In a fascinating paper to the journal British Wildlife in 2004, ornithologists Chris Hewson, Robert Fuller and Ken Smith with squirrel biologist Brenda Mayle presented data suggesting that Grey squirrels can, on a local scale at least, be significant predators of bird nests. Amongst other results, the researchers found that as many as 27% of total nest failures of tit species (Parus) in Nottingham were attributable to Grey squirrels. Overall, the authors classified the impacts of Greys into three categories: Predation; competition for breeding holes; and competition for food. (Note: for a comprehensive review of the literature, the reader is directed to the BTO’s research report on the impacts of Grey squirrels that is available from their website.)
One particular example from Norfolk, saw the predation rate for 38 species of open-nesting birds by Greys decline from 85% (114 out of 135 nests) in 1983 to between 5 and 10% in 1984 after the population of Grey squirrels had been greatly reduced through shooting. Norfolk does, however, seem to be a rather extreme case and the rates of direct predation (i.e. killing of chicks or stealing of eggs) varies considerably according to region.
In the BTO report mentioned above, Hewson and Fuller give figures of 4-7% of occupied nest boxes of blue tits (Cyanistes caeruleus), great tits (Parus major), pied flycatchers (Ficedula hypoleuca) and redstarts (Phoenicurus phoenicurus) predated, or suspected to have been predated, by grey squirrels in a woodland in Stirling. In Nottingham, 3-6% of chaffinch (Fringilla coelebs), greenfinch (Chloris chloris) and thrush nests were predated, while 8-10% of long-tailed tit (Aegithalos caudatus) nests were lost in Yorkshire. During a study of nest box use by Red squirrels in Formby, Lancashire between 1994 and 1997, Craig Shuttleworth found evidence that squirrels had consumed 29% of great tit clutches, and either predated or scavenged about 17% of nestlings. Overall, the suggestion is that the squirrels’ proclivity to forage at almost all elevations (i.e. from the ground to right up into the canopy), puts birds of all nesting ilks at risk of predation.
Several studies have shown that squirrel presence on bird feeders can exclude small passerine birds (see below) and the intimation is that this can subsequently increase mortality. More recently, however, it has been suggested attraction of predators to feeders may also pose a risk to nests and eggs. In a paper to the journal Ibis in 2016, a team of biologists at the University of Reading led by Mark Fellows reported the results of their study on how the presence of bird feeders affected local nest predation on the university’s Whiteknights Campus.
Fellows and his colleagues studied the rates of predation on 102 artificial nests situated near guarded (which squirrels could not access) and unguarded feeders between May and June 2014. In total, 74 of the nests were predated and, of these, eight (11%) were predated by Grey squirrels. Interestingly, there was no difference in survival of nests around guarded or unguarded feeders, suggesting that feeders attract nest predators regardless of whether they can be accessed. Nests placed near unguarded feeders, however, were 20% less likely to survive than those adjacent to empty feeders and the researchers concluded:
“Overall, nest predation was associated with increased predator visits, particularly by Grey Squirrels, to feeders.”
Competition for breeding holes
In the British Wildlife paper, it is suggested that the instance of squirrels denning in trees that would otherwise have been suitable for nesting birds is sufficiently high to suppress the breeding of some species, such as tawny owls (Strix aluco), kestrels (Falco tinnunculus), and jackdaws (Corvus monedula). Such intimation is based on observations of Grey squirrels inhabiting nest boxes created for birds – in some instances, the squirrels apparently killed the occupants and consumed the eggs before taking over the box. The paper also contains a communication from one nest recorder of a Grey squirrel consuming the eggs in a rook’s (Corvus frugilegus) nest, before building its drey on top of it – this provides circumstantial evidence to suggest that Greys may also be competitors for nest sites with “open-nesting” species.
During his study at Formby, Shuttleworth found that Red squirrels regularly used almost half of the bird nest boxes during spring and the presence of great tit clutches did not preclude squirrels setting up home. Reoccupation of the box by squirrels caused the nesting great tits to abandon their clutches. Interestingly, the establishment of a jackdaw colony on the site resulted in most nest boxes being used by the corvids, with fewer than 10% occupied by squirrels.
While directly competing with some birds for nesting sites is one problem, it seems that squirrels might change the dynamics of the nest, rendering it uninhabitable for subsequent avian use. Hewson and his colleagues report that by gnawing the edges of old Greater spotted woodpecker (Dendrocopos major) holes, Greys expand the entrance and make the nest unsuitable for any further use by this species, although whether this affects the suitability of the hole for other bird species is unknown. Similarly, if high densities of squirrels hamper forest development the result is an increase in scrub habitat, which is detrimental to species such as tawny owls (Strix aluco) and lesser-spotted woodpeckers (Dendrocopos minor) as well as cavity-nesting bats.
There is also concern that Greys could take over the roost sites of some woodland bat species, although this has yet to be subjected to any rigorous investigation. Equally, however, it should be noted that there are anecdotal reports of some birds, including kestrels, hobbys (Falco subbuteo), wood pigeons (Columba palumbus), robins (Erithacus rubecula) and even long-eared owls (Asio otus), using squirrel dreys as nests.
Competition for food
Squirrels compete with birds and other native small mammals for food resources. In their British Wildlife paper, Hewson and his colleagues suggest that, because as much as 95% of the Grey squirrel’s diet can be seeds and, moreover, that the squirrels will often harvest these seeds (e.g. hazelnuts, acorns and sweet chestnuts) several weeks before the ‘seed rain’ (i.e. the time when the seeds fall off the tree), they are removing a significant food resource for native small mammals such as mice, voles and the Red squirrel. Indeed, a paper to the journal Mammal Review in 1996 reported that two-thirds of hazelnuts returned during a dormouse (Muscardinus avellanarius) survey had been opened by squirrels.
Along with the direct competition associated with reaching or utilising a food resource before any other species, there is also the problem of cache robbing by squirrels. Hewson and his co-workers note that Grey squirrels will rob the caches of birds, such as jays (Garrulus glandarius), and may even be dominant over some birds when foraging in the same area. The authors cite Derek Goodwin’s 2002 communication to the journal British Birds, in which a Grey squirrel was seen to chase away a Eurasian jay attempting to re-locate a seed cache in a lawn. My personal experience has been the exact opposite of this – I have observed jays, crows (Corvus corone) and magpies (Pica pica) raiding caches made by Greys, even sidling up and pulling the squirrel’s tail while it is trying to cache the nut. In a paper to the journal Ibis in 2014, a team led by Sheffield University biologist Colin Bonnington observed that Grey squirrels could dominate bird feeders such that they spent up to 16% of daylight hours on them, and reduced the total number of bird visits by 98% and the amount of food taken by birds by 97%.
As Hewson and his colleagues point out, the biggest problem with trying to gain a definite grasp of the impact that Grey squirrels might be having in and on our native woodlands, is that there are no long-term, systematic studies from Britain. Much of the data we have comes in fragmented studies from around the globe and anecdotal observations from naturalists. Nonetheless, in the most recent study I am aware of, published in the Journal of Ornithology in 2010, a team of biologists led by the BTO’s Stuart Newson concluded that:
“… grey squirrels are very unlikely to have driven observed declines of woodland birds in recent years, although the number of associations, positive as well as negative, between grey squirrels and woodland birds is greater than expected.”
The researchers found that blackbirds (Turdus merula), collard doves (Streptopelia decaocto), green woodpeckers (Picus virdis), long-tailed tits and particularly jays were negatively affected by Grey squirrel presence, while stock dove (Columba oenas), dunnock (Prunella modularis), coal tit (Parus ater), goldcrests (Regulus regulus), jackdaw, greenfinch and lesser redpoll (Carduelis cabaret) populations increased in the presence of squirrels. The suggestion is that bird and squirrel populations are both responding in a similar way to variability in habitat quality.