Aggressive Behavior of the Female 264 Interspecific Relationships 264 Discussion 265 COURTSHIP BEHAVIOR 267 Displays and Postures 268 Discussion 270 SELECTION OF NEST-SITE AND NESTBUILDING 272 Building 274 Gathering of Nesting Materials 276 Length and Hours of Nestbuilding 277 Abortive Nestbuilding Efforts 277 Renesting 277 The Nest 277 EGGLAYING AND INCUBATION 278 Egglaying 278 Clutch-size 279 Incubation 280 The Roles of the Sexes in Incubation 280 Relief of Partners in Incubation 283 NESTLING PERIOD 283 Hatching Sequence 283 Development of the Nestlings 284 Parental Behavior 285 Feeding of the Nestlings 286 Nest Sanitation 287 Fledging 287 Nest Parasites 287 FLEDGLING LIFE 288 Second Broods 288 REPRODUCTIVE SUCCESS 289 Behavior 290 Predation 291 Cowbird Parasitism 291 SUMMARY 292 LITERATURE CITED 294 INTRODUCTION The Bell Vireo (_Vireo bellii_ Aud.) is a summer resident in riparian and second growth situations in the central United States south of North Dakota. In the last two decades this bird has become fairly common in western, and to a lesser extent in central, Indiana and is apparently shifting its breeding range eastward in that state (Mumford, 1952; Nolan, 1960). In northeastern Kansas the species breeds commonly and occurs in most tracts of suitable habitat.