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

[Tringa]

Cats catch birds and other animals. I don't doubt this for a second and I think every cat that is not indoors 24 hr/day will catch and kill some animals, some of which I'm sure will be birds. I don't have any evidence to back that up, but I still think it is a reasonable assumption. I don't know how many birds are killed by cats every year, but 55 million is a figure that seems to be mentioned.

The important thing is, as has been said earlier in this thread, whether, in catching birds, cats have an effect on the populations of birds, rather than do they have an effect at all.

I can imagine that a cat close to an area of ground nesting birds could have an impact on the population, similar to the effect of non-native species introduced (either deliberately or accidentally) to islands.

How much of an effect cats have on bird populations in general is less clear. Although cats are not native they have been in Britain for many hundreds of years so their effect on bird populations could have been going on for years and to some extent birds and cats have developed together.

However the number of cats has increased over the years and this could have an impact on bird populations. Cat numbers are also artificially maintained by feeding by humans which could also have an impact, but commercial cat food has been produced for many years, so any impact of a well fed cat population should, perhaps, have been seen some time ago.

The only information I have on cats and birds comes from home. I have lived in my current house for 31 years and have seen a few changes, though these are obviously limited in area.

Over these years I've seen a decline and then, recently, a small increase in house sparrows. The numbers of many garden birds have remained largely unchanged. The changes have been

a decrease in kestrels, tawny owls and house martins, and

an increase in sparrowhawks, goldfinches and green woodpeckers.

For ten of these years a next door neighbour had seven cats. Three of the cats because, I assume, they did not like the other cats, almost lived in our garden and house. They caught birds and other animals but I did not see a reduction in numbers.

I think that cats do not have an impact on the populations of birds but that is a view based on limited information and I accept that my view may be incorrect. I do not know the number of cats in Britain but about 8 million is suggested by pet food organisations. This means each cat kills about 7 birds/year.

However, I think that we (ie every contributor to the forum) should resist going one road or the other because, I think, there is not enough evidence to support either position.

I have looked for studies on cat predation. Here are a couple

Domestic Cat Predation on Wildlife


Urban bird declines and the fear of cats - Beckerman - 2007 - Animal Conservation - Wiley Online Library

They have some interesting observations but, I think we still have a way to go before we have a definitive line on cats and birds.

Dave

[Dorts]

We live in a small village in the middle of the Snowdonia National Park. The population density here is very low. When we first moved here 6 years ago there was a small population of House Sparrows of around 10-15 birds. This population is now zero. I have not seen a single Sparrow Hawk since being here. We have no cats. We do have a road through the village, though traffic volume would by most standards be classed as low.
Nowhere in this part of the country are House Sparrows common.
Apart from last years very hard winter, I have no answer as to why our Sparrows have disappeared. Food supply should not have been a problem as we have chickens, so there is always plenty of spare grain around.
I think the reasons for their disappearance here would be pure guesswork, but seem most unlikely to be down to any of the reasons so far raised.
Dorts.

[King Edward]

Tringa,
Good post, and two very interesting links.

I have no desire to reopen the debate into the particulars of the Sparrowhawk paper, since that's been well discussed on Birdforum, but reading Tringa's two cat-related links is quite informative about the possible role of Sparrowhawks in the decline of Sparrows.

Firstly, cats are estimated to kill around 27 million birds/year (plus other species), while Sparrowhawks are likely to be taking somewhere in the region of 50-100 million (as a conservative estimate, it could quite possibly be more). Admittedly cats are likely to kill a far greater percentage of sparrows, since both cats and sparrows are concentrated around houses/gardens, but even so it seems difficult to point the finger at one while ignoring the other.

Secondly, the figures for cat/hawk predation are additive - I find it quite plausible that, for a sparrow population already under pressure from cats and declines in food availability and (maybe) habitat quality, the added pressure from hawk predation could tip the balance from population stability into rapid decline.

Thirdly, the abstract in Tringa's second link would make just as much sense if 'cat' was replaced by 'hawk' (or even 'cat & hawk') - the indirect effects of both predator species need to be considered. Indeed, it would be surprising if this wasn't the case - for a small bird, needing to continually find large amounts of food to survive and breed, I can easily see that the need to remain constantly vigilant, keep body fat levels low (for rapid escape flight) and forage only near cover could be at least as important as direct predation effects.

[tigertom]

I have already posted that house sparrow are increasing round were I live yet between the ten houses nearest to me their are 15 cats+ 2 ferals and nine of these cats are on my block alone 3 of them mine out of the nine on my block 4 of them roam Free day and night and so do 4 others along with the 2 ferals thats 9 cats roaming and free to hunt yet house sparrows hear are on the up and as far as sparrows visiting my garden by 100% in 2 years which would surgest to me cats is not the reason for their decline in the 1st place.
MIKE

[RoyW]

Originally Posted by Deb London

Apart from the cat talk, nice discussion all. Ta.

Yeah, sorry about that...

Originally Posted by Tringa

However, I think that we (ie every contributor to the forum) should resist going one road or the other because, I think, there is not enough evidence to support either position.

I agree with this.
A lack of evidence either way means that any opinion is largely guesswork.

Originally Posted by King Edward

Secondly, the figures for cat/hawk predation are additive - I find it quite plausible that, for a sparrow population already under pressure from cats and declines in food availability and (maybe) habitat quality, the added pressure from hawk predation could tip the balance from population stability into rapid decline.

I also agree with this.
My personal opinion is that no good case has really been made for any single factor being responsible for the fall in House Sparrow numbers. If a combination of different factors are involved, which seems plausible, this would be very difficult to prove - and potentially the balance of each could be different in different areas (it's even possible that petrol additives are part of the reasons, but there is no good reason to consider this to be the case based on available evidence!).
It is perhaps encouraging that the majority of observers that have answered the poll are under the impression that House Sparrow numbers have recently increased in their area, perhaps a new population balance has been reached?

[Deb London]

No worries Roy. Since I don't blame cats in this instance (having tried to imagine how a cat would predate a nest of House Sparrows and failed ) I just glance at the cat discussions and move on.

Originally Posted by Dorts

We live in a small village in the middle of the Snowdonia National Park. The population density here is very low. When we first moved here 6 years ago there was a small population of House Sparrows of around 10-15 birds. This population is now zero. I have not seen a single Sparrow Hawk since being here. We have no cats. We do have a road through the village, though traffic volume would by most standards be classed as low.
Nowhere in this part of the country are House Sparrows common.
Apart from last years very hard winter, I have no answer as to why our Sparrows have disappeared. Food supply should not have been a problem as we have chickens, so there is always plenty of spare grain around.
I think the reasons for their disappearance here would be pure guesswork, but seem most unlikely to be down to any of the reasons so far raised.
Dorts.

INTERESTING! Cases like these need to be studied. As do populations in other countries.

Crazy ideas
I found the following article, in which Pattazhy points the finger at mobile phone towers as the reason for the decline in sparrows there:

House sparrow listed as an endangered species

I don't for one second think it's this, but this example serves to illustrate that the cause is such a mysterious, worldwide problem that weird and wonderful ideas have been and will be proposed by some people.

Turning the problem on its head
It was not until I followed a programme on why thin people were thin that I suddenly understood how obesity works. If the sparrows number are really starting to increase in some areas (I'm not saying they are, just speaking hypothetically) then I think this should be studied too. I think it is fair to say that while some people have ideas as to why House Sparrows declined, nothing is known for sure, leading to complicated hypotheses implicating multiple causes. If numbers of these birds recover in some areas, I think it's an exciting opportunity to put some of these ideas to the test. A second chance to discover what really controls the numbers of these birds in different environments. And not just because it matters from an ecological point of view, but also because it is such an interesting conundrum in its own right.