28 IOWA STUDIES IN NATURAL HISTORY 
line along most of the small creeks. They are indicated on the 
township maps in stippling. The willows have been elevated in 
this treatise to a place of a separate heading chiefly because they 
form the only natural trees for the nesting of birds along streams 
which wander through the broad cultivated areas that once were 
prairie. Along these smaller streams they frequently occur in 
pure formations; along the larger water courses they usually 
occur mixed with birch and a scattering of other trees. Their 
trunks and foliage provide for a number of insects, the most of 
which are specific feeders upon willow. As these growths are 
usually so limited in extent, they provide insufficient shade for 
undergrowth and are commonly too scattered to have any effect 
upon the soil. Therefore their life strata may be said to be lim- 
ited to two, the tree-crown stratum and the tree-trunk stratum. 
In many instances, there are thickets of willow which are only 
shrub-like in size; in these cases there will of course be no tree- 
crown stratum, and the whole formation can be roughly divided 
into what would correspond to an undergrowth stratum and a 
sou stratum. 
(5) Hazel. Hazel has already been mentioned as forming a 
part of the undergrowth in the oak forest. However, in many 
cases it occurs unprotected by trees. It is one of the plants quick 
to establish itself after the removal of the oak. In fact, the 
greater number of hazel formations throughout the county will 
be found on ground that was once occupied by oak or oak-hickory. 
The hazel seldom attains a height of more than six feet, and as 
it is usually pastured it occurs in clumps between which is a 
firm blue-grass sod. It is found regularly on high, well-drained 
land where the humidity is low as compared, for example, with 
the elm formations. It provides, by its annual crop of nuts, food 
for the white-footed mouse and the chipmunk. The latter, how- 
ever, is not a regular resident of hazel thickets except in the 
more rocky places where the crevices provide it with easily pre- 
pared shelter. The hazel has, of course, its own set of insect 
feeders, and the hazel brush is regularly inhabited by such birds 
as the thrush. These formations permit of subdivision into not 
more than two life strata or sub-habitats. These are the soul 
stratum and that stratum composed of the brush itself, which, as 
