728 U.S. NATIONAL MUSEUM BULLETIN 23 7 pari- 2 



male returned to his old territory on the study area. A second male 

 arrived two days later, but a short cold spell delayed the arrival of 

 the rest of the population until April 12. 



Cruikshank (1942) states that the species arrives in a marked wave 

 about New York City diu-ing the first week of May, and stragglers 

 pass through as late as the first week of June. I have never observed 

 any marked wave in central or western Pennsylvania; there a few 

 birds appear first, then the population buUds up over a period of 

 1 to 2 weeks. My observations indicate that the first arrivals are 

 males. They generally do not appear on the nesting areas until 

 the grass is tall enough to conceal them. 



Territory. — Upon arrival at their nesting grounds, male grasshopper 

 sparrows undertake territorial establishment. The first arrivals have 

 the area to themselves and generally confine their singing to the 

 morning hom'S. As more birds return, territorial activity increases 

 in intensity, reaching a climax about 2 to 3 weeks after the first 

 birds arrive. Then song is heard throughout the day. 



The male proclaims territory by singing the "grasshopper" or 

 territorial song (see Voice). When engaged in a song duel, the male 

 alternates song with display. Crouching with head lowered between 

 the shoulders, he raises and flutters one or both wings. Then after 

 hearing the song of his neighbor, he stands erect and sings back. 

 The song completed, he again crouches and flutters his wings while 

 his rival sings. The wing fluttering, conspicuous only during terri- 

 torial establishment, is never accompanied by a song or a call, and 

 is confined to the intervals between songs. 



I regard the wing fluttering of the grasshopper sparrow as a hostile 

 display. During the period of territorial establishment the song 

 of a rival is a sufficient stimulus to release it. Often the birds are 

 hidden by the vegetation or the topography of the field so they can 

 not see one another. They sense the presence of a rival by the 

 sound of his song and manifest this by a hostile display, as if the rival 

 were nearby in the grass. 



I have never observed a territorial dispute that elicited an intimi- 

 dation display of high intensity, although some could have taken 

 place in the grass, out of sight. The only physical encounters I 

 observed during hundreds of hours spent with the species occurred 

 after a bird saw another invade its aerial territory. In each instance 

 the bird chased the intruder, then retired to a singing perch, fluttered 

 his wings, and sang the grasshopper song. I have witnessed a num- 

 ber of such clashes at disputed boundaries. Since the deep grass con- 

 ceals territorial infringements on the ground, this mode of defense 

 could be most important. Perhaps the grasshopper sparrow recog- 

 nizes the limits of its territory only from a grasstop point of view. 



