The density dependence tentatively concluded
above implies that the presence of individuals
of a species makes the environment less suitable
for other individuals of that species. It would
also be expected then that the presence of
individuals of one species may make the environ
ment less suitable for individuals of a different
species. This is called interspecific competition.
As mentioned above, this seems to mean that two
syuipatric species will have their populations
limited by different factors so that each species
inhibits its own population growth more than it
inhibits that of the others. The factors inseparably
hound to a species' persistence in a region are,
then, its relation to other species and the presence
of food, proper feeding zone, shelter from weather,
and nesting sites (Andrewartha and Birch 1954,
Grinnell 1914). In this section these factors as
observed during the breeding season of the five
species of warbler will be discussed.
The summer of 1956 was devoted to observa
tions upon the four species, myrtle, black-throated
green, blackburnian, and bay-breasted warblers,
on their nesting grounds. The principal area
studied was a 9.4 acre plot of mature white spruce
(Picea qlauca] on Bass Harbor Head, Mt. Desert
Island, Hancock County, Maine. On 7 July 1956
the site of observations was changed to the town
of Marlboro in Windham County, Vermont, where
a red spruce (P. rubens] woodlot of comparable
structure was studied. In the summer of 1957
more plots were studied. From 30 May until 5
June, eighteen plots of balsam fir (A bies bal
samea), black spruce (P. mariana), and white
spruce near Cross Lake, Long Lake, and Mud
Lake in the vicinity of Guerette in Aroostook
County, Maine, were studied. The remainder of
the hreeding season was spent on Mt. Desert Is
land, Maine, where five plots were censused.
These will be described later.