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On 03/03/2017 11:14, Huge wrote:
On 2017-03-03, Don Pearce wrote:
On Fri, 03 Mar 2017 01:09:42 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Woody wrote:
More likely the abundance of grey squirrels and magpies that steal the
eggs out of the nests - and not just sparrows.

That wouldn't explain the sparrows round here disappearing so quickly.
I assumed it was some sort of virus or whatever. And if it were predators
stealing eggs, why are there so many pigeons?

No, it's farmers. They have killed all the small insects which are the
sparrows' main food source.

In Central London, where Dave lives? Lots of farms there. Not.


There are quite a few, often underground. http://growup.org.uk

--
Graeme Wall
This account not read.

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On Fri, 03 Mar 2017 15:05:57 +0000, Tim Streater
wrote:

In article , Don Pearce
wrote:

On Fri, 03 Mar 2017 14:15:17 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Don Pearce wrote:
Just did some Googling


http://www.independent.co.uk/environ...-vanishing-spa
rrow-1026319.html

Yes - I've read that before. But no explanation for the suddenness of it.
Did farmers start using a new and very powerful insecticide?

And adult birds can live for 20 years (in theory). So a lack of young
replacements would have made their decline gradual?

Yes to the first. For the second, sparrows have their predators like
any other bird and as soon as a decline sets in the balance is upset
to a point where the predators make short work of the remaining birds.

There's no such thing as a "balance" as you put it. Populations of
everything rise and fall for a variety of reasons.

There is a balance- it is dynamic, affected by all sort of factors.
The good old unstable Xi+1 = r * Xi * (1 - Xi) equation describes this
more or less. But when suddenly new generations are not being raised
because a food source has all but vanished, that is severely upset.
The normal predation cycle no longer applies and the remaining birds
are picked off ever more rapidly. Total extinction is rare, and
normally a new dynamic equilibrium will be restored at a much lower
population level. This will be matched some time later by a reduction
in the number of predators.

That is where we are now. The number of garden sparrows is way down,
but not zero. It is at a sustainable level given the number of insects
available.

d


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Dave Plowman (News) said:
Don Pearce wrote:
On 3 Mar 2017 11:14:44 GMT, Huge wrote:
On 2017-03-03, Don Pearce wrote:
On Fri, 03 Mar 2017 01:09:42 +0000 (GMT), "Dave Plowman (News)"
In article ,
Woody wrote:
More likely the abundance of grey squirrels and magpies that steal
the eggs out of the nests - and not just sparrows.

That wouldn't explain the sparrows round here disappearing so
quickly. I assumed it was some sort of virus or whatever. And if it
were predators stealing eggs, why are there so many pigeons?

No, it's farmers. They have killed all the small insects which are
the sparrows' main food source.

In Central London, where Dave lives? Lots of farms there. Not.

Doesn't matter. The surrounding countryside is where the insects bred.
Do you have another explanation for the disappearance of flying
insects?

But other bird species didn't get wiped out so quickly. Do sparrows have a
particular and picky diet? That would be odd for such a previously
successful species? In the average London garden they outnumbered all
others by a very large percentage.

It's a long time since I was in London - what's the pigeon population like ?
Is it possible they're outcompeting other species for the food ? Or seagulls
?



--
Richard Robinson
"The whole plan hinged upon the natural curiosity of potatoes" - S. Lem

My email address is at http://qualmograph.org.uk/contact.html

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On Fri, 03 Mar 2017 09:22:37 -0600, Richard Robinson
wrote:

Dave Plowman (News) said:
Don Pearce wrote:
On 3 Mar 2017 11:14:44 GMT, Huge wrote:
On 2017-03-03, Don Pearce wrote:
On Fri, 03 Mar 2017 01:09:42 +0000 (GMT), "Dave Plowman (News)"
In article ,
Woody wrote:
More likely the abundance of grey squirrels and magpies that steal
the eggs out of the nests - and not just sparrows.

That wouldn't explain the sparrows round here disappearing so
quickly. I assumed it was some sort of virus or whatever. And if it
were predators stealing eggs, why are there so many pigeons?

No, it's farmers. They have killed all the small insects which are
the sparrows' main food source.

In Central London, where Dave lives? Lots of farms there. Not.

Doesn't matter. The surrounding countryside is where the insects bred.
Do you have another explanation for the disappearance of flying
insects?

But other bird species didn't get wiped out so quickly. Do sparrows have a
particular and picky diet? That would be odd for such a previously
successful species? In the average London garden they outnumbered all
others by a very large percentage.

It's a long time since I was in London - what's the pigeon population like ?
Is it possible they're outcompeting other species for the food ? Or seagulls
?

Pigeons are almost gone - certainly compared to 30 years ago. There
are some seagulls, but they aren't the menace I was expecting them to
be.

d

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On Fri, 03 Mar 2017 13:15:28 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Fri, 03 Mar 2017 11:36:25 GMT, (Don Pearce) wrote:

On Fri, 03 Mar 2017 11:27:38 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article , Don Pearce
wrote:
That wouldn't explain the sparrows round here disappearing so
quickly. I assumed it was some sort of virus or whatever. And if it
were predators stealing eggs, why are there so many pigeons?

No, it's farmers. They have killed all the small insects which are
the sparrows' main food source.

Again, near overnight? And in central London?

Yes.

d

Just did some Googling

http://www.independent.co.uk/environ...w-1026319.html

What isn't clear from a quick read of that is why the effect might be
disproportionately severe in London, which probably had fewer insects than
the countryside before the decline outside London.

The thought that occurred to me is that the air pollution might also weaken
the birds. But I didn't see any attempt to correlate against that.

Jim

I don't think it is, particularly. It is just that London gardens are
the places most people are used to seeing sparrows up close. In the
countryside they are just another small bird. I don't think air
pollution has much to do with it. I live on top of the hill in
Hampstead where the air is particularly good (for London), and
sparrows are rare here. But apart from bees and butterflies, flying
insects are all but non-existent. So the sparrows have no food here.

And of course as far as insects are concerned, London is just an
island in their world of farmed land surrounding us. If the insects
vanish from the farmland, they are gone from London in short order.

d

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In article ,
Don Pearce wrote:
They may theoretically live to 20, but very few did so a decline over
four or five years fits the bill very well.

But they didn't decline over 4 or 5 years here - that was my point. It
happened very quickly. Over a matter of months at best - maybe even
shorter.

--
*Why is a boxing ring square?

Dave Plowman London SW
To e-mail, change noise into sound.

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In article ,
Don Pearce wrote:
It's a long time since I was in London - what's the pigeon population
like ? Is it possible they're outcompeting other species for the food ?
Or seagulls ?

Pigeons are almost gone - certainly compared to 30 years ago.

Not round here. Still as common as ever. Got parakeets now too.

--
*Red meat is not bad for you. Fuzzy green meat is bad for you.

Dave Plowman London SW
To e-mail, change noise into sound.

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On Fri, 03 Mar 2017 17:08:55 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Don Pearce wrote:
They may theoretically live to 20, but very few did so a decline over
four or five years fits the bill very well.

But they didn't decline over 4 or 5 years here - that was my point. It
happened very quickly. Over a matter of months at best - maybe even
shorter.

Maybe where you are - here it took a little longer. But if the food
disappears during one season, then that is exactly what will happen.

d

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Don Pearce said:
On Fri, 03 Mar 2017 09:22:37 -0600, Richard Robinson

It's a long time since I was in London - what's the pigeon population like ?
Is it possible they're outcompeting other species for the food ? Or seagulls
?

Pigeons are almost gone - certainly compared to 30 years ago. There
are some seagulls, but they aren't the menace I was expecting them to
be.

Okay ... London really is a unique place, then. Would you like some of ours
?


--
Richard Robinson
"The whole plan hinged upon the natural curiosity of potatoes" - S. Lem

My email address is at http://qualmograph.org.uk/contact.html

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On Fri, 03 Mar 2017 17:19:36 GMT, (Don Pearce) wrote:

On Fri, 03 Mar 2017 17:08:55 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Don Pearce wrote:
They may theoretically live to 20, but very few did so a decline over
four or five years fits the bill very well.

But they didn't decline over 4 or 5 years here - that was my point. It
happened very quickly. Over a matter of months at best - maybe even
shorter.

Maybe where you are - here it took a little longer. But if the food
disappears during one season, then that is exactly what will happen.

d

There are plenty of population graphs out there. It seems that the
decline was pretty much complete over a span of ten years (which was
about what I remembered) starting about 1977 and reaching the present
level about 1987. So clearly it is Maggie Thatcher to blame.

d

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