In the description of the subspecies the two sexes are treated as one because no significant secondary sexual variation was found. Only fully adult specimens of age groups 3 to 5, as defined on pages 377 and 388, have been considered.
Unless otherwise indicated, specimens are in the University of Kansas Museum of Natural History. Those in other collections are identified by the following abbreviations:
| AMNH. | American Museum of Natural History. |
| CAS. | California Academy of Science. |
| CM. | Carnegie Museum. |
| Chic. AS. | Chicago Academy of Science. |
| Clev. MNH. | Cleveland Museum of Natural History. |
| LMH. | Collection of Lawrence M. Huey. |
| JKJ. | Collection of J. Knox Jones, Jr. |
| CMNH. | Colorado Museum of Natural History. |
| FM. | Chicago Museum of Natural History. |
| HM. | Hastings Museum, Hastings, Nebraska. |
| ISC. | Iowa State College. |
| MCZ. | Museum of Comparative Zoology. |
| MO. | University of Missouri Museum of Zoology. |
| MVZ. | Museum of Vertebrate Zoology, Berkeley, Calif. |
| NMC. | National Museum of Canada. |
| NGFP. | Nebraska Game, Forestation, and Parks Commission. |
| NCS. | North Carolina State College. |
| OHIO. | Ohio Wildlife Research Unit, Ohio State University. |
| OKLA. | Oklahoma Agricultural and Mechanical College. |
| PM. | Provincial Museum of British Columbia. |
| ROM. | Royal Ontario Museum of Zoology. |
| SDM. | San Diego Natural History Museum. |
| SITC. | Southern Illinois Teachers College. |
| USBS. | United States Biological Surveys Collection. |
| USNM. | United States National Museum. |
| UCM. | University of Colorado Museum. |
| UIM. | University of Illinois Museum of Natural History. |
| UM. | University of Michigan Museum of Zoology. |
| UU. | University of Utah Museum of Zoology. |
The species are arranged from least to most progressive, and the subspecies are arranged alphabetically.
The synonymy for each subspecies includes first a citation to the earliest available name then one citation to each name combination that has been applied to the subspecies and, finally, any other especially important references.
Marginal records of occurrence for each subspecies are shown on the maps by means of hollow circles and these localities are listed in clockwise order beginning with the northernmost locality. If more than one of these localities lies on the line of latitude that is northernmost for a given subspecies the western-most of these is recorded first. Marginal localities have been cited in a separate paragraph at the end of the section on specimens examined in the account of a subspecies. Localities that are not marginal are shown on the maps by solid black circles. Localities that could not be represented on the distribution map because of undue crowding or overlapping of symbols are italicized in the lists of specimens examined and in the lists of marginal records.
The localities of capture of specimens examined are recorded alphabetically by state or province, and then by county in each state or province. Within a county the specimens are recorded geographically from north to south. The word “County” is written out in full when the name of the county is written on the label of each specimen listed for that county, but the abbreviation “Co.” is used when one specimen or more here assigned to a given county lacks the name of the county on the label.
The following account has been made possible only by the kindness and cooperation of those persons in charge of the collections listed above. For the privilege of using the specimens in their care I am deeply grateful, as I am also to Prof. A. Byron Leonard for assistance with [figures 35-37], to Dr. Rufus Thompson for [figures 16-21], and to Mr. Victor Hogg who made all of the other illustrations. My wife, Dorothy Krutzsch, helped untiringly in assembling data, in typing the manuscript, and gave me continued encouragement. Finally, I am grateful to Professor E. Raymond Hall for guidance in the study and critical assistance in the preparation of the manuscript and to Professors Rollin H. Baker, Robert W. Wilson, and Robert E. Beer for valued suggestions.