52 Trans. Acad. Sci. of St. Louis 
number was higher, proportionately, than at Ithaca, the average 
territory was only about 100 yards in diameter whereas at Ithaca 
they extended out to lengths of 300 yards and breadths of 200 
yards in some cases. The territory of pair ‘‘D’’, a short dis- 
tance from ‘‘A’’, ‘‘B’’, and ‘‘C’’, was a field of plowing just 
six acres in area which pair ‘‘D’’ called their own exclusively. 
But though some boundaries might be established by the mar- 
gin of unsuitable territory there was a limit even where suitable 
ground still persisted. Thus the old stubble field, which was 
later plowed and sowed, extending off to the northwest from 
territory ‘‘C”, was eminently suited for Larks yet the ‘‘C”’ 
male was never seen farther than the boundary noted in figure 2. 
That invisible boundary, that the male birds put up between 
themselves where their territories coincided, was of the greatest 
interest. The boundary between ‘‘A’’ and ‘‘B’’, ‘‘C’’ and 
‘*A’’ was a ridge, extending into a fair hill on the east. It is 
not strange that a natural marker, such as this, should be used 
to delimit the area. But the line that was laid down between 
“‘B”’ and ‘“‘C”’ was over a perfectly smooth stretch of ground 
with no natural indications of any kind. Yet these two birds 
recognized it within twenty or thirty feet for a length of more 
than 100 yards. And here they posted themselves for the greater 
amount of time, to see that the other respected that invisible, 
but to them, quite definite boundary. 
_ History of territories in subsequent nestings—The observa- 
tions at Ithaca followed Larks ‘‘A’’, ‘‘B’’ and ‘‘C” throughout 
the season from March to late June and through a number of 
nestings (see Tables 4, 6, 8, 9, 12, 13, 24, ete.). It is apparent 
that, except for one influence, the territories would have Tre- 
mained unmodified from first to last. That modifying influence 
was vegetation, here tall wheat. Thus territory ‘‘A’’, entirely 
within the fall wheat, was completely abandoned at the close of 
the second nesting in May. Territory ‘‘B’’, partly on the garden 
and partly on the fall wheat, was reduced in May on the east 
in the fall wheat portion, to little strips running up and down 
from the ditch there which had been washed bare of seed in the 
fall rains (Fig. 3). These areas, in June, were finally no longer 
tenable and the ‘‘B”’ pair was forced to a territory on the garden 
less than 50 yards wide (see Fig. 4). The territory of ‘‘C”’ 
was shortened to the ditch on the south but otherwise remained 
