months have often been colder than previously. Sparrows start breeding earlier than previously, but the cold weather in May may have had an adverse effect on the breeding success. Counterargument: Not all birds breeding in early spring have decreased.

3. Because of changed practices in building houses and in gardening, there are less roosting and nesting sites for Sparrows, as well as less hedges where they seek cover during the days. Counterargument: The decline started pretty suddenly, whereas the changes in the suburban and urban environment have accumulated over a much longer period, and Sparrows have also declined in areas where the environment has changed very little.

4. Competition with Tree Sparrow: Since the decline of House Sparrow started, the population of Tree Sparrow has increased very dramatically. Counterargument: Some researchers suggest that Tree Sparrow is not a direct competitor with House Sparrow. It is also possible that the decrease of House Sparrow has allowed Tree Sparrow to increase. Additionally, the decline of House Sparrow started already in the late 1980s, whereas Tree Sparrow has become common in many areas only during the 21st century. House Sparrow is also declining in other parts of Europe, where Tree Sparrow has not increased.

5. Changes in the winter bird feeding practices: Previously oat was commonly feeded to wintering birds, but nowadays mainly sunflower seeds and peanuts are given. Unlike many other birds, Sparrows prefer oat over sunflower seeds and peanuts. Counterarguments: Sparrows may also eat sunflower seeds. Yellowhammer has not decreased, although it also prefers oat.

6. Some unknown disease or parasite, affecting mainly or only Sparrows, may have caused the decline. Counterargument: No such disease or parasite has yet been detected.

 

[CPBell]

CAU – The factors you’ve listed are similar to the ones proposed in the British literature, though I haven’t seen the ‘cool May’ argument before. Does this mean that winter conditions have been ruled out? The most marked declines appear to have been between 1990-91 and 1998-99, particularly for rural sites in the north, both of which were particularly harsh winters.

Speckled Wood – You mentioned the former habit of House Sparrows of moving into the country to feed on ripening grain. It’s worth pondering the fact that Sparrows feeding in wheat fields are wide open to Sparrowhawk attack, so the disappearance of this habit following the increase in Sparrowhawk numbers is understandable.

CAU, Redeyedvideo and Jos Stratford all mention Tree Sparrows. I would guess the increase in Tree Sparrows in Finland and the Baltic region is part of the general march northward of species with the recent warming trend. Like many other species in the region, Tree Sparrows are migratory at the northern edge of their range, so wouldn’t suffer so badly from cold winters as resident House Sparrows. In Britain the decline in Tree Sparrows began in the mid-1970s at the same time as that in rural House Sparrows, and equally abruptly, but in the end was much more severe, of the order of 90%. The decline betrays the tell-tale pattern of beginning in the west and moving east, so it’s a prime candidate as another Sparrowhawk-related pattern. I would like to get access to monitoring data for the species and do a similar analysis to the one in the House Sparrow paper, but unfortunately the BTO are refusing to release this to me at the moment.

Jos Stratford questions the 20-30% mortality figure in our model, but I don’t think this is unreasonable. The Dutch study that I quoted earlier with 400 sparrows brought to an urban nest in one summer estimated a 20% local Sparrow mortality as a result. To back this up, consider some back of the envelope calculations: A typical rural Sparrow density is 30 per square km, urban density 300 per sq km. If we take a Sparrowhawk hunting territory to cover 4 sq km, this gives a sparrow population of 120 for rural territories, and 1200 for urban territories, so to reach 30% mortality a rural pair would need to take 36 Sparrows in a year, and an urban pair 360. Remember the rural pair would have a much greater diversity of prey to choose from, and the urban estimate is pretty close to the figure emerging from the Dutch study.

http://www.cpbell.co.uk

 

[Jos Stratford]

I would guess the increase in Tree Sparrows in Finland and the Baltic region is part of the general march northward of species with the recent warming trend. Like many other species in the region, Tree Sparrows are migratory at the northern edge of their range, so wouldn’t suffer so badly from cold winters as resident House Sparrows.

Sorry, but for Baltic region under discussion, this is not true. I live here in the Baltics and Tree Sparrows are not migratory, regardless of severity of winter. Furthermore, even further north in Finland, the expansion of population is, in part at least, attributed by some authors (eg here) to relate to increased winter feeding, ie. illustrating they are not migratory there.

In Britain the decline in Tree Sparrows began in the mid-1970s at the same time as that in rural House Sparrows, and equally abruptly, but in the end was much more severe, of the order of 90%. The decline betrays the tell-tale pattern of beginning in the west and moving east, so it’s a prime candidate as another Sparrowhawk-related pattern.

Possible contributing factor maybe, prime candidate I think not. In the UK, I stand to be corrected, but are there not examples where increased winter-feeding and provision of nestboxes has reversed local declines? i.e. suggestive that the cause of decline is not predation.

Moreover, here in the Baltics, Tree Sparrow is abundant, yet so too is Sparrowhawk. At one feeding station I operate, constant year-round feeding has boosted the Tree Sparrow population many times over (from average winter counts of approx 30 birds to counts now reguarly over 120, with corresponding increases in the number of breeding pairs). Yet Sparrowhawk occur throughout the year and, attracted by the large number of birds at the feeders, are particularly common in winter.

Jos Stratford questions the 20-30% mortality figure in our model, but I don’t think this is unreasonable. The Dutch study that I quoted earlier with 400 sparrows brought to an urban nest in one summer estimated a 20% local Sparrow mortality as a result. To back this up, consider some back of the envelope calculations: A typical rural Sparrow density is 30 per square km, urban density 300 per sq km. If we take a Sparrowhawk hunting territory to cover 4 sq km, this gives a sparrow population of 120 for rural territories, and 1200 for urban territories, so to reach 30% mortality a rural pair would need to take 36 Sparrows in a year, and an urban pair 360. Remember the rural pair would have a much greater diversity of prey to choose from, and the urban estimate is pretty close to the figure emerging from the Dutch study.

You have taken one example (where the study estimated 20%) and extrapolated to reach a figure of 20-30% for the level of predation as a whole. Where is the the evidence to suggest that one Dutch example is representative? Even taking your calculations, where is the evidence to support that an average pair of Sparrowhawks is taking 360 House Sparrows? It may be true, but in the absence of evidence, I would be very cautious before accepting an average pair of Sparrowhawks is taking a House Sparrow every day of the year.

Incidently, even though Tree Sparrows are abundant at my feeders, predation by the regular Sparrowhawks appears very low, Great Tit being by far the most frequent casualty (PS Great Tits do not show decline as a result).

 

[lewis20126]

Nickderry, Deboo and Lewis20126 and others mention cats. Cat ownership figures are surprisingly hard to come by, but there is tentative evidence that their numbers might have approximately doubled in Britain since about the 1950s. However there is no reason to believe that the pattern of increase in cats corresponds with that of Sparrow decline in the way that Sparrowhawk does. Why would cats have increased less in the North and West of Britain to produce a less severe Sparrow decline? Why would they have increased later in urban areas than rural areas producing the later urban Sparrow decline? Why would there be a greater increase in cat numbers in the more affluent parts of town? The spurious correlation argument is much more cogent when applied to cats than Sparrowhawks.

http://www.cpbell.co.uk

I would expect cat populations to have greatly increased in urban areas (cf. rural areas) as urban human populations & densities have increased - that's what regeneration of urban brownfield land has created.

Increases in more affluent areas of town? Well people with degrees tend to earn more and are more likely to own cats than dogs (conversely I suspect the rural rich are more likely to own dogs...). Regional variations could be explained in the same way - cat ownership may be a good (if bizarre) proxy for educational attainment and thus salary?

cheers, alan

 

[CPBell]

Jos Stratford – One has to be careful when discussing the migratory status of a population, since it’s not an all or nothing question. According to all the range maps I have to hand, Tree Sparrow populations breeding from Karelia east to at least the Pechora valley are fully migratory. Populations south of about 60-65 degrees north are mapped as resident, but this just means that the species occurs there all the year round, not that the whole population stays put. Migratory behaviour often varies greatly within breeding populations – you can learn more about this here . The reference you quote does mention that Tree Sparrows are more mobile than House Sparrows, which could be crucial in finding food in severe winter weather, such as heavy snowfall.

You’ve made a common error (affirming the consequent) in deducing that if a change in a particular variable appears to reverse a population decline, it must have also caused the population decline by previously moving in the other direction. However, there is no reason why a decline caused by predation cannot be reversed by an increase in food availability. Again though, you are in good company – the RSPB in particular appears blissfully unaware of this principle, as illustrated by its London Sparrow projects here and here. I’ve also no doubt your bird table has stable or increasing Tree Sparrow numbers despite Sparrowhawk attacks – there is nothing contrary to such an outcome in the conclusions that we draw in our paper.

I think you’re being excessively cautious if you doubt the likelihood that a typical pair of urban Sparrowhawks can take 360 Sparrows in a year. The Dutch nest in question received an average of 8, rising to 17 deliveries per day by the male. A bird under such pressure is likely to focus on small birds that can be taken without a struggle, which in an urban environment means House Sparrows. Regardless of this each bird requires 2-3 sparrow sized birds per day, so maintenance for the pair, the growing brood in the nest, and any fledged offspring – do the math.

Lewis20126 – Nice story! However, all we really know is that cats have probably doubled, compared with a four-fold increase in Sparrowhawk, and we have actual data to show that Sprawks vary in parallel with the Sparrow trends, rather than a speculative socio-economic model of cat ownership.

 

[Himalaya]

I'll say my reason for questioning it wasn't because I think Sparrowhakws are cute and fluffy, it just seems bizarre and stupid a predator would overhunt its prey. However, considering how tame sparrows have become, and the fact they were overpopulated, it seems plausible(mind you there are other theories, sadly more glum ones, which are just as possible IMHO), and I would prefer this to a serious problem which might actually take them to below a healthy population.

Some predators can have negative effects on their prey species.

 

[fugl]

Well, this has been a most interesting & illuminating thread. I didn't get around to reading the article until today, but now that I've done so, I must say that I find the authors'conclusions quite convincing. In fact, I find them chastening, since up to now bird of prey predation would been the very last thing I would have guessed responsible for the decline either in the UK or in the States.

 

[DunnoKev]

A very enjoyable debate, with the arguments for both sides being well put forward. As a data footsoldier, I'm interested in one thing CPB said a little while back

..Tree Sparrows.. ..I would like to get access to monitoring data for the species and do a similar analysis to the one in the House Sparrow paper, but unfortunately the BTO are refusing to release this to me at the moment.

Refusing? Have they given grounds for refusal? If so, why?

 

[DunnoKev]

Refusing? Have they given grounds for refusal? If so, why?

Oh, hang on, don't worry, I think I've found out why.

Just delved down through the sub-sub-sub-pages on your website c/o the link in your first post under 'House Sparrow decline' and found where you felt the need to publish your interesting personal correspondence with both the BTO and RSPB after they tried to dismiss your paper as just not good quite enough to win the Independent newspaper's prize re. decline of the House Sparrow..
http://www.cpbell.co.uk/home/House-...awk-hypothesis/-the-independent-sparrow-prize

Especially liked the bit where they point out the same data has already been used elsewhere to show no correlation and you then shoot them down by pointing out how they used unsophisticated methods. (Interesting lead author on their paper btw..) Also perhaps explains why you used terms that I thought were perhaps a little scathing about RSPB's efforts as well in one of your earlier posts, thought that sounded a little less than scientific, but can understand why now.

Look forward to reading more of this debate if it continues, although I suspect these bodies will now defer from further correspondence as public spats like this generally dilute the science for the laymen and footsoldiers out there who they want to have faith in the BTO/RSPB Scientific departments generally getting it right.

(Also hugely disappointed to note Dennis Summers-Smith didn't go along with you. I used to think he was a top chap on Sparrows as well.)

So can I swap my question for another one?

You mention how hard it is to find funding for certain research. Out of interest, was yours funded, if so, are you able to say who funded yours?

DunnoKev
---------
Footsoldier, second-class

 

[Mike Price]

One last thought from me is that I would like to say that I appreciate Dr Bell taking the time to answer my questions and whilst you haven't changed my view that lots of things can and probably do effect the population of House Sparrows, I have enjoyed reading the paper and learning from it.
I do understand that your evidence does fit provided you ignore the variables you cannot measure and that is something that you can live with in the name of science, I am afraid I cannot find someone guilty unless there is 0% doubt and so I am afraid we will have to agree to disagree.
Regards

Mike Price

 

[CAU]

CAU – The factors you’ve listed are similar to the ones proposed in the British literature, though I haven’t seen the ‘cool May’ argument before. Does this mean that winter conditions have been ruled out? The most marked declines appear to have been between 1990-91 and 1998-99, particularly for rural sites in the north, both of which were particularly harsh winters.

I don't know if winter conditions have been ruled out in a peer-reviewed publication, but at least I don't personally buy that reason. Here are the winter mean temperatures for the last 110 years in Finland (Sodankylä is in Lapland and Helsinki at the south coast):
http://www.fmi.fi/kuvat/talven_keskil_1900.gif
As you can see, there were some cold winters at the end of the 1980s, but the decline of the Sparrow continued through the 1990s, and the winter of 1998-99 wasn't particularly cold in Finland. House Sparrow is one of the most hardened Finnish bird species, being resident and still fairly numerous even at Sodankylä (the warmest winters at Sodankylä equal the coldest at Helsinki, so why wouldn't the Sparrows survive such winters at Helsinki when they survive them at Sodankylä?). Additionally, more susceptible species have not started to decline in a similar manner.

The cold May argument may be more specific to Finland, as the May months have perhaps not been unusually cold in other parts of Europe, and the breeding period of Sparrow is also different and longer in the more southern countries. It also seems like at least during the 2000s the mean temperature of May has actually risen in Finland (but perhaps May is still rainier than previously, don't have any statistics on that). If the weather is cold and rainy in May, the Sparrows won't find too many insects to feed to their nestlings, but probably also a lot of other species should be affected in a similar manner, if the argument were true.

One has to be careful when discussing the migratory status of a population, since it’s not an all or nothing question. According to all the range maps I have to hand, Tree Sparrow populations breeding from Karelia east to at least the Pechora valley are fully migratory. Populations south of about 60-65 degrees north are mapped as resident, but this just means that the species occurs there all the year round, not that the whole population stays put. Migratory behaviour often varies greatly within breeding populations – you can learn more about this here . The reference you quote does mention that Tree Sparrows are more mobile than House Sparrows, which could be crucial in finding food in severe winter weather, such as heavy snowfall.

Tree Sparrow is indeed partly migratory in Finland, but only a relatively small fraction of the population migrates (though a larger fraction than of House Sparrow, which also seems to be partly migratory to a very slight degree). The species is resident in at least the Finnish part of Karelia, for example flocks of up to 130 individuals were seen last winter in Northern Karelia. It's true that Tree Sparrow is more mobile than House Sparrow, as overflying flocks are fairly common all year, and after the breeding season some Tree Sparrows disperse to areas where they don't breed.

Anyway, the discussion of the decline of the Finnish House Sparrows is not completely on-topic, as the decline in Britain isn't necessarily related. However, as the species declines almost all over Europe, and the timing of the decline has been about the same, one still wonders whether there should also be some common reason for the decline.

 

[Jos Stratford]

Jos Stratford – One has to be careful when discussing the migratory status of a population, since it’s not an all or nothing question. ...

I was careful, you talked of Tree Sparrows as migratory in Finland and the Baltic region. As CAU said, partly migratory in parts of Finland, but resident elsewhere. Where I reside in the Baltics, the population is not as you suggested migratory, carefully said.

You’ve made a common error (affirming the consequent) in deducing that if a change in a particular variable appears to reverse a population decline, it must have also caused the population decline by previously moving in the other direction. However, there is no reason why a decline caused by predation cannot be reversed by an increase in food availability. Again though, you are in good company – the RSPB in particular appears blissfully unaware of this principle....

Happy to be in the company of such groups as the RSPB. We will have to agree to differ on this, I actually agree with the RSPB and other bodies that believe your research is flawed.

I think you’re being excessively cautious if you doubt the likelihood that a typical pair of urban Sparrowhawks can take 360 Sparrows in a year. The Dutch nest in question received an average of 8, rising to 17 deliveries per day by the male. A bird under such pressure is likely to focus on small birds that can be taken without a struggle, which in an urban environment means House Sparrows. Regardless of this each bird requires 2-3 sparrow sized birds per day, so maintenance for the pair, the growing brood in the nest, and any fledged offspring – do the math

You are merely repeating your earlier conclusion, again based on the example of one single pair, no indication whatsoever if representative.

You now go on to say Sparrowhaws will focus on small birds which in an urban setting means House Sparrows. Does it necessarily mean House Sparrows? Funny, my mum lives in a UK urban setting and has Chaffinches, Greenfinches, Great Tits, etc, etc.

So, I do your maths - if we exclude all other possible prey items (I appreciate you like to exclude other possiblities), then the figure may just reach the result you wish it to.

 

[CPBell]

DunnoKev – Our project wasn’t funded. I feel it’s appropriate to publish the reasons given by the RSPB/BTO as this was done in the course of judging a public competition. Denis Summers-Smith has also offered a critique, but this was indeed private correspondence done offline from the competition, and will remain so unless Denis agrees to make it public.

I can assure you that I’m not motivated by the prospect of personal financial gain. I’ve previously agreed with my co-authors that if we did happen to win the £5,000 prize we would put it towards further research on the topic. In fact my intention had been to challenge the RSPB/BTO/English Nature to each match the £5k, and perhaps fund a PhD student somewhere. A much more important reason is that the Independent declined to report the entry in any way, which is in marked contrast to the extensive coverage it provided to the only other entry their competition has received, the paper by Peach of the RSPB. I’ve been criticised elsewhere on this thread for giving the story to the Daily Mail, but the fact is that the liberal press won’t cover it because it doesn’t follow the narrative of wicked humans despoiling pristine nature. Instead the Guardian/Observer was yesterday still recycling the BTO’s spin on their paper published back in March in the Journal of Applied Ecology, which was linked to by Boy Wonder earlier in the thread.

On the matter of the Tree Sparrow data, I should probably have spelled out in the earlier post that what the BTO has done is to demand money in return for the data. Unfortunately money is the one thing I don’t have, and even if I did I wouldn’t be inclined to pay for the privilege of doing the BTO’s job for it. You can’t blame them for trying to monetise their assets in these tough financial times, but maybe you should also think about enclosing an invoice next time you send them your data? In the meantime, if you’re content to follow the line laid down by the BTO/RSPB as an act of faith, that is your choice. My objective is to engage with those who prefer to make up their own mind.

Mike Price – I’m glad you enjoyed the paper, though I’m sorry it failed to convince you. In the end everything comes down to a matter of personal judgment, and in science above all conclusions are always held provisionally. The most important thing about our conclusions is that they are refutable – data may come to light at any time which disprove them, and this is why I’ve appealed for any data that appear contrary.

CAU – Curious that the temperature data don’t reflect the perception of weather conditions reported elsewhere. Again, however, one has to be careful interpreting averages, since it can often be a short spell of bad weather that does the damage, and cold per se isn’t necessarily the most relevant variable. Ground feeders like the House Sparrow are perhaps more likely to be felled by snowfall than anything else, and there does seem to be a hint of a north-south contrast, with the rural/north contingency indicating two well-defined population crashes dividing periods of slow increase.

It is true that our hypothesis does not have to explain every Sparrow decline everywhere to retain credibility. For this to be the case every Sparrow population outside the area of Sparrowhawk induced decline would have to necessarily be stable, and of course wild bird populations fluctuate all the time. However the challenge is to explain House Sparrow decline as a general phenomenon, and clear exceptions such as you have demonstrated in Finland raise a question mark at the very least. My provisional assessment is that the Finnish decline is qualitatively different from that observed in western Europe and elsewhere, and represents ‘normal’ variation despite being large in terms of proportion of the population. Populations which, like Finnish House Sparrows, are near the edge of a species’ range are always liable to fluctuate more.

 

[Mary]

I agree with many folk- interesting debate. My question would be, is there sufficient data on which to base such a definate conclusion? I believe there are less than 300 contributors to the GBFS throughout the whole of G.B, each of whom spends varying amounts of time recording which species actually take food provided, not just useing the garden. e.g. in the case of house sparrows in my garden, usually only a couple will be feeding at one time, whilst others 'wait their turn' nearby, so the actual population is probably much higher that it appears. Has the same interpretation been applied to GBW data, which has a far higher number of surveyors, and includes the whole garden, all year round?

 

[Capercaillie71]

In Britain the decline in Tree Sparrows began in the mid-1970s at the same time as that in rural House Sparrows, and equally abruptly, but in the end was much more severe, of the order of 90%. The decline betrays the tell-tale pattern of beginning in the west and moving east, so it’s a prime candidate as another Sparrowhawk-related pattern.

Or it could simply be related to the decline of arable farming in the wetter, western parts of Britain that occurred over the same time-scale.

I have no particular problem with the idea that Sparrowhawks may be a factor in the decline of sparrows - after all, predation has been shown to be a factor in regulating other bird populations (particularly gamebirds and waders).

However, it seems to me that predation (by native species at least) very rarely affects prey population in isolation from other factors, particularly those associated with habitat quality. I could envisage that house sparrows are only able to thrive in some 'sub-optimal' urban habitats (with limited cover for example) in the absence of significant predation, and that the recolonisation of sparrowhawks might have tipped the balance. However, is the same likely to be true of all suburban and rural habitats that sparrows inhabit, and if so, why do we not see similar declines in species like Chaffinches and Greenfinches that are likely to be equally vulnerable to sparrowhawks in these situations?

Of course it could be that the population densities of sparrows in some urban areas prior to the decline were 'un-naturally' high (whatever that means in our un-natural environment) in the absence of predation and have simply been reduced to a more 'natural' equilibrium. In the same way, Red Grouse and Grey Partridge populations prior to the 1940s were un-naturally and pathologically high in the almost total absence of predation.

All in all I think it is an interesting paper that does suggest that Sparrowhawks may be an important factor, but I'm not entirely convinced that they are the sole cause of the sparrow decline.

 

[captaincarot]

question withdrawn

 

[CAU]

CAU – Curious that the temperature data don’t reflect the perception of weather conditions reported elsewhere. Again, however, one has to be careful interpreting averages, since it can often be a short spell of bad weather that does the damage, and cold per se isn’t necessarily the most relevant variable. Ground feeders like the House Sparrow are perhaps more likely to be felled by snowfall than anything else, and there does seem to be a hint of a north-south contrast, with the rural/north contingency indicating two well-defined population crashes dividing periods of slow increase.

Well, global weather phenomena are not always observed on a local level. However, like on the global scale, the winters are becoming milder in Finland, and the majority of the other species depending on winter feeding are increasing (the rest are stable).

The winter bird census is performed three times in Finland during the winter, in early winter, mid-winter and late winter, in order to reveal the mortality. Here's a figure for House Sparrow, not showing any large mortality, and no big differences between the different periods:
http://www.fmnh.helsinki.fi/seurannat/talvilintulaskennat/artikkelit/33lajia/kuva29.htm
Again, syys = early winter, talvi = mid-winter and kevät = late winter. The different graphs represent different regions of the country according to this figure:
http://www.fmnh.helsinki.fi/seurannat/talvilintulaskennat/artikkelit/33lajia/02.gif
The dots show the locations of the routes in the census.

Here's the figure for Greenfinch, which is also extremely dependant on winter bird feeding (being more comon in Finland than in Estonia, where winter feeding is less common):
http://www.fmnh.helsinki.fi/seurannat/talvilintulaskennat/artikkelit/33lajia/kuva32.htm
Again, no clear winter mortality is observed, and the numbers are strongly increasing.

Here's the figure for Goldcrest, which is very susceptible to cold winters:
http://www.fmnh.helsinki.fi/seurannat/talvilintulaskennat/artikkelit/33lajia/kuva16.htm
A significant winter mortality is observed.