Friday, 1 April 2011

Response by Chris Bell

I have had a Youtube response to my posting on House Sparrows by the author. It analyses my posting and discusses in depth the idea I touched upon of House Sparrows as indicators of a depleted ecosystem.


P said...

Seems like a decent chap, and good on him for replying, and taking the rough with the smooth there, arguing his point in a well-mannered way.

I would have hoped he'd have known the cat situation better than he did, and everything he said there about them seemed to be guesswork. Saying cats are fed Whiskas every night is true in the sense they're not killing for every meal, but even so everyone who's owned or lived near a cat knows they can (depending on the individual cat's nature, its age, its sex, whether its been neutered or not, whether it wears a bell etc.) take a regrettable number of birds and small mammals, simply to satiate their hunting instincts, not matter how full their bellies. My instinct is that cats can't be a main factor in any particular species' decline (hypocritically, with no data to back me up!), due to the factors of variance between cats I've already mentioned (some cats kill nothing, some kill loads), and also their indiscriminate prey tastes. But if pushing causal links between a particular predator in the decline of sparrows, ruling out the significance of an increase in cat ownership, without exploring the data, seems rash.

Richard said...

The Mammal Society's study found that birds accounted for a quarter of all prey items brought in by cats, and they extrapolated this to a figure of 27 million birds killed annually by domestic cats. House Sparrows were by far the most numerous prey species of bird. Using their figures, one can estimate that domestic cats alone would kill about 8 million House Sparrows annually.

CP Bell rightly states that many of these cats will be fed, and this may well reduce their predation overall. But that's still 8 million sparrows. Then we have feral cats, of which defra estimates there are 1.2 million in the UK. These live in rural and urban areas, and are pervasive enough to threaten the Scottish wildcat (so one cannot claim that they're just an urban phenomenon). Feral cats are generally not fed, so one can assume that their individual predation rates will be higher than for domestic cats. Domestic cats outnumber ferals by about 7 to 1, but feral cats outnumber Sparrowhawks by about 10 to 1 in summer. If we conservatively assume that ferals kill sparrows at the same rate domestics (1/year), that's another million = 9 million sparrows falling prey to all cats annually.

There is little information on the number of sparrows in the Sparrowhawk diet. Newton, in SCotland, found it was in the single-figure % of the diet. If we assume that sparrowhawks only eat sparrow-sized birds, and extrapolate that across the population (39,000 pairs) and their annual requirments (2,200 'sparrow-sized' birds, cf Newton 1986), that gives us a total number of c.5 million Sparrows killed by Sparrowhawks. In other words, half the number killed by cats each year. So, far from 'not stacking up', these numbers start to look interesting, even if we allow much more sparrow predation from Sparrowhawks. The Mammal Society study discussion also contains some references on sparrows, quoting a study that concluded cat predation accounted for 30% of sparrow mortality in a typical English village.

So, empirical evidence has shown that cats can be a significant cause of sparrow mortality. Sparrows are their most common avian prey. Both domestic and feral cats vastly outnumber Sparrowhawks. And simple calculations give a figure for cat predation that is double that for Sparrowhawks. Yet this is not worth considering further? That set of circumstantial evidence is at least as compelling as the set of cirumstantial evidence underlining the Auk paper.

James said...

I am inclined to agree that dismissing the cat situation was a large oversight - I was recalling those figures as I typed the original criticism in my first posting. Also rural cats are employed as vermin killers and thus are usually extremely adept at catching birds.

Unknown said...

I have in the past done a bit of research on the cat thing, but unfortunately there doesn't seem to be any good information on trends in cat numbers and distribution, so the theory is difficult to test.

The pollution/urban bird diversity theory is eminently testable however. It shouldn't be too difficult to get breeding bird lists for most cities around the world, and then it would just be a matter of regressing against a pollution index, along with confounding factors such as local avian diversity, city area, proportion of green space etc. With a bit more work it might be possible to look at the effect on diversity indices such as evenness.

It's such an obvious thing, I'd be amazed if it hasn't already been done.

Alan Whitehead said...

Totally unrelated to your post James. I couldn't accessa nother way of contacting you! I'm off to SW Spain on Sat and noticed that you had some info on Rufous Bushchat. I wondered if you could help with possible sites?

Cheers Alan

How brains and birds become mutually exclusive