Grey squirrel  (Sciurus carolinensis)

Gestation: The state of being pregnant; the period from conception to birth.
Oestrus: The time of ovulation (release of an egg from the ovary) in female mammals, when the female becomes receptive to males, also known as 'heat'.

1. Animal Diversity Web (August, 2002)
   http://animaldiversity.ummz.umich.edu/accounts/sciurus/s._carolinensis$narrative.html
   
2. Burton, J.A. (1991) Field guide to the mammals of Britain and Europe. Kingfisher Books, London.
   
3. Gurnell, J. (1987) The natural history of squirrels. Christopher Helm (Publishers) Limited, Kent.
   
4. Genovesi, P. and Bertolino, S. (2001) Human Dimension Aspects in Invasive Alien Species Issues: The Case of the Failure of the Grey Squirrel Eradication project in Italy. In: McNeely, J.A. Ed. The Great Reshuffling Human Dimensions of Invasive Alien Species. IUCN Biodiversity Policy Coordination Division, Gland, Switzerland. Available at:
   http://www.iucn.org/dbtw-wpd/edocs/2001-002.pdf
   
5. The Invasive Alien Species Project (March, 2008)
   http://138.253.199.114/IAAP%20Web/IAAPwebsite/FactSheet/Squirrel.doc
   
6. Macdonnald, D.W. and Tattersall, F.T. (2001) Britain's mammals- the challenge for conservation. The Wildlife Conservation research Unit, Oxford University, Oxford. Available at:
   http://www.wildcru.org
   
7. National Biodiversity Network Species Dictionary (August, 2002)
   http://www.nhm.ac.uk/nbn/