A study done by the Mammal Society http://www.mammal.org.uk/sites/default/files/Domestic%20Cat%20Predation%20on%20Wildlife.pdf
animals per year. The study was done in 1997, and is only a sampling
of slightly more than 900 people (not uncommon for such a test).
However, it's problematic to blame cats for the sole decline of
species such as birds. For one thing, most baby wild chicks
will die before they can leave the nest.
The RSPB states that millions of baby birds will die
before they have a chance to leave:
http://www.rspb.org.uk/makeahomeforwildlife/advice/gardening/unwantedvisitors/cats/birddeclines.aspx
to be caught, and killed by cats, are birds which won't
survive anyway, whether they are; sick, too young, or
too old. It's not only common in bird species, but, it's
also common throughtout the natural world for this to
happen.
The numbers of birds declinging, doesn't match with the Mammal Society report. The mammal society reports that there are more birds being lost (without any evidence, that doesn't record and analysis the whole UK bird poppulation decline), than the numbers of birds actually being lost.
This report goes back even further, to 1966, it is by the RSPB, and it's finding where that the UK has lost 44 million birds, which is a lot lower than the Mammal Society's 250+ non-scientific findings.
This is still a massive decline, and a worrying one. But, cats are not the reason for the sharp decline in bird spcies, it is us how are the reason behind it.
Other human practices can also, and do also have a negative effect on birds. Such as; farming, bird persecution, wind turbines, less natural gardens/suitable gardens for birds.
Another reason for their decline is harsh winters, says the RSPB. The cold winters can kill many birds, especially if they have a lack of food and shelter.
You can see some of the RSPB's findings below