Predation of woodland songbirds: grey squirrels have a case to answer

Two years’ research by the Game & Wildlife Conservation Trust indicates that grey squirrels have a significant effect on certain woodland birds’ fledging success. In a pioneering project, which tested the effects of removing grey squirrels from groups of woods, some birds experienced a better success in rearing fledged young than in a comparable group of woods where no squirrel control was undertaken.

“The results from this research work indicated that grey squirrels at particular densities have a noticeable effect on some birds, particularly those which have open nests, rather than nesting in holes or hollow trees,” said George Farr Chairman of the European Squirrel Initiative who commissioned the research.

The project used a method, known as a ‘randomised removal experiment’ never attempted before, rather than only looking at possible correlations between the presence of grey squirrels and birds in woods [footnote 4].

“The aim of the work was to test the idea that removing grey squirrels from woods might lead to a consequent increase in fledging success of woodland songbirds”, continued     Mr Farr.  “We also wanted to see if there were any differences between the effects on birds which build open nests, and those which are hole nesters”.

Hole nesting species included great tit and nuthatch, while open nesting species included chaffinch and blackbird.  An improvement in productivity was 20% or more in some open-nesting species in woods where squirrels were removed.

“The study indicated when grey squirrels were at a relatively high density and were then removed there was a measurable positive effect on the post nesting fledging of the woodland bird community,” said Prof Nick Sotherton of the Game & Wildlife Conservation Trust.  “The outcome would, then suggest that the grey squirrel has a case to answer.  However more work needs to be done in order to build up a definitive picture of the full impact grey squirrels have”, he added.

Notes to editors

1)  The Game & Wildlife Conservation Trust is an independent wildlife conservation charity, which carries out scientific research into Britain’s game and wildlife. The GWCT advise farmers and landowners on improving wildlife habitats and it lobbies for agricultural and conservation policies based on science. It employs 14 post-doctoral scientists and 50 other research staff with expertise in areas such as birds, insects, mammals, farming, fish and statistics.  It undertakes their own research as well as projects funded by contract and grant-aid from Government and private bodies.

2) The grey squirrel, Sciurus caroliensis, a native species of North America, was introduced toEngland from the late 19th century, until 1938 when it became illegal to import or keep the species in captivity.

3) The European Squirrel Initiative was founded June 2002 by a group of concerned conservationists and foresters. The organisation seeks the restoration of the native Red Squirrel and the protection of the natural environment by removing the impact of the alien Grey Squirrel inEurope.

Its role is to:

Persuade conservation bodies and governments of the absolute necessity of riddingEurope of the Grey Squirrel.Continue to commission research into the impact of the Grey Squirrel on local ecosystems.

4) In this case, the expression randomised removal experiment’ describes the process whereby a species, (the grey squirrel), was removed throughout the period of a research project, from a group of woods, where previously, the species had been present.  At the same time, the species was not removed from a comparable group of woods, again, where the species was already present. The twenty woods involved in the project were grouped in ten pairs and the choice of the wood in each pair in which to remove squirrels was at random. 

Issued on behalf of the European Squirrel Initiative byKendalls.

For more information, please contact Andrew Kendall, telephone 01394 610022. 


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