BIRD LIFE IN THE UEUBAMBA VALLEY OF PERU. 31 



At other localities, notably inner valleys with comparatively low 

 rainfall, the Temperate Zone is characterized by a scrubby vegeta- 

 tion restricted largely to the borders of streams, up which the arid 

 portion of the zone extends Gnger-like projections well into the Puna 

 Zone. 



Conditions of this kind can be understood only by one who has 

 observed them in the field. They can not be expressed by the most 

 careful labeling of specimens. A collection from Ttica-Ttica (alti- 

 tude 11,900 feet), for example, contains a mixture of forms apparently 

 not susceptible of zonal interpretation. With such characteristic 

 species of the Puna as UpucertJiia pallida, Geositta tenuirostris, Agri- 

 ornis solitaria insolens, Muscisaxicola rufivertex, etc., there are pre- 

 sumably associated Anaeretes flavirosiris, Serpophaga cinerea, Elaenia 

 alhiceps, Saltator alhociliaris, Diglossa, hrunneiventris, etc., but in the 

 field it was found that the first group was largely restricted to the 

 grassy slopes, while the second was found only in the narrow fringe 

 of bushes at the borders of streams. 



The collection from Cedrobamba contains a similarly confusing 

 assemblage of Temperate and Puna Zone forms, the occurrence of 

 which within a restricted area is explained by Heller's description of 

 the strildng conditions which exist at that locality. The upper limit 

 of the Temperate Zone, therefore, coincides with the upper limit of 

 tree or bush growth, and this may often be at a higher altitude than 

 the lower limit of the succeeding or Puna Zone. 



On the eastern slopes of the Andes the lower limits of the Temper- 

 ate Zone correspond with the upper limits of the Subtropical Zone, 

 and although forest may stretch continuously from timberline to 

 the Amazonian plains, the limit between the two zones is here uni- 

 formly about 9,000 feet. Where, however, lack of rain prevents the 

 development of the forest which so strongly distinguishes the Subtrop- 

 ical Zone, the Temperate Zone in its arid phase may descend much 

 lower. In the Urubamba Valley it reaches Torontoy at 8,000 feet 

 and on the treeless Pacific slope of the Andes it actually descends 

 to sea level. The influence exerted by the Humboldt current must, 

 however, be taken into consideration here, an inquiry which would 

 lead us far beyond the scope of this paper. 



The assemblage of species characterizing the Temperate Zone is 

 intensely interesting. Being either tree or bush inhabiting, it is 

 clear that they must have had their geographic origin in tree or bush- 

 grown regions. The humid South Temperate Zone is separated from 

 the district under consideration by 1,500 miles of treeless country, 

 which has proved an effective barrier to the northward extension of 

 the forest-inhabiting species of southern Chile.** 



^* Scytalopus is an exception: but it is not improbable that Scyialnpus originated in the mountains of 

 southeastern Brazil where its present isolation from the Andean forms is paralleled among trees by AraW 

 earia. 



