V ° L 1920 CVI1 ] Lincoln, Birds of the Clear Creek District, Colo. 



61 



between the Transition and Upper Sonoran life zones, in which 

 latter the entire valley is located. Here agricultural activity with 

 much irrigation has long been carried on effectively, but until 

 recently considerable areas were in their natural wild state, with 

 masses of impenetrable bushes, extensive swamps and groups of 

 large trees, usually immediately adjoining the cultivated fields, 

 thus offering ideal environments for many varieties of birds. 

 On the north and south bluffs overlooking the valley, are num- 



erous thickets of Wild Plum (Prunus amerlcanus), Choke-cherry 

 (Prunus melanocarpa) , and Hawthorn or Thorn-apple {Crataegus), 

 where small streams fed by springs and seepage from the irrigated 

 tracts, find their source. These streams form sloughs, heavily 

 fringed with Willow (Salix amygdaloldes), and Birch (Betula fonti- 

 nalis), and supporting masses of Watercress (Roripa). In places 

 they widen out into swampy ponds containing dense growths of 

 cattails and tules with small areas of open water. Small groves of 

 Broad-leaved Cottonwood (Populus occld entails), and Box Elder, 

 (Rulac texanum), are also of regular occurrence throughout the 

 length and breadth of the valley. 



