Each year, monarchs congregate in colonies along the California coasts. Although not as well-known as the colonies in Mexico, the overwintering grounds of the western population are also critical for their survival and monitored each year by volunteers for the Xerces Society. The "Thanksgiving" Counts are performed each year in the fall after monarchs have returned from their breeding grounds. Although 450 sites have been documented as having supported monarchs at some time, under 300 sites are regularly counted. The main colonies are monitored every fall and other colonies less frequently. This program keeps track of the overwinter populations. Formerly known as the Thanksgiving Monarch Counts, this program has expanded to include New Years counts and is now part of the Western Monarch Count Resource Center.
Schultz, C.B., L.M. Brown, E. Pelton, and E.E. Crone. 2017. Citizen science monitoring demonstrates dramatic declines of monarch butterflies in western North America. Biological Conservation 214:343-346
Espeset, A.E., J.G. Harrison, A.M. Shapiro, C.C. Nice, J.H. Thorne, D.P. Waetjen, J.A. Fordyce, and M.L. Forister. 2016. Understanding a migratory species in a changing world: climatic effects and demographic declines in the western monarch revealed by four decades of instensive monitoring. Oecologia 21:1-2
Pelton, E., Jepsen S., Schultz C., Fallon, C., and S.H. Black. 2016. State of the Monarch Butterfly Overwintering Sites in California. 40+vi pp. Portland, OR: The Xerces Society for Invertebrate Conservation
Jepsen S, Black SH. 2015. Understanding and conserving the western North American monarch population in Oberhauser KS, Nail KR, Altizer SM, eds. Monarchs in a Changing World: Biology and Conservation of an Iconic Butterfly. Ithaca, USA: Cornell University Press.
Stevens SR, Frey DF. 2010. Host plant pattern and variation in climate predict the location of natal grounds for migratory monarch butterflies in western North America. Journal of Insect Conservation 14: 731-744.
Frey D, Schaffner A. 2004. Spatial and Temporal Pattern of Monarch Overwintering Abundance in Western North America. Pages 167-176 in Oberhauser KS, Solensky MJ, eds. The Monarch Butterfly: Biology and Conservation. Ithaca: Cornell University Press.