Springwatch investigates: The corvid cull
Martin has been out mystery-busting, exploring the arguments for and against the corvid cull: plans to cull crows and magpies to test if they are contributing to a decline in songbird numbers.
But are these changes a coincidence? And what research has been done? Unfortunately there are some fundamental differences in opinion...
In January 2011 it was announced that the Game and Wildlife Conservation Trust (GWCT) would conduct a large-scale trial corvid cull.
GWCT says that it is confident that corvid control is important in managing wild populations of game birds such as partridge, and that there is a need to control crow and magpie populations to look after these birds. However, it says that it doesn't know how important predation control might be in relation to other species.
The key funder of the trial cull is Songbird Survival, a UK-wide charity of ornithologists, conservationists, gardeners, farmers and members of the public who are concerned with the effects of increasing predation on songbird populations. It says that although the organisation is not stating that predation is the major factor affecting declining songbirds, it is an important factor that hasn't been looked into thoroughly.
In 2010 the BTO published an analysis of songbirds and their predators, pulling on information that has been collected over 40 years. This analysis looked at 30 species of songbird and seven predator species and found very few significant relationships between growth in predator populations and declines in songbird populations. The conclusion the BTO made was that magpies probably have little or no significant effect on the overall country-wide populations of our songbirds. It believes that declines are most likely the result of changes in land use and other habitat issues.
The RSPB states that magpies have no overall effect on garden bird populations. It says that corvids have lived alongside songbirds for millennia and cope with predation by having lots of youngsters so that some survive to adulthood. It argues that there is no evidence to support Songbird Survival's claims that corvids are responsible for songbird decline. For the RSPB it is changing farming techniques that is most likely to blame.
The RSPB does however cull magpies and crows at some of its reserves where it thinks that predation is a problem at a local level. In some cases, it says, bird populations have fallen to such an extent (due to other reasons) that predation can be the final straw. It is using these control measures to reduce the potential impact of the predators on these already vulnerable isolated populations.
The debate continues. What do you think?
Martin's film will be on Springwatch, 8pm BBC Two 14 June.