76 SHARP-BILLED WARBLERS. 



state which form I saw, thus I am unable to give the exact 

 winter range of our Blue Yellow-back. One or both forms oc- 

 cur at Key West, on the Bahamas and in the West Indies, 

 They begin their northward migration in early April, arriving 

 in Massachusetts the first week in May. Almost all depart for 

 the south about the first of September, but I found a few lin- 

 gering about Watsontown, Pennsylvania as late as the twenty- 

 ninth of that month. 



Fig. 39. 



Sternum of Blue Yellow-back, 



Genus. SHARP-BILLED WARBLERS. Helminthcphila, 



Size, rather .small, 4.25 to 5.25 Ions. Colors, 1)luisli or ji^reeiiislt 

 above; yelloAV, greenish-yellow and white beneath. AVing bands 

 and tail si>ots, present or absent. l?ill abont equal in length to the 

 liead and v<>ry sharp. Tarsus, longer than middle toe and claw. 



Members of this genus may be divided into two groujjs: 



A, wing bands and tail spots present, and with black mark- 

 ings on the sides of head ( dusky in females ) , and sometimes 

 w ith black breast or throat ])atch (absent or dusky in females) ; 



B, wing bands and tail spots, absent ( sometimes the feath- 

 ers of the latter are margined with Avhite ), and there ai-e no 

 ])rominent ])lack markings anywhere. The height of the keel 

 is equal to one-half the width of the sternum. Coracoid bones 

 shorter in length than the top of the keel. The tongue and 

 some other structural characters appear to vary somewhat 

 specifically. The nests of all our ^ew^ England species are 

 j)laced on the ground. The species are mainly confined dur- 

 ing the breeding season to temperate ]Srorth America ; thus 

 none are tropical. 



