102 The Wilson Bulletin— No. 105 



thoiiotary warbler, orioles, and many others leaving' just 

 ■\vUen food, becomes most plentiful. Thus we see that mi- 

 gration is not merely a response to appetite. 



II. 



Materials and Methods. 



The data upon which this article is based are from two 

 .sources: nuiseum records and field observations. My 

 thanks are due to Mr. liunker in charge of the museum 

 collection here, who has placed at the writer's disposal 

 the records of the museum collection which show the date 

 upon which each specimen in the museum was taken. 

 These records have supplied many data, all of which are 

 of absolute reliability. My thanks are also due to Pro- 

 fessor Frank Smith, of the University of Illinois, for the 

 use of his unpublished data on the migration of Illinois 

 birds covering a period of nine years. The writer, likewise, 

 made (hiily trips in the fall migration season of 1016 and 

 the s])ring of 1017, making careful records of all birds ob- 

 served. In this work only absolute and unquestionable 

 identifications have been recorded. Many birds seen, es- 

 pecially warblers and sparrows, had thus to be left off of 

 the records even when the identification was fairly posi- 

 tive. For many of the more difficult birds dependence was 

 placed entirely upon nuiseum specimens. The field records 

 here reported are consi<lered to be above question. Cir- 

 cumstances, likewise, did not permit much study of the 

 water birsls. Records were also kept of the weather con- 

 ditions and of the places visited and of the likelihood that 

 birds would be found and allowances made for these. Like- 

 wise, it was recorded, wliere significant, whether the birds 

 were seen in one flock or in difierent places. 



In keeping records, close attention Avas given to the 

 question Avliether the bird was to be expected in the local- 

 ity visited and whether the bird Avas seen or lieard, since 

 the activitiy of the bird has much to do with its discovery. 

 If a given bird was looked for in its proper habitat and 

 not found a circle with a dot in the center Avas used to in- 



