3 April 2012

Cat Predation Of Birds

Naughty cat, leave those endangered sparrows alone.

I'm going to get serious now...

As bird numbers have been falling, with the House Sparrow in particular danger, some have looked to predation by domestic cats as a possible cause. There have been a number of research projects that have looked into this. One concentrating on a single village did notice cats had an effect on sparrow numbers, but many other projects show cats are not necessarily a significant cause of bird decline.

A survey in 1997 of the number and type of creatures brought home by cats showed mammals were in more danger than birds.

For the purposes of the project it was assumed that what the family cat dumped on its owner’s bed related to the actual number it killed and perhaps ate outside (a recent study however suggests this is only 30% of the catch). 986 cats took part with 91% bringing something home. Not surprisingly older, fatter cats caught less than more spritely kittens.

Mammals i.e., mice and rabbits (I hope that doesn’t mean the neighbour’s pet rabbit), were the greater animals killed at 69%. Birds were only 24% of the total kills, with amphibians just 1%. [The figures for another survey conducted in 2005 had the numbers as 77%, 15% and 5% respectively]. As you can see birds are not the main prey for cats.

However if the figures are applied to the national population of cats of approximately 9 million then that would mean 27 million birds are killed in a similar period of time.

So what can be done to reduce this predation?

Not attracting birds to your garden in the first place would seem a good idea, but counter intuitively, the number of birds brought home in the study was less where households put food out for birds, though the number of species increased. It is speculated that the reasons for this are:

  • A greater number of birds round a feeding area helps in ‘group vigilance’ 
  • Extra food means birds do not need to forage on the ground 

Maybe putting a bell on kitty’s collar would warn of her coming? It seems not:

"It is possible to speculate that this may be because birds rely largely on visual cues in predator avoidance behaviour, or because the acoustic qualities of cat bells may not lend themselves to warning bird"

You would think feeding your cat up so she isn’t hungry would help, but even this has no effect on the hunting instinct.

"Most urban cats are well-fed, but this does not stop them hunting birds and small mammals. Cats' hunting instincts are largely unconnected to how hungry they are; a well-known experiment established this by putting cats in a box with a large quantity of tasty food, before putting a rat into the box with them. Invariably the cats would find the rat, kill it, carry it back to near their bowl of food, and then continue eating the food."

It seems cats will be cats and there is little to be done bar reducing their number. It has also been argued that cats and birds have evolved together for many thousands of years so birds would have developed strategies to outwit their natural predator.

It will come as no surprise that recent research points the finger at pollution as the main reason for bird population decline, not the domestic cat. A study on the reproductive success of house sparrows within the urban-suburban fringe of Leicestershire shows a more complex problem. It concludes that increased pollution from the greater number of cars is affecting insect numbers

"UK traffic volume has increased by 82% since 1980 (20% since 1990)"

Not only are the chemicals decreasing the availability of insects to eat, but are directly creating smaller birds that are more prone to die from cold or other causes.

"Nutritional stress is also the most plausible explanation for the recent reduction in average maximum brood size from an extensive sample of house sparrow nests from across Britain (Baillie et al., 2007)."

I’m going to include the researchers recommendations in full here (even though it makes this post everso long) because it seems important to me and it surely has implications for many other birds and animals in the urban environment.

"Management to promote invertebrate abundance in urban–suburban habitats is an obvious recommendation to draw from this study, although other unknown limiting factors might still constrain population recovery. House sparrow parents collect most invertebrate prey from deciduous woody vegetation, trees and grassland and make relatively little usage of evergreen or ornamental shrubs (Vincent, 2006). There is evidence of widespread recent losses of greenery from British cities as a consequence of development on green space, tree removal and the conversion of front gardens for parking (e.g. London Assembly, 2001, 2005, 2007). Management should therefore include the planting of native deciduous shrubs and trees especially those providing canopy cover above 2m (Smith et al., 2006). Allowing a diversity of grasses and forbs to set seed and remain in situ throughout the winter should promote the abundance of a range of foliar invertebrates including Dipterous flies, Lepidopteran larvae, spiders and Homopteran bugs (McNeill, 1973; Purvis & Curry, 1981; Morris, 2000). Given the uncertainty concerning the possible impacts of air pollution and/or vehicles on house sparrows, management to promote invertebrate abundance should initially be concentrated away from busy roads. Such advice needs to be targeted at the owners of private gardens and could be incorporated into local authority green space management plans."

It seems if we want to help birds it is not the cats we should limit but our own use of the car... and plant more trees. The eco-warriors are right!


Cat Predation