By Matt Walker
Editor, Earth News
Eating eggs does no long term harm to birds
Grey squirrels do not have a significant impact on the populations of many of England's woodland bird species, a study has found.
Researchers from the British Trust for Ornithology and Natural England examined the impact of grey squirrels on 38 bird species across the country.
They found some evidence that grey squirrels may locally suppress the populations of some species.
But they do not appear to cause the birds any widespread or lasting harm.
The findings of the study, which contradict the view that grey squirrels have caused declines in British bird numbers over the past 40 years, are published in the
Journal of Ornithology.
It has long been known that grey squirrels, an invasive species to the UK, predate on bird eggs and will also kill and eat fledglings.
the UK's native squirrel species, do the same.
Over the past four decades, many
wild bird species have declined in abundance in the UK.
Many of these declines have been associated with changes to agricultural practice.
But it has often been argued that predators, such as grey squirrels, are also killing off native birds.
This threat - combined with the damage that grey squirrels can do to native trees, and the impact that grey squirrels may be having on the remaining populations of red squirrels - had led to calls last year, backed by Prince Charles, to
cull the invasive species.
Blackbirds thrive despite the grey squirrels' taste for their eggs
However, firm evidence that predation by grey squirrels has caused national declines in the numbers of native bird species has been lacking.
So researchers at the British Trust for Ornithology (BTO) and Natural England (NE) reviewed a wealth of data on bird and grey squirrel numbers in England to search for evidence that greys were having an impact.
BTO ecologists Dr Stuart Newson, Dr David Leech and Dr Chris Hewson and NE's Dr Humphrey Crick and Mr Phil Grice examined 38 bird species associated with woodland, including common starlings, wood pigeons, wrens, woodpeckers, thrushes, warblers, tits and finches.
Data on their population sizes and that of grey squirrels was gathered by the
Breeding Bird Survey.
Around 160,000 red squirrels remain in the UK, compared to 2.5 million grey squirrels
Over 2 million of these grey squirrels live in England
In the past, grey squirrels have been blamed for significant declines in now rare birds such as lesser spotted woodpeckers, common nightingales, tree pipits, willow warblers, song thrushes, willow and marsh tits and hawfinches
They found very little evidence that grey squirrels were suppressing bird numbers.
"Grey squirrels are very unlikely to have driven observed declines in woodland birds in recent years," Dr Newson told the BBC.
Of the 38 bird species, a statistically significant relationship between grey squirrel and bird population sizes was found for 12 species.
Of those, squirrels appeared to have a positive impact on seven bird species, a correlation probably caused by both mammal and bird species benefiting from similar changes to their habitat.
Grey squirrels had a negative impact on just five: the common blackbird, Eurasian collared dove, green woodpecker, long-tailed tit and Eurasian jay.
"Of these species, the most convincing evidence is for blackbird and collared dove," says Dr Newson.
For these two species, the researchers found a weak but significant relationship between the abundance of grey squirrels and a failure of the birds' nests.
However, while grey squirrels may be predating on these two species, the overall number of blackbirds and collared doves has gone up nationally, and even locally where grey squirrels are common.
It is just that the numbers of these bird species have increased even more where grey squirrels are absent.
"This was the first national analyses of its type to be carried out, so I was open-minded as to what I might find," says Dr Newson.
One explanation for why grey squirrels eat birds' eggs, yet do not appear to cause any decline in bird numbers, is that birds compensate for the loss to their nests.
If the number of fledglings falls, for example, the survival rate of those remaining often goes up, as they have less competition for food in the nest.
Surplus non-breeding birds may also step in to bolster numbers if a population does begin to decline.
The researchers do not rule out the possibility that grey squirrels may be having an impact on other high-risk species, such as hawfinches, which could not be studied as a result of a paucity of data.
But they say it does reinforce the view that the most pressing conservation issue relating to grey squirrels is the impact they are having on native red squirrels rather than woodland birds.
There is good evidence that grey squirrels have played an important role as a vector in the spread of parapoxvirus, a form of pustular dermatitis that does not affect greys, yet can kill a red squirrel within four to five days.