298 Grinnell, Range of the Mockingbird in California. Ljuly 



true with regard to many other parts of the San Diegan faunal 

 district. 



The so-called citrus belt seems to be the metropolis of the Mock- 

 ingbird. The citrus belt lies in a portion of the Lower Sonoran zone 

 possessing a semi-arid climate, and in which, although the summers 

 are hot, the annual range of temperature downward is not so great 

 as to bring killing frosts. A law was long ago formulated by C. H. 

 Merriam (Nat. Geog. Mag., VI, 1894, p. 236) to the effect that 

 the northward distribution of animals and plants is determined by 

 the sum of the positive temperatures for the entire season of 

 growth and reproduction, and that the southward distribution 

 is governed by the mean temperature of a brief period during the 

 hottest part of the year. The range of the Mockingbird appears 

 to be accounted for under the first portion of this law, though not 

 altogether. The bird is of Austral origin, and in California is but 

 slightly or not at all migratory. The upward extension of its 

 breeding range is clearly limited by the temperature conditions 

 obtaining for a large portion of the year, including the summer, at 

 the upper edge of the Lower Sonoran zone. Its winter range is the 

 same except (1) that there is a scattering movement of birds-of- 

 the-year in the autumn, leading to their appearance during the 

 early winter in the warmer central coast belt of California; and (2) 

 that there is a vertical movement in the Mojave Desert and Death 

 Valley regions as an escape from the cold of the interior concomi- 

 tant with altitude. It is doubtless the severity of the winter 

 climate, in other words the normal dropping of the temperature 

 below the freezing point, that accounts for the relative scarcity 

 of Mockingbirds on the higher deserts of southeastern California, 

 where Lower Sonoran conditions find their extreme in summer. 



That food is not a prime factor in the case, as it clearly is in 

 controlling the winter distribution of certain other birds, is shown 

 by the fact that the Mockingbird is pre-eminently a berry-eater, 

 especially throughout the fall and winter. On parts of the Mojave 

 Desert mistletoe thrives and produces enormous crops of its 

 berries which in other localities, namely those of warmer winters, 

 form a favorite food of the Mockingbird. But this abundant food 

 is still not a sufficient attraction to overbalance the repelling effect 

 of the cold. Yet the latter is not sufficient to affect adversely other 



