160 BULLETIN 19 7, UNITED STATES NATIONAL MUSEUM 



these two birds or were inactive sexually. It is significant that some 

 pairing activity occurs previous to any perceptible increase in the 

 gonads. 



In the vicinity of the Pinole, Contra Costa County, Calif., early 

 in January 1930, I located several single shrikes and studied and 

 mapped their territories. Later, at the time when these shrikes paired, 

 and nested, there was no change in the size or limits of the territories, 

 two birds and finally the entire family remaining within the confines 

 of the area once occupied by but one of the parents. In most cases 

 the sex of the original owner of the territory used for nesting was 

 not known with certainty. In one instance a territory was first lo- 

 cated when seemingly but one bird was present. This bird later proved 

 to be a female, fortunatelj'^, in this case, mating with a bird sufficiently 

 different in coloration as to be individually distinguishable in the 

 field. The female, when discovered on February 7, I followed about 

 over her territory for some time, and so I was able to outline the 

 area occupied. Had the bird been paired at this time, I am confident 

 from other experiences that the mate would have been seen. By 

 February 10 a male had joined this female and they were constantly 

 close together. 



Other solitary birds were found to be joined by mates late in January 

 and still others not until March. Occasionally some minor shifts in 

 the territorial limits may be made when the winter territories are 

 converted into breeding territories, in one observed instance with the 

 seeming purpose of including a desirable nest tree. The complete 

 abandonment of some winter territories is inevitable if pairs are to be 

 formed ; disappearances of this kind were noted in a few instances in 

 January. 



The size and shape of the territory, whether breeding or feeding, 

 are dependent upon several factors, namely : the floral habitat occu- 

 pied, whether an open prairie or a moderately wooded area ; the con- 

 centration of the food supply and the provision of nest sites; the 

 local abundance of the species ; and the local associational or physical 

 barriers. 



There is a marked difference in sizes of territories in California, and 

 this apparently is correlated with habitat and concomitant variations 

 in food supply. Territories in grassy hills and meadows with scat- 

 tered oaks, eucalyptus, and lines of willows ranged from 11 to 14 acres 

 in size in Contra Costa County, but in semidesert terrain consisting 

 of much bare ground, widely spaced bushes, and few trees ; in Kern 

 County (race Lanius ludovicianus nevadensis) territories consisted 

 of 25 to 40 acres. 



In parts of the northern San Joaquin Valley, where the shrike 

 population is large, nearly every individual's territory is bounded on 

 all sides by other shrikes. But in the neighborhood of Pinole shrikes 



