BIRDS : GENERAL CONSIDERATIONS. 
133 
In watching, as there related, from the first ray of dawn on an elevat- 
ed summit the effect of the gradually increasing light upon bird life, 
the thought is naturally suggested that, the light of morning reaching 
first the mountain tops thence gradually descending, not improbably 
exerts an influence in attracting avian-life to high summits; for at the 
early hour at which they stir, the birds, it would seem, must uncon- 
sciously be guided upward toward light rather than downward to- 
ward darkness. 
The longer period of daylight, also, upon high as compared with 
lower elevations, furnishes another point of similarity between such 
situations and the northern habitats ol those species which extend 
southward on mountain ranges. 
This difference in the length of the period of daylight in valleys 
and on mountains is not improbably a cause of some variation in 
color of species inhabiting both situations; and from the known 
effect of the action of light on organic color, the varying period 
of light in different regions would seem properly to be a matter 
for consideration in connection with the subject of geographical vari- 
ation of species, as well as their seasonal movements. In regions 
where occur great seasonal changes of light, there, also, take place 
the most complete seasonal color-changes in resident animals. 
Before passing to a formal consideration of the birds of the region 
a few remarks upon the subject of its Avi-fauna in general will not 
be out of place. 
Although along the more cultivated portions of the valleys the 
familiar birds were not different from those which abound at the 
same season in the valley of the lower Hudson, a marked difference 
in the Fauna resulted from the absence or rarity of many species com- 
mon in the latter region. Aside from those southern species now 
well known to characterize the Fauna of the lower Hudson Valley, but 
which would obviously be excluded from this territory, there were 
absent other species, less definitely restricted in their northward 
range, as well as certain Alleghanian forms which, from the latitude 
