There are certain difficulties concerning these
measurements. Since the forest was very dense,
certain types of behavior rendered birds invisible.
This resulted in all species being observed slightly
disproportionately in the open zones of the trees.
To combat this difficulty each bird was observed
for as long as possible so that a brief excursion
into an open but not often-frequented zone would
be compensated for by the remaining part of the
observation. I believe there is no serious error
in this respect. Furthermore, the comparative
aspect is independent of this error. A different
difficulty arises from measurements of time spent
in each zone. The error due to counting should
not affect results which are comparative in nature.
If a bird sits very still or sings, it might spend a
large amount of time in one zone without actually
requiring that zone for feeding. To alleviate this
trouble, a record of activity, when not feeding, was
kept. Because of these difficulties, non-parametric
statistics have been used throughout the analysis
of the study to avoid any a priori assumptions
about distributions. One difficulty is of a dif
ferent nature; because of the density of the vegeta
tion and the activity of the warblers a large number
of hours of watching result in disappointingly few
seconds of worthwhile observations.