1914] Grinnell: Mammals and Birds of the Colorado Valley 71 



been swept by channel-swinging for a long period of years. However, 

 in the vicinity of the Colorado River, we failed to observe the screw- 

 bean outside of the willow association, although it is known to occur 

 in the mesquite and even the catclaw associations elsewhere ; so that 

 here the screwbean may be considered a restrictedly riparian plant. 



One other plant of the willow association requires mention — the 

 cane (Phragmites communis), which grows in dense jungles on per- 

 manent portions of the river bank from the vicinity of Picacho down. 

 The tracts of cane are usually narrow, but. as in Canebrake Canon, 

 below Picacho, may extend continuously close along the river for miles. 

 At the higher stages of the river the bases of the stalks are submerged, 

 while the drooping ends trail in the water (see pi. 4, fig. 3). A verdant 

 screen on either hand thus intercepts the view of the voyager. 



The exogenous vegetation of the willow association is all of it 

 deciduous, so that before the time of leafing-out in early March (see 

 pi. 5, fig. 4) a very different appearance is presented from that later 

 in the season. The spring growth is luxuriant, and accompanying it 

 is a crop of insects which offer prolific food-supply to the abundant 

 bird population of that season. Practically all of the birds listed for 

 the willow association are either insectivorous or raptorial. Gramin- 

 ivorous or spermophilous species are notably absent. 



The greater part of the fifty passerine birds listed for this asso- 

 ciation are transients or winter visitants. Only three are permanent 

 residents. The most notable characteristic summer visitants are : 

 Dendroica aestiva sonorana, Vireo belli arizonae, Piranga rubra coop- 

 eri, Archilochus alexandri, and Molothrus ater obscurus. 



Since the willow association includes practically the only forest 

 elements in the region, we find here a concentration of dendrophilous 

 birds, other than foliage-feeders, such as woodpeckers and flycatchers. 

 The latter, in particular, are abundant in both species and individuals, 

 and contribute to the great contrast afforded between the life of the 

 riverside and that on the desert which in large part lacks them. 



While bird-life is conspicuously more abundant in the willow asso- 

 ciation than in any one of the desert associations, just the reverse 

 obtains with the mammals. With the exception of the insect-feeding 

 bats, which share with the smaller birds the benefits of the insect 

 supply, there is but one rodent of wide and plentiful occurrence — 

 Peromyscus maniculatus sonoriensis. Three other rodents occur locally, 

 notably Sigmodon. Otherwise the only mammals of the willow asso- 

 ciation are far- ranging predators. 



