The term _Life-Zone_ is used by scientists in referring to a region
where environmental conditions so react upon each other as to form a
suitable home for certain plant and animal forms. A Life-Zone naturally
has no hard and fast boundaries as does a geographical zone; its
boundaries are determined by temperature, rainfall, soil, altitude,
drainage, and innumerable other factors, which so create a certain
average whole as to attract certain species of plants, birds, mammals,
and so forth, which in turn themselves become part of the environment,
and are responsible for the presence of certain forms. These Life-Zones
are, then, associations which naturally develop in sections where
similar conditions exist. In Pennsylvania there are three or four of
these associations. One, noticeable in the southern and southwestern
counties, has been called the Carolinian Life-Zone. Here such birds as
the Carolina Wren, Cardinal, Tufted Tit, and Red-bellied Woodpecker
live. In much of Pennsylvania the Alleghenian Life-Zone occurs, where
the Least Flycatcher, Wilson’s Thrush, Swamp Sparrow, and Rose-breasted
Grosbeak are to be found in summer. Higher in the mountains is the
Canadian Life-Zone; here the Junco, Hermit Thrush, Red-breasted
Nuthatch, Wilson’s Snipe, and Northern Water Thrush nest. As you read
this book, you will notice that many birds are to be found in _northern
or mountainous counties_; others in _southern or less mountainous
counties_, and so forth. This is an indication of Life-Zone
distribution.