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Grey squirrels – the science behind the impact of the invasive species

Not all scientists agree but those who shoot know there is plenty of evidence that grey squirrels are an invasive species than threatens our wildlife, says Richard Negus

It can be no coincidence that songbirds thrive where grey squirrel control is regularly carried out

I imagine most people reading this will agree that grey squirrels are a wholly unwelcome addition to our national fauna. However, the majority of Britons aren’t rustics or readers of Shooting Times. More than 80% of the UK population lives in towns and cities and for the majority of them the grey squirrel is one of the few ‘furry and cute’ mammals they get to see in their parks and gardens.

The wholesale damage grey squirrels cause to woodlands by stripping thin-barked species such as sycamore, beech, oak, sweet chestnut, pine, larch and Norway spruce is of little consequence to them — townspeople tend not to own commercial forestry nor manage woodland for a living. And most city dwellers are ambivalent towards squirrel pox, the fatal disease spread by grey squirrels that has devastated our native reds (Want tips on controlling grey squirrels? Click here).


The grey squirrel’s impact on our native reds

How many families enjoying their weekend stroll through Richmond Park have even looked a red squirrel in the face, let alone one with oozing pustules, lesions and scabs covering it? No, the grey squirrel has — largely thanks to its gregarious nature, twinkly eyes and luxuriant tail — received a free pass from any meaningful lethal control in urban and suburban areas.

This has led to booming numbers that spill out from the metropolises and into the green belt, causing a perpetual and growing problem.

The matter of educating the general public in the ways and management of wildlife is a challenge at the best of times. Harder still to make a case for shooting or trapping an animal that is perceived to not interfere with your day-to-day life or, indeed, adds to your free-time enjoyment of open spaces. So if ruined trees and squirrel pox won’t shake the majority of Brits from their toleration, or even love, for the three million non-native grey squirrels infesting our country, will the knowledge that they pose a real threat to our song and farmland birds change their minds?

A good trapping line is essential

Talk to almost anyone from the shooting community about grey squirrels and, while they will cite tree damage and mutter about the threat they pose to reds, the greatest complaint relates to the impact the American interlopers make on songbirds through nest predation.

It has also been suggested, anecdotally, by pigeon shooters — notably Tom Payne (of this parish) — that grey squirrels predate on woodpigeon eggs and have consequently cleared out old roosting woods. Tom believes that greys are the reason some woods he enjoyed shooting as a boy no longer have much to offer.

The anecdotal corroboration in reference to songbirds is plentiful, well founded and documented. However, there is an increasing reticence from some groups with an interest in wildlife — including many of Britain’s three million birders —to believe the evidence of their own eyes or the testimony of practical conservationists regarding nest predation by grey squirrels. “Show me the science” has become a phrase to hide behind when it comes to serious decision-making in wildlife conservation, particularly when lethal control is involved.


Their impact on the bird populations

Some recent research into grey squirrel predation by Oxford ecologist Richard Broughton appears to exonerate the grey squirrel from any predatory impact on songbirds. His research has been used by a number of animal rights groups to stop urban councils undertaking lethal grey squirrel control. Dr Broughton’s study suggests that grey squirrels rarely plunder eggs or chicks, affecting, he claims, only 0.5% of nests. He also states that if they do attack nests, only common woodland species such as the thrush and blackbird are affected — this seems to imply that such losses are of no consequence.

Grey squirrels will predate on woodpigeon eggs as well as passing on pox to our native reds

The same excuse is used by the RSPB for its stance on songbird predation by cats, claiming they generally only take common species such as blue and great tits. The flaws in Dr Broughton’s research are, like many studies, that he uses data from a relatively small number of bird species — a mere 12 in this instance — and over a relatively small timescale. This highlights the contrast between the first-hand year-round evidence provided by practical conservationists and the starkly clinical — some might say, one-dimensional — data collected by some academics in such matters.

Dr Broughton’s data seems to tell an entirely different story from the one inferred by those of us who work year round in woods and hedgerows.


The problem with providing evidence

One of the challenges of scientifically proving the impact grey squirrels have on nesting birds is due to the range of habitats where they live and feed. They are as happy in the uppermost tree canopy as they are at all points downwards and on the ground. The canopy is where red-listed bird species such as hawfinch and spotted flycatcher build their nests.

In the British Trust for Ornithology’s review on the decline in woodland birds by Robert J Fuller, David G Noble, Ken W Smith and Des Vanhinsbergh, these highly regarded conservation scientists believe there is a correlation between the increase in the grey squirrel population and declines in species such as hawfinch, chaffinch and spotted flycatcher. They state that: “Predation is the largest cause of nest failure in most birds, and… it may be sufficient to suppress or reduce the breeding population.”

Dr Roger Draycott of the GWCT researched grey squirrel predation at Loddington. They discovered that greys have a much larger scavenging range than previously thought and found that, in some years, concerted squirrel control appeared to contribute directly to an increase in songbird numbers. Yet in other years, despite the ongoing cull of the rodents, bird numbers declined, due to avian predators or weather/food-related factors.

Dr Draycott is satisfied — scientifically, anecdotally and practically — that grey squirrels do pose a credible threat to a number of red-listed bird species both in woodland and field margins.


Hungry gap

The hedgerow/field margin zone is an area where grey squirrels pose a threat to the grey partridge, yellowhammer and linnet. Grey squirrels readily scavenge and the thicker the hedge is, the more the squirrel favours it. Particularly at
this time of year, when the hungry gap is at its starkest, grey squirrels, like every other farmland bird and mammal species, seek to boost their food intake to prepare for breeding.

Drawn out into the margins by the sustenance in supplementary bird feeders, they discover early clutches of eggs. Once they glean that eggs provide them with considerably more protein than they can obtain from seed, a prey memory is made and the die is cast for repeated nest raiding.

At Flea Barn farm I have set numerous New Zealand Department of Conservation (DOC) traps, baited with eggs for stoats. In three years I have yet to catch a mustelid, but I’ve accounted for double figures of grey squirrels using this combination.

Songbird decline in the UK cannot solely be laid at the door of the grey squirrel. Habitat loss, farming practices, climate change, feathered predators and cats all contribute.

However, it is beyond question that the grey squirrel does not, like the domestic cat, belong in our landscape. Every native bird nest grey squirrels raid and every tree they bark was never theirs by right to assail. Regardless of what some scientists may say, I believe it is high time the grey squirrel went the way of the coypu in Britain.