Determination of long-term population trends by State, 1961 to 1971 
Short-term (year-to-year) population changes are based, as indi- 
cated, on data from comparable routes only. Since the composition of 
these comparable routes changes with each 2-year comparison, a Base Yea; 
(BY) has been chosen for each State. Long-term trends are shown by q 
applying the percent change in the BDI from year to year to this index, 
Before 1967, the BY was generally the first year that the State's call- 
count routes were randomized. In order to provide a more uniform basis 
for evaluation of long-term trends, 1967 has been selected as the BY 
for all States except Maine, New Hampshire, Rhode Island, and Vermont. 
The index for this Base Year was obtained by taking the mean of com- 
parable routes run in both 1966 and 1967 (Ruos and MacDonald, 1968). 
The four excepted States have been assigned a 1971 Base Year, repre- 
senting the mean number of doves heard on comparable routes run in 
both 1970 and 1971. These BY values are thought to provide a mean- 
ingful refinement over the previous method. Not only are two "random- 
route" years averaged to reduce the influence of a possible atypical 
year, but the choice of a uniform BY for most States reduces possible 
bias in overweighting a State by the selection of a BY in a peak year. 















As in the past, the BDI value for each State for each year is 
adjusted to the BY value and weighted to provide management unit values, 
This weighting is based upon differences in land area among States. 
The land area values and the adjusted BDI values for States and manage 
ment units are presented in table 3. 
Determination of long-term trends by physiographic region, 1965 to 1971 
Dove BDI's have been determined for each physiographic region since 
1965. The 7-year trend has been obtained by adjusting these values to 6 
base year similar to the procedures described in the preceding section, 
"Determination of long-term population trends by State, 1961 to 1971." 
For all regions, 1969 has been selected as the Base Year. In order to 
reduce the influence of low sampling intensity within regions, the 1969 
BYI represents the average of the mean number of doves heard per route 
in 1968, 1969, and 1970. 
The land area values and the BDI's adjusted-to the 1969 BY are pre= 
sented for each physiographic region within management units (table 4) 
