The fall and rise of House Sparrows - is petrol involved?

[Deb London]

Nearly a year ago now I heard it suggested on a bird walk that the decline in House Sparrows had something to do with an additive in petrol. Is this a chemical called MTBE? Does it affect the abundance of insects for feeding young? Does anyone support this idea?

Furthermore, I'm fairly certain that House Sparrow numbers are higher in my street than at this time last year. And I have heard them in areas where I haven't before. There seems to be hope. I have read similar in other member's posts. Does anyone else agree with this observation?

If there really has been a rise in numbers of House Sparrows, does this support or go against the idea that an additive in petrol is to blame? Has anyone any petrol-based theories?

[poschiavanus]

Declines in House Sparrow populations are, I believe, mainly a British phenomenon.

In Swiss cities House Sparrows are very apparent even in the middle of town. Often the smallest shrub would be full of Sparrows (admittedly probably subsisting off McDonalds, Pizza and Bratwurst ). We had a huge breeding population in the building opposite our office which was in an industrial area between two arterial roads. The fledglings would usually manage their first flight on to the window ledges of the office.

Also Tree Sparrows are still common in Switzerland despite a highly-manicured countryside. It was always noticeable to walk out of a village and after 100 m or so House Sparrows would be replaced by Tree Sparrows.

[RoyW]

The "petrol hypothesis" was that methyl tertiary-butyl ether, an additive to unleaded petrol, was somehow affecting the sparrows. It is apparently a known carcinogen, but was expected to burnt up completely during the combustion of the petrol. As far as I know there is no good evidence that this hypothesis has any real substance to it.



Declines in House Sparrow numbers is not solely a British thing, and large falls in numbers have been noted in many other European cities:
First they disappeared from Britain. Now Europe's house sparrows have vanished - Environment - The Independent

[rossy]

one of the reasons that house sparrows are not as numerous in the cities is , nesting sites, and habitat ie food source, round our end they are plentiful, and numbers have increased , just had 40 in the garden again, and roy this isnt having a go, but i read what you posted on the rspb garden watch, stating their is no evidence that cats have any impact on garden birds, well over 55 million that have been predated, is a unbelievable figure, and thats just an estimation, so i do think they are a huge problem, and think of this house sparrows nest in mainly houses , and feed mainly in our gardens, to where cats roam , hide cause damage to property, and KILL BIRDS. ie house sparrows, in particular the young ones, because obviously they are very vulnerable when they leave the nest. its not rocket science, its fact. its not natural ,read one article of where they did a study , to where there was a high density of cats, and bird numbers where significantly lower, than in areas where there was no cats. rossy.

[tigertom]

Plenty of house sparrows now round my way very much on the up hear 2 years ago would see very few also of note tree sparrows not doing to bad on my local patches as well with good numbers now again a few years back you would be lucky to see any.
But there is plenty of nest sites for them bird box's and nice gaps under a lot of the roofs down to type of roofing tiles used (something to do with fact there are bats in some roofs on my estate in summer)
Would also state that were I live cats do not seem to have much affect on them 15 cats + 2 feral toms all live within 20mts of me though only 8+the 2 feral's roam freely the 2 feral's also do get fed well by one locals.
MIKE

[Tringa]

The evidence of the decline of House Sparrows and MTBE seems circumstantial. Even the main proponent of the theory accepts this -

Lead-free petrol may be villain in mystery of demise of the world's most familiar bird - Environment - The Independent

If MTBE was the cause of the decline in House Sparrow then would it not also have affected the numbers of other birds in cities - correlation does not imply causation.

It would have be more useful if the bird count in Kensington Gardens, as reported in the above article, had included other birds and not just house sparrows.

Around my way house sparrows appear to have increased over the last few years. About five years ago we hardly saw any, now, while not the commonest bird in the garden, they are regular visitors. Of course this could be an illusion, there may have been no increase in the wider area and the sparrows I see now have simply moved in from somewhere else.

Dave

[RoyW]

Originally Posted by rossy

and roy this isnt having a go, but i read what you posted on the rspb garden watch, stating their is no evidence that cats have any impact on garden birds, well over 55 million that have been predated, is a unbelievable figure, and thats just an estimation, so i do think they are a huge problem

Cats certainly kill large numbers of birds (and other wildlife), but even though studies have suggested that 55 milion birds a year might be killed by cats (it may be many more, but it could also be considerably less - estimates can be too high as well as too low), there is still no scientific evidence that they are causing a decline in bird populations that would give the RSPB sufficient grounds to campaign for changes in the law to try and prevent this killing (which is the point I was making on the other thread).
55 million birds killed might sound like a lot, but if we take just the ten top garden birds from last years Big Garden Birdwatch, these 10 combined have a UK breeding population of about 30 million pairs, so 60 million adults. Each breeding pair will probably fledge at least 5 young on average each year, in many cases 10 or more, so that's in excess of 150 million young, perhaps as many as 300 million or more, in addition to the 60 million adults. Millions of immigrants of these 10 species will then arrive from the north and east during the autumn to spend the winter here further adding to the numbers.
...and then we need to take into account all the other species of bird that are common and widespread in towns and countryside.

It is quite likely that cats do have an effect on some local bird populations, and it may well be that they are having an effect on House sparrow populations that are already struggling, but overall there is no evidence that the number of birds killed is a significant cause of the decline of any species. This doesn't of course mean that it is OK to let cats out so that they can kill wildlife (and there is evidence that they can have a significant detrimental effect on lizard populations in some areas!).

[RoyW]

Originally Posted by Tringa

If MTBE was the cause of the decline in House Sparrow then would it not also have affected the numbers of other birds in cities - correlation does not imply causation.

Correlation certainly doesn't mean that there really is a link.

Another researcher has tried to argue that the post DDT increase in Sparrowhawk numbers is the cause of the decline in House Sparrows, suggesting that living without Sparrowhawks for many generations may have made them less wary than they need to be now that Sparrowhawks have moved back into towns. This is another case where there is a correlation (decreasing House Sparrow numbers, correlate well with increasing Sparrowhawks), but again their is likely to be no actual link.

A lot of studies have been carried out on House Sparrow in recent years, but we still don't really have any idea why they are falling in numbers - it's most likely due to a combination of factors.

[King Edward]

One needs to be careful with the word 'correlation'. In the petrol case, the 'correlation' is mainly that the introduction of unleaded petrol (containing potentially toxic additives) coincided broadly in time with the decline in Sparrow numbers, and also that the decline was apparently greater in large cities (with more cars, hence more pollution). The evidence here is not very strong, and may well be entirely coincidental.

In the Sparrowhawk study, the correlation was derived from counts of both species at 264 individual garden feeding stations i.e. the decline of Sparrows at a particular location was strongly associated with the arrival (recolonisation) of Sparrowhawks at the same location. This is quite a robust methodology, and not easily dismissed on the grounds of being 'just a correlation'. There is also, of course, a very plausible mechanism for the link in that Sparrowhawks have been found (in other studies) to feed preferentially on Sparrows (eating many more than would be predicted by their abundance in relation to other prey species).

This has been discussed extensively already on Birdforum (here) with just about every conceivable argument / counter argument raised.

[Kayleigh]

The Spawk has not had a sparrow in my garden for well over a year I moved the bird table as it was in the open to the about a 1ft away from cover.
The Sparrows seem happier.