1914] (IritincU: Mammals and Bhrh of llir Colornrlo Valley S7 



Neotonia intermedia desertorum: Corynorliinus macrotis pallescens: 



excl. excl. (?) 



Perognathus formosus: max. Pipistrellus hesperus hesperus: max. 

 I'erognathus intermedius: max. (foraging everywhere else) 



I'erognathus spinatus spinatua: max. 



h'( marks uput) the Encclia (Rocky Hills) Association. — None of the 

 hills or "mountains" in the near vicinity of the lower Colorado River 

 i.s of such great altitude as to bring a reduction in temperature to 

 an extent sufficient to modify its biotic complexion. The tallest of 

 The Needles rises to less than 2,000 feet above the level of the river. 

 In other words, as far as observed, the plants and animals of the hills 

 show no distributional behavior other than as explained on associational 

 grounds. 



But the hill country does exhibit a distinctive a.ssociation of plants 

 and animals, .setting them apart sharply from the desert mesa, or the 

 riverside. A conspicuous shrub on the rocky steeps, especially at The 

 Needles, wa.s the dense Encelia farinosa, with its light gray foliage, 

 growing on talus slopes and even in crevices of the cliffs (pi. 10, fig. 

 15). Other plants of the same locality were: Atriplex confcrti folia. 

 Lama diraricata (sparsely intermixed), Asclepias subulata (visited 

 regularly tiy the Costa hummingbird), Fagonia calif ornica (on the 

 hottest slopes of broken rock), Hyptis emoryi (a "sage-bu.sh" five to 

 six feet high growing on the sides of ravines and at time of l)lossoming. 

 in ]\rareh. frequented by hummingbirds). I'l rityle emoryi (an abund- 

 ant composite annual, the seeds of which were much sought after in 

 .March by fringillids), and MUhloibergia dcbilis (a grass growing in 

 shaded ravines and providing forage for graminivorous rodents, like 

 Perognathus). 



While to distant view the hills seemed more barren of vegetation 

 than most of the other as.sociational areas, nearby inspection showed 

 abundant remains of inconspicuous annual plants. This in part would 

 seem to account for the great numbers of mammals present, as shown 

 by our trapping. The additional favorable factor was evidently the 

 abundance and availability of natural retreats afforded in the talus 

 and fractured outcrops. 



"With insectivorous and raptorial species, such as the swifts and 

 bats, hawks and owls, the rocky hills served merely as home retreats, 

 foraging being doubtle.ss carried to the more productive lowlands. 



