Winter Trend Data ~ Pacific Flyway 
The winter surveys during the year 1958 and 1959 were 
conducted under reasonably comparable conditions throughout all areas 
that are covered regularly during the survey. In Alaska and British 
Columbia the total wintering habitat is not completely covered. ‘The 
areas surveyed are selected on the basis that there is a fair degree 
of certainty that they can be covered each year. In the United 
States and Mexico an effort is made to cover all concentration areas 
completely. In the Central Valley in California, aerial photography 
is used to check the accuracy of visual estimates of the number of 
birds in the larger concentration. 
The tables and figures which follow present the waterfowl 
data in two categories. The first tables presents a comparison of 
1958 and 1959 data. Following this are presented comparisons of 
data taken during the period 1949 through 1959. When arriving at a 
trend figure for each of the past 11 years, it was recognized that 
the winter survey data were not comparable. For example, it was 
not possible for the Bureau to conduct a survey in Mexico during 
January 1957. When there were differences in coverage between one 
year and the next, an estimate was calculated based on the assump- 
tion that areas where comparable surveys were conducted, yielded an 
accurate measure of percent change in population from one year to 
the next. Although in most instances, comparable coverage between 
one year and the next does provide a useable measure of percent 
change in population, there is question regarding the estimated 
duck figure for 1957 based on this method of estimation. It will 
be noted in the graph which follows that there was a considerable 
decrease between 1956 and 1957 and an increase between 1957 and 
1958 which more than compensated for the decrease. At thia point 
it seems doubtful that the trend in duck populetion was downward in 
1957. 
Among the ducks there was a small decrease recorded in the 
number of pintails, which is the most important bird in the flyway. 
It is perhaps significant to note that the total pintail breeding 
population index has been steadily decreasing since 1956, and during’ 
this past spring the index was more than 40% below the 1956 level. 
Since the Pacific Flyway regularly winters from half to two-thirds 
of the pintail population, there appears to be a discrepancy between 
the population trend as portrayed by the winter data and the informe- 
tion from the breeding ground, 
Smell to moderate increases were recorded for most other 
important duck species with the exception of canvasback, which 
decreased approximately one-fourth. 
