YELLOW-BILLED MAGPIE l63 



their own program to pay much attention to other birds except on special 

 occasion. Ordinarily this stage of nesting is reached nearly at the same 

 time by almost all the pairs of magpies in a colony. 



Nesting, — Habits connected with nesting in the yellow-billed magpie 

 are, in general, like those of the black-billed species, but they contrast 

 in several ways. Possibly the difference in nesting behavior between 

 the two kinds seems more marked than in other types of behavior be- 

 cause the results of it, being definite objects, are more easily perceived 

 by the human observer. 



First, the nesting colonies are more compact and the nests are closer 

 together in the yellow-billed form. This may have some connection 

 with a more favorable foraging habitat that permits the yellow-billed 

 form to live in a more gregarious society. Or it may reflect the result 

 of some different need for group response to disturbance by intruders. 



The actual position of the nest provides one major difference between 

 the two kinds. In the yellow-billed one the nest is nearly always in 

 some tall tree and far out on the limbs, or if in a medium-sized tree, it 

 is likely to be in the periphery. Thus it is regularly at a different level 

 and in a different site from the low, bushy one occupied by the Ameri- 

 can black-billed birds in their nesting. 



Trees prominent among the ones nested in by yellow-billed magpies 

 are sycamore, valley oak, live oak, blue oak, poplar, cottonwood, locust, 

 and willow. One colony observed near Oroville by W. B, Davis tried 

 repeatedly to nest in a clump of digger pines but with poor success, for 

 these trees provided poor anchorage for the nests. They were easily 

 dislodged by the wind, and sometimes the weight of the nest itself was 

 enough to change the slope of the limb so that the structure would slide 

 off to the ground. (See Linsdale, 1937.) 



A curious item of yellow-billed magpie nesting, commented upon at 

 length by Dawson (1923), is the resemblance of a nest to a clump of 

 mistletoe. It happens that in California the area occupied by this bird 

 and that occupied by two kinds of mistletoe are closely similar in their 

 boundaries. Not only are the areas nearly the same, but the species of 

 trees concerned are the same. Cottonwoods, sycamores, and valley oaks 

 are the kinds of trees mainly involved in this peculiar relationship. The 

 bird not only nests in trees having many clumps of this plant, but also it 

 often builds actually within a mistletoe clump. Whether or not intentional 

 selection is made by the bird for this purpose, it is obvious that the close 

 resemblance of the two objects helps to screen the presence of nesting 

 birds. Even a person experienced in detecting the nests is unable to dis- 

 tinguish one from a mistletoe cluster at a distance, so nearly alike are 

 they in size, shape, and position. It seems possible that such a lot of 



