26 
Influence of the Size of the stacy Area -= Owing to the small number of 
broods involve 1¢ smail size o e study area, chance wanderings were 
well able to produce the population fluctuations noted. If the population 
and/or the area had been larger, chance movements onto and off the area should 
have cancelled each other and no fluctuation would have been apparent, 
There was no reason to believe that there was mass migration either onto 
or off the area, The surrounding terrain had some dry regions which were 
probably not as attractive to broods as the area itself, but there was no 
factor discovered which would stimulate general migrations. 
These graphs not only indicate that the species which were rated as most 
mobile in Table 7 showed the greatest population fluctuation due to movement of 
broods, but also furnish a better picture of the details of these fluctuations, 
Although it is impossible to determine accurately the speed and extent of 
movement from the data at hand, they do suggest that many broods may have 
traveled farther and more rapidly than any of those which were actually traced. 
This may also serve to account for the poor success in locating marked broods, 
since it seems quite possible that they may have left the area before they 
could be located, 
The Three Methods compared @=- The first method, involving a study of 
marke ividuals, presents conclusive data on the details of movement, such 
as distances moved and number of potholes used. However, it is extremely 
time-consuming and gives no indication of the percentage of broods which move. 
The second method involving the determination of average term of residence 
gives accurate information on the relative mobility of species and the 
frequency with which broods moved but none concerning the details of movement, 
This method is also time-consuming, requiring close coverage of the study area, 
The third method, involving census by age classes, is the most easily 
applied, but furnishes the least conclusive and least detalled information, 
It does give an indication of the effect of movement on the population of a 
small area. | 
Table 7, showing the mobility rating of the pintail as 0.20, indicates 
that on the average broods of this species used one pothole only 5 days at a 
time. If, as Hochbaum (19h) reports, the time of development from hatching 
to flying requires 45 to 58 days, the average pintail brood made use of nine 
different water areas before flying. The average canvasback, with a mobility 
rating of 0.15 and requiring 63 to 77 days, uses the same mumber of potholes. 
The baldpate, with a mobility rating of 0.05 and requiring 5 to 58 days, 
would not be expected to use more than two potholes, 
