440 BULLETIN 17 0, UNITED STATES NATIONAL MUSEUM 



milion, the various chollas blossomed in different colors, the prickly- 

 pears glowed with great yellow blossoms, and along the desert trails 

 two species of rainbow cactus were masses of gorgeous flowers, dark 

 crimson with white centers and magenta with j^ellow centers. 



A. J. van Rossem (1936) found elf owls in the cottonwood groves 

 along the Santa Cruz River, at a point some 14 miles from the nearest 

 giant cactus; he "saw at least a dozen birds within half a mile", on 

 May 14, where they were doubtless breeding; this locality is "nearly 

 40 miles from the extensive groves" of giant cactus near Tucson, 

 where we found them breeding. 



Nesting. — The abundance of the elf owls in the region described 

 above can be appreciated from the fact that we collected 15 sets of 

 their eggs in the parts of three days, May 21, to 23, 1922, that we were 

 able to devote to the saguaro plains, and much of this time was taken 

 up with other things. The nests, with one exception, were all in the 

 giant cacti, in old or abandoned holes made by Gila woodpeckers 

 and Mearns's gilded flickers. Nearly all these giants had one or more 

 of these holes, and many of them were fairly riddled with them. 

 Most of the holes were from 15 to 20 feet up, within eas}'- reach from 

 our 18-foot ladder, but some were lower and some as high as 30 feet. 

 One old veteran giant cactus, from which I took one of my sets, had 

 been visited off and on by my companion, Frank C. Willard, since 

 1904; during that period he had taken from its numerous holes the 

 eggs of desert sparrow hawk, saguaro screech owl, elf owl, ash-throated 

 flycatcher, Arizona crested flycatcher, Lucy's warbler, Gila woodpecker, 

 Mearns's gilded flicker, and cactus wren; on one day he found both 

 woodpeckers and the elf owl nesting in it at the same time ; I think it 

 holds the record for all this popular resort. 



The prominent ribs of the giant cactus are armed with rows of long, 

 stiff, sharp spines, so that climbing them is a painful process; but, 

 from a ladder, it is easy to chop out the holes, as the pulp is not much 

 harder than watermelon rind. Woodpeckers find it easy to excavate 

 their holes in this pulp, which largely accounts for, the popularity of 

 the saguaros as nesting sites. The sap that exudes from the fresh 

 cuts soon hardens around the entrance hole and around the walls of 

 the cavity, forming a hard, firm nesting box, which lasts for many 

 years, furnishing an ideal home for any of the various species that 

 prefer to nest in these cactuses. I have seen skeletons of these giants 

 lying on the ground, in which all the pulp had disappeared, leaving 

 only the hard ribs intact and the gourd-shaped shells of the nesting 

 holes still retaining their shape. 



Two surprises that we encountered illustrate the overcrowded 

 condition and the competition for nesting sites in this populous 

 community. One day, we saw an elf owl looking out of a hole in a 

 giant cactus and took a photograph of it. On climbing up to it, we found 

 a saguaro screech owl sitting on a set of three elf owl's eggs. On 



