68 University of California Publications in Zoology [Vol.12 



Pisobia minutilla: excl.; winter Sayornis nigricans: excl.; winter 



Actitis macularius: excl.; winter Corvus corax sinuatus: max.; resident 



Oxyechus vociferus vociferus: min.; Petrochelidon lunifrons lunifrons: 



transient excl.; summer 



Circus hudsonius: min.; winter Stelgidopteryx serripennis: min.; 



Pandion haliaetus carolinensis: excl.; summer 



transient Anthus rubescens : excl. ; winter 

 Ceryle alcyon: excl.; transient 



MAMMALS 



Castor canadensis frondator: excl. Procyon pallidus: max. 



Ondatra zibethica pallida: max. 



Remarks upon the River Association. — For reasons already 

 explained there is relatively little eryptogamic aquatic flora in the 

 Colorado River. There is therefore little or no food-supply from this 

 source to attract plant-eating ducks. This category of water-birds was, 

 in fact, very sparsely represented. 



On the other hand, herons were notably plentiful because of the 

 supply of catfish and carp made abundant at intervals by the drying-up 

 of overflow ponds. While fishes were not abundant in the main stream, 

 they were plentiful in backwater sloughs, where, too. the water was 

 more nearly clear because the sediment had a chance to settle out. 



The ornis of the river appeared to owe its proportionate consistency 

 in large measure to the above two circumstances, namely, poverty in 

 aquatic plant life, and sporadic abundance of certain fishes in the 

 lateral sloughs (see p. 62). 



The single carnivorous mammal {Procyon) belonging chiefly to the 

 river association was piscivorous in food habits, foraging along mud 

 bars (see pi. 4, fig. 2) and at the margins of overflow ponds and 

 sloughs, as do the herons. The two rodents of the river and larger 

 paralleling sloughs, which are here included as part of the River 

 Association, lived in banks immediately adjacent to the water. The 

 beaver fed chiefly upon bark, twigs and foliage of such willows and 

 cottonwoods as had fallen over into the water through being under- 

 mined by the current. 



