Sciurus carolinensis | Eastern Grey Squirrel | Iora glas



Conservation status

Least Concern

Legal status

Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011. (Note: Regulation 50 not yet enacted).

Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).

Regulated invasive  species of Union concern under the European Regulation on the prevention and management of the introduction and spread of invasive alien species [1143/2014].

Native status


First reported in the wild



Invasive species - risk of High Impact

Irish status


Introduction pathways - 1

Release in Nature

Introduction pathways subclass - 1

Landscape/flora/fauna improvement

Invasive score


NAPRA Ireland risk assessed


Species Biology


Small arboreal mammal, larger than red squirrels, predominately grey in colour and lacking the prominent ear tufts of red squirrels.


Omnivorous, feeding mainly on seeds and plant material, though they will consume bird eggs and nestlings it is not known if this constitutes a significant proportion of their diet. Grey squirrels range expansion has been associated with a range contraction of ~20% of red squirrels in Ireland since 1911 (Stritch et al, 2015). This is thought to be as a result of competition between the two species and the transmission of squirrel pox virus (SQPV) to immunologically naive red squirrels (Lawton & Sheehy, 2014; Stritchen et al, 2015; Teangana et al., 2000). Range, tree use and foraging overlaps between the species in Italy suggest there is no niche partitioning between the species so competition will occur as grey squirrel densities increase (Wauters et al., 2002). In the presence of the SQPV, displacement of red squirrels by grey squirrels via competition is 17-25 times the rate in uninfected areas (Rushton et al., 2006), with infection rates in grey squirrels of 55% detected in Wicklow (Stritch et al., 2015). Furthermore grey squirrels have introduced an endoparasite Strongyloides robustus and the prevalence of it and a native endo parasite of red squirrels were higher in areas with grey squirrels than red only areas (Romeo et al., 2015).


Woodland, forest and other wooded land; Constructed, industrial or other artificial habitats; Regularly or recently cultivated agricultural, horticultural or domestic habitat; Miscellaneous


Promiscuous or polygynous mating system, producing litters in either spring or summer of 1-7 pups (Gurnell et al, 2008).

Pathway and vector description

Released at Castle Forbes in Co. Longford in 1911, they have since naturally dispersed across the country at an estimated rate of 0 km/year to an 13.4 km/year, with various rivers and other geographical barriers slowing dispersal (O Teangana et al., 2000). They are seemingly restricted to the east of the country by the river Shannon, though this is likely to be the result of other factors than an inability to physically cross the river.

Mechanism of impact

Competition, Disease transmission

Broad environment


Habitat description

Predominately found in woodlands, either semi-natural or coniferous, but also found in parks and urban areas (Gurnell et al., 2008).

Species group


Native region

North America


World distribution(GBIF)

Irish distribution

Established - Widespread & Common, though absent from the west of the country. Since 2007, the grey squirrel has retreated in range from several midland counties (Lawton et al., 2015). This decline is though to be as a result of a strong negative association between the grey squirrel and the recovering pine marten population (Lawton & Sheehy, 2014), which may be as a result of predation or modification of grey squirrel behaviour.

Native distribution

Native to Canda and the United States.

Temporal change

Date of first record category


Fifty year date category


Records submitted to Data Centre in 2017

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How can you help

Report any sightings to the National Biodiversity Data Centre.

Further information

Invasive Species Specialist Group (ISSG) of the SSC- Species Survival Commission of the IUCN -International Union for Conservation Nature 100 Worst Invaders globally. Delivering Alien Invasive Species In Europe (DAISIE) project list this as one of the 100 Worst Invaders in Europe. The collective name for a group of squirrels is a scurry, though as they are predominately solitary animals scurries rarely occur.



Gurnell, J., Kenward, R.E., Pepper, H. & Lurz, P.W.W. (2008). Grey Squirrel Sciurus carolinensis In: Mammals of the British Isles : Handbook, 4th edition. Harris, S. & Yalden, D.W. (eds) The Mammal Society, UK.

Lawton, C., Flaherty, M., Goldstein, E.A, Sheehy, E. and Carey, M. (2015) Irish Squirrel Survey 2012. Irish Wildlife Manuals, No. 89. National Parks and Wildlife Service, Department of Arts, Heritage and the Gaeltacht, Ireland.

Romeo, C., Ferrari, N., Lanfranchi, P., Saino, N., Santicchia, F., Martinoli, A., & Wauters, L. A. (2015). Biodiversity threats from outside to inside: effects of alien grey squirrel (Sciurus carolinensis) on helminth community of native red squirrel (Sciurus vulgaris). Parasitology  Research, 1-8.

Rushton, S. P., Lurz, P. W. W., Gurnell, J., Nettleton, P., Bruemmer, C., Shirley, M. D. F., & Sainsbury, A. W. (2006). Disease threats posed by alien species: the role of a poxvirus in the decline of the native red squirrel in Britain. Epidemiology and Infection, 134(03), 521-533.

Sheehy, E., & Lawton, C. (2014). Population crash in an invasive species following the recovery of a native predator: the case of the American grey squirrel and the European pine marten in Ireland. Biodiversity and Conservation, 23(3), 753-774.

Teangana, D. O., Reilly, S., Montgomery, W. I., & Rochford, J. (2000). Distribution and status of the red squirrel (Sciurus vulgaris) and grey squirrel (Sciurus carolinensis) in Ireland. Mammal Review, 30(1):45-56.

Watt, H.B. (1923) The American Grey squirrel in Ireland. Irish Naturalist Journal, 32:95–353.

Wauters, L. A., Gurnell, J., Martinoli, A., & Tosi, G. (2002). Interspecific competition between native Eurasian red squirrels and alien grey squirrels: does resource partitioning occur?. Behavioral Ecology and Sociobiology, 52(4), 332-341.

IUCN Red List of Threatened Species

Global Invasive Species Database

Atlas of Irish Mammals

DAISIE Factsheet

CABI Datasheet