It is very difficult to assess if domestic cat predation of birds makes a difference to population sizes of the various species in the long term. I’ve said it before that some scientists are frankly biased and their studies are unreliable partly because they do small scale studies and then scale up the figures to represent the entire country. Some scientists have an axe to grind, a hidden agenda to follow.
The fairly recent Smithsonian study produced an estimate of between 430 million and 1.1 billion birds killed annually by domestic cats in mainland USA. The numbers sound startling. But what is actually happening on the ground and is this level of predation (if correct) damaging to bird population sizes?
Taking any one species of bird it may be that there are sufficient numbers of breeding pairs to produce sufficient offspring to maintain population levels despite cat predation. And of course there are the other man-made treats to wildlife such as wind turbines and skyscapers, which impact on bird population sizes. It is a complex issue.
However, many young birds don’t make it out of the nest and many starve during the cold winter months. This is because their metabolism runs very fast and they struggle to store enough food to keep themselves alive overnight.
When a domestic cat returns from a hunting trip, the bird he has in his mouth when he comes through the cat flap may have died from natural causes during the night. The cat simply picked up the body. Cats may simply be collecting birds and other animals that were destined to soon die anyway.
These animals are easy prey for a cat. It is the sort of prey a cat would go for. These animals may be malnourished and ill. A study which examined the condition of prey brought in by cats found that the animals were in general underweight and ill.
The conclusions of studies that I have read have not referred to the possibility that the cat’s prey, birds or small mammals, may have been ill or malnourished. There should be some sort of estimate of the percentage of cat prey items that are in this condition at any one time. This should then be factored in to the study’s conclusion.
If domestic cats are taking birds which are dying anyway of natural causes their impact on bird numbers is much less than as stated.
Source: myself and Cat Sense by Dr Bradshaw.