igrill.a), myrtle (D. coronata), black-throated
green (D. cirens ), blackburnian (D. fusca), and
bay-breasted (D. castanea), are sometimes found
together in the breeding season in relatively homogeneous mature boreal forests. These species
are congeneric, have roughly similar sizes and
shapes, and all are mainly insectivorous. They
are so similar in general ecological preference, at
least during years of abundant food supply, that
ecologists studying them have concluded that any
differences in the species' requirements must be
quite obscure (Kendeigh. 1947; Stewart and
Aldrich, 1952). Thus it appeared that these
species might provide an interesting exception to
the general rule that species either are limited by
different factors or differ ill habitat or range
(Lack, 1954). Accordingly, this study was undertaken with the aim of determining the factors controlling the species' bundances and preventing
all but one from being exterminated by competition cross-breeding cannot be
excluded.