23 
Relative Mobility of Species 
Duration of Residence <= As a basis for a further study of brood movement, 
a residence chart was constricted for each pothole on the study area, as shown 
in Plate XJa for pothole No. 33. Each bar on the chart represents the period 
that one brood occupied the pothole. Where two bars are on the same level, 
they represent two visits by the same brood. : 
Since all broods seen at a pothole and the quality of the coverage given 
it on each visit had been noted as "complete", "fair", or "visit only", a good 
estimate could be made of the number of broods occupying a pothole and the 
length of time they remained there. Thus a brood which was once seen on a 
pothole but could no longer be found there was assumed to have departed from 
it. This assumption was made if it was not seen during one "complete" coverage, 
or two "fair" coverages, or three "visits." The date on which it left the 
pothole is assumed to be half way between the date it was last seen and the 
date at which it was judged to be absent. This takes inte account the fact 
that none of those potholes was observed every day and some were not observed 
for a week or more, The date of arrival of a brood in a pothole was calculated 
in the same manner, If a feigning hen of the species in question was seen, she 
was assumed to indicate the presence of the brood provided that she had not 
flown in from another pothole as often occurred. In spite of its inherent 
inaccuracies, this residence chart not only provides an index of the popula- 
tion of the pothole at any date, but shows the length of time that any one 
brood spent there. 
For further study of the brood population a chart such as is illustrated 
in Plate %Ib was drawn up, This is merely the breakdown of all the residence 
charts into the separate species, There was no attempt made here to trace 
individual broods, and one which was seen in several different potholes may 
appear several times on the chart. It was usually fairly simple to determine 
how long a brood remained in one pothole, but impossible in most cases to tell 
where it went after leaving. 
Calculation of Mobility -- By use of the information illustrated in 
Plate Yib, the average period that broods of a species spent in any one pothole 
serves as an index to the mobility of the species. Each bar is considered to 
be one "residence" which consists of a certain number of "brood-days" (the 
presence of 1 brood for 1 day), The mobility of a species is indicated by 
the total number of residences divided by the total number of brood days the 
species spent on the study areas in other words, the reciprocal of the average 
term of residence. 
Table 7 shows the relative mobility of the species found on the study 
area. Those which furnished less than 10 observations were separated from the 
rest since it is doubtful whether they have much significance. The shoveller 
brood using potholes Nos. 29, 30, and 31, whose movements have already been 
described, was separated from other broods of the species when calculating 
the mobility rating. The fact that more observations were made on what was 
known to be one brood than on all of the other shovellers, gave it too mch 
weight. The mobility rating was calculated for this one brood and averaged 
in with that of other shovellers. The average observed travel from the 
case histories already described is included in the last column for comparison, 
It was impossible in many cases to determine whether a new brood in an 
area was recently hatched or, on the other hand, whether the departure of 4 
brood indicated that it had walked away or flown off, Since all broods both 
hatch and fly, those which never moved out of one pothole have their residence 
terminated at both ends and are given a higher rating than they deserve. Thus 
the difference between the mobility ratings of the various species is slightly 
greater than indicated in Table 7. 
