The Causes of British House Sparrow Declines (732 hits)
Category: Science & EnvironmentalRating: 1.25 on 10 reviews (Rate this item) (V)
Submitted by Martyn Steiner (View user info) at 2004-09-29 16:05:36 EDT
More and more, interested members of the British public are noticing the national decline in house sparrow numbers. This once common species has suffered severe population set-backs in the last twenty years, with numbers falling from 25 million in the early 1970s to just over 13 million in 2004. Despite such shocking statistics, the cause of the fall in numbers is still not fully understood, though many theories have been proposed.
In order to get a clearer idea of exactly what it is that has created the problem, it is perhaps more sensible to consider two distinct groups of house sparrow - rural and urban. The issue is one of far greater significance in towns and cities, and given the habits of sparrows, it is clear why. One of the most frequently used explanations of the sparrow reduction is the blocking-up of eaves in modern house construction, thereby removing the number of possible sparrow nest-sites. And nest sites are not just lost in and around houses. Fashionable garden makeovers that remove potential nest-sites are increasingly popular, forcing birds to search further afield for locations.
An extremely emotive issue has been created in the debate over the effects that domestic cats may have on songbird numbers, with feline-fanciers vehemently denying any involvement that their pets have in the loss of birds. However, the evidence supporting claims that sparrow and cat numbers are inextricably connected does seem overwhelming. There is no denying that cats will hunt and kill sparrows, and in the same time period that sparrow numbers have fallen by twelve million, domestic cat numbers have risen by 2.6 million; If each cat killed just five sparrows in its lifetime, the decline is already fully accounted for.
It is not only sparrow numbers that have fallen in urban gardens; evidence suggests that the small invertebrates on which they feed are also suffering from the changes brought about by modern human lifestyles. It only takes a simple understanding of food chains to realise that this loss can only make the sparrow's troubles worse.
The removal of a large part of their diet is perhaps an even greater problem for country sparrows than it is for city-dwellers, with the quantity of chemical pesticides used in agriculture rocketing in recent times. In the past, sparrows would feast on seeds dropped by farm machinery and were able to enter grain stores to supplement their diet. However, modern combine harvesting produces less spillage and strict farm hygiene rules result in tightly sealed silos that prevent sparrow access.
An almost equally contentious issue takes flight in the form of the sparrowhawk. Intense arguments are fought with regard to how much difference the hawk has on its namesake. Equally impressive links can be shown between sparrow and sparrowhawk numbers as for cats and sparrows, though in this case there is evidence that suggests the figures may be coincidental. Sparrowhawks eat most small birds, and thus one would expect that numbers of all their prey would fall as hawk populations increased, yet this is not the case. The RSPB contend that as with all predatory-prey relationships, a decrease in sparrow numbers would decrease sparrow hawk numbers and that the sparrow hawks would "starve to death before a significant drop in overall prey numbers" occurred. However, organisations such as SongBird Survival argue, "predation by sparrowhawks is one major factor in... the disappearance of the house-sparrow".
Although house sparrows are able to raise up to three broods per year, their major difficulty is in ensuring chicks survive to maturity. Sparrows struggle to find nest sites and then once they do, food shortages result in parents and or chicks starving. It is an ironic twist of fate that an animal whose success has been built on its relationship with humans should suffer so much at the hands of our success.
Sources;
www.rspb.org.uk/birds/guide/s/sparrowhawk/sparrowhawk_-_a_top_predator.asp
news.bbc.co.uk/1/hi/uk/1965499.stm
www.defra.gov.uk/wildlife-countryside/ewd/sparrows.pdf
www.bto.org/appeals/HS%20info.pdf.
www.rspb.org.uk/birdwatch/2003/house_sparrows_and_starlings.asp
User Reviews
Submitted by GodLovesALittleLovin (user info) at 2004-09-29 22:50:16 EDT (#)
Ranking: 1
Yeah, I'm a tad confused why anyone would care about these ugly birds, but your past stuff was good and I can see much thought was put forth.
Need to just fry 'em all up and make some delicious chicen wings for all. I would eat a parrot wing if it was fried and dipped in Buffalo sauce. That would taste amazing i'm sure.
Am I rambling? Yeah, well fuck it.
Submitted by Random Joe at 2004-09-29 17:25:46 EDT (#)
Ranking: 0
For whatever it's worth, that is my take on it. If I had some good references I could have expounded the problem a little more. oh... I'm not shooting down your references, I just haven't looked at them.
By the way...good job, I really enjoyed reading and responding to this.
Submitted by Random Joe at 2004-09-29 17:25:34 EDT (#)
Ranking: 0
I'm not too familiar with the sparrowhawks life history. I'm going to assume that it is a generalist predator and I do not believe that "a decrease in sparrow numbers would decrease sparrow hawk numbers." Reason being that anyone who studies predator-prey relationships understands functional responses(type I, II, and III) and numerical responses, which are ways that predator populations change in respect to prey denisty. Simply, being a generalist, which many birds of prey are, they would simply switch to another prey item more available. Predator-prey theory is complex and I'm going to go into it, but when house sparrows numbers fall significantly enough that the encounter rate drops the sparrow hawks will switch to another prey item. Once the house sparrow population increases and the encounter rate follows, sparrow hawks will switch back. Now, throwing the house cat into the picture (a predator that can locate nests and inflict mortality on chicks, something sparrow hawks don't) then the population can be held in low densities or a predator pit. This may be occuring because of the high density of feral cat populations. Sparrow hawks are not a new predator to house sparrows and they probably have evolved to deal with sparrow hawk pressures but not feral cats.
Submitted by Random Joe at 2004-09-29 17:24:38 EDT (#)
Ranking: 0
The cat arguement holds up well. Feral cats pound the hell out of local populations of birds. The best statistics to get on this is to check with wildlife rehabilitation centers. I've seen them and they are outrageously high. I would lean towards this as an influential factor on your house sparrow's decline.
But the arguement that you make about a decline of sparrow's due to less urban gardens and less spillage from modern agricultural practice is one that is ambiguous because these are not natural food sources for the birds and keep them artifically high above a natural systems capacity to hold them. Perhaps because of these factors the population declining to a more natural size.
Submitted by Random Joe at 2004-09-29 17:23:44 EDT (#)
Ranking: 2
Before I go on with my reply, when you folks find the problem put in a bottle and send it over here to the States because we are infested with these exotics. They are everywhere over here.
Decline in species populations are often hard to pin point because the decline is usually caused by a number factors that affect those populations survival rates, birth rates, death rates, migration among and within populations, resource quantity and quality, territory size, sex ratio, etc.
I wouldn't consider house sparrows that occupy rural areas and urban areas as two distinct populations because they overlap and interbreed with each other. If they were sexually isolated and differed slightly in some traits I would classify them as two distinct populations. What you should consider is a species that is occupying two different types of habitats... rural and urban. Even so, those two terms are vague.
Submitted by dodahdave (user info) at 2004-09-29 16:35:32 EDT (#)
Ranking: 2
Well written and referenced! I agree that diversification is what's needed here, and you've made a good start.
+2.
Submitted by MaximusPadus (user info) at 2004-09-29 16:15:39 EDT (#)
Ranking: 2
We need more people like you. Next do a piece on "Those really long turds that, to your dismay, automatically go down the hole of the toilet, and when you get up, it is not there leaving you to say wtf!!"
Submitted by Jack_McCallum (user info) at 2004-09-29 16:14:39 EDT (#)
Ranking: 0
The old cat argument is total bullshit. We have cut into their environment, reduced their food and living space, and poisoned them with pesticides. That's why they are dying.
Global warming isn't helping either. Many UK bird species (pity the seabirds) are having a terrible year because their food sources are not longer available.
Submitted by Random Joe at 2004-09-29 16:09:53 EDT (#)
Ranking: 0
do not care
Submitted by Martyn_Steiner (user info) at 2004-09-29 16:07:16 EDT (#)
Ranking: 0
I realise this is not likely to interest many ubersite people, but I felt that the type of submissions needed to be diversified. If you want to read some mindless entertainment, check out my previous posts. Love you.