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University of California Publications in Zoology [Vol.12 



association. It is not improbable, however, that further work would 

 disclose the presence of a number of species of both mammals and 

 birds, at least on the Arizona side, not now known so far west. 



It is clearly apparent that the critical feature of the saguaro which 

 prescribes its avian dependents is nothing else than the favorable 

 opportunity offered for the excavation of safe retreats in its trunk. 

 And only the two woodpeckers are equipped for making these exca- 

 vations. So that, without the woodpeckers to make holes, the other 

 birds would be no better off for the presence of the saguaro. As it 

 happens, at least one of the species of woodpeckers (Centurus aropy- 

 gialis) invariably accompanies the cactus. 



This is a most interesting form of contingent or incidental inter- 

 dependence of animal and plant. Since the giant cactus during the 

 greater part of the year produces no fruit and harbors no insect life, 

 it follows that all the birds using its cavities as roosting or nesting 

 places glean their livelihood from the surrounding desert. The latter, 

 as already stated, presents conditions which seem to be practically 

 identical with the creosote association, which is so nearly barren of 

 bird life, and of vast extent beyond the limits of the saguaro. One 

 form of associational restriction is hereby proven, namely, that by 

 dependence upon safe home retreats. 



Encelia (Rocky Hills) Assoclvtion 

 BIRDS 



Buteo borealis calurus: max.; resident 

 Falco mexicanus: excl.; resident 

 Bubo virginianus pallescens: min.; 



resident 

 Aeronautes melanoleucus: excl.; resi- 

 dent 

 Calypte eostae; max.; summer 

 Sayornis sayus sayus: max.; resident 

 C'orvus corax sinuatus: min.; resident 

 Carpodaeus mexicanus frontalis.- 



min.; resident 

 Astragalinus psaltria hesperophilus: 

 min.; resident (?) (as a forager 

 only - ) 



Spizella passerina arizonae: max.; 



winter 

 Spizella breweri: min.; winter 

 Spizella atrogularis: excl.; transient 

 Amphispiza bilineata deserticola: 



min. ; summer 

 Stelgidopteryx serripennis: max.; 



summer 

 Salpinctes obsoletus obsoletus: max.; 



resident 

 Catherpes mexicanus conspersus: 



excl.; resident (?) 



MAMMALS 

 Ovis canadensis nelsoni: excl. Ammospermophilus leucurus leucurus: 



Ammospermophilus harrisi harrisi: max. 



max. Peromyseus crinitus stephensi: excl. 



