i 914 ] Grinnell: Mammals and Birds of the Colorado Valley 87 



Neotoma intermedia desertorum: Corynorhinus macrotis pallescens: 



excl. excl. (?) 



Perognathus formosus: max. Pipistrellus hesperus hesperus: max. 

 Perognathus intermedius: max. (foraging everywhere else) 



Perognathus spinatus spinatus: max. 



Remarks vpon the Encelia (Rocky Hills) Association. — None of the 

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

 is 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 association 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, was 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 confertifolia, 

 Larrea divaricata (sparsely intermixed). Asclepias subulata (visited 

 regularly by the Costa hummingbird). Fagonia calif ornica (on the 

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

 six feet high growing on the sides of ravines and at time of blossoming, 

 in March, frequented by hummingbirds), Perityle emoryi (an abund- 

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

 March by fringillids), and Muhlcnbergia debilis (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 associational 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 doubtless carried to the more productive lowlands. 



