1914] Grinnell: Mammals and Birds of flu Colorado Valley 77 



the conditions above the Laguna Dam. Here the whole association 

 had been effaced by drowning. On the other hand, the mesquite is 

 unable to exist on the desert proper, even in the larger washes. Only 

 at the mouths of these did scattering examples extend away from the 

 actual river bottom, and then, in the most favored places, for not more 

 than a quarter of a mile. 



Alan's occupancy of the region has affected the mesquite associa- 

 tion more than any other. The great value of the mesquite trunks 

 for fuel has led to its practical disappearance as a tree along much 

 of the lower course of the river. The steamboats which once plied 

 regularly between Needles and Yuma are said to be chiefly respon- 

 sible for this depletion. Several pumping plants contributed to the 

 demand for fuel. Mesquite trees are very slow of growth ; tracts of 

 stumps now mark many areas where luxuriant groves once stood. 



A considerable number of low-growing plants find places as elements 

 in the mesquite association, but by far the most important to the 

 animal life is the mesquite itself. This shrub, or tree, provides both 

 shelter and food, the latter through its fruit and foliage (see pi. 6, 

 fig. 7) either directly or by way of insects. The mesquite serves also 

 as the host of a parasitic plant (pi. 6, fig. 6), a species of mistletoe 

 (Phoradendron calif or nicum) , which when in blossom is visited by 

 myriads of insects, and which produces an abundant and almost con- 

 tinuous crop of berries. Several of the winter and resident birds of 

 the mesquite association depend almost wholly on these mistletoe berries 

 for their food. Notable among these are: Phainopepla, Mimus, Oreo- 

 scoptes, Planesticus and Sialia. 



Four species of breeding birds (Pipilo aberli, Toxostoma crissale, 

 Vermivora luciae, and Phainopepla nitens) find in the mesquite asso- 

 ciation the center of their abundance. One mammal is characteristic 

 of the same association, namely Neotoma albigula venusta. 



Saltbush Association 

 birds 



Lopliortyx gambeli: min.; resident Phalaenoptilus nuttalli nitidus: min. 

 Zenaidura macroura marginella: resident (?) 



max.; winter ( 'hordeiles aentipennis texensis: min. 

 Geococcyx calif ornianus: min.; resi- summer (only as a forager) 



dent Sturnella negleeta: excl.; winter 



Phalaenoptilus nuttalli nuttalli: Pooecetes gramineus eonfinis: excl.; 



min.; winter winter 



