The First Christmas Bird Count was run on Christmas Day in 1900. Later, the count period was extended to allow counts on dates from mid-December through early January.
Each yearly cycle is published separately. We will use
the term year number to mean 1
for the first year of the CBC, 2 for the second, and so
on. In general, year number
includes dates in December of year 1899+
N and January of year 1900+