Such corrected dates can be used in making histograms that show the time of primary breeding activity, or the "egg-season." All such schedules are generalizations; data are used for a species from any year of observation, whether 50 years ago or less than one year ago. One advantage of such procedure is that averages and modes are thus more nearly representative of the basic temporal adaptations of the species involved, as is explained below. When information on the schedule of a species from one year is lumped with information from another year or other years, two (and ordinarily more than two) frequency distributions are used to make one frequency distribution. The great advantage here is that the frequency distribution composed of two or more frequency distributions is more stable than any one of its components. Second, the peak of the season, the mode of egg-laying, is represented more broadly than it would have been for any one year alone. Third, the extremes of breeding activity are fairly shown as of minute frequency and thus of limited importance, which would not be true if just one year were graphed. All these considerations combine to support the idea that general schedules in fact represent the basic temporal adaptations of a species much better than schedules for one year only. Variation in Breeding Seasons In the chronology of breeding seasons of birds, there are three basic variables: time at which seasons begin, time at which seasons end, and time in which the major breeding effort occurs. These variables have been examined in one population through time (Lack, 1947; Snow, 1955; Johnston, 1956), in several populations of many species over wide geographic ranges (Baker, 1938; Moreau, 1950; Davis, 1953), and in several populations of one species (Lack, _loc. cit._; Paynter, 1954; Johnston, 1954). The analysis below is concerned with breeding of many kinds of birds of an arbitrarily defined area and with the influence of certain ecologic and zoogeographic factors on the breeding seasons for those several species. THE INFLUENCE OF SEASONAL STATUS.--Here we are interested in whether a species is broadly resident or migrant in Kansas; 70 species are available for analysis. _Resident Species_ Twenty-four species, furnishing 875 records of breeding, are here considered to be resident birds in northeastern Kansas. These species are Cooper Hawk, Red-tailed Hawk, Prairie Chicken, Bobwhite, Rock Dove, Great Horned Owl, Red-bellied Woodpecker, Hairy Woodpecker, Downy Woodpecker, Horned Lark, Blue Jay, Common Crow, Black-billed Magpie, Black-capped Chickadee, Tufted Titmouse, Carolina Wren, Bewick Wren, Mockingbird, Eastern Bluebird, Loggerhead Shrike, Starling, House Sparrow, Eastern Meadowlark, and Cardinal. The distribution of completed clutches (Fig. 1) runs from mid-January to mid-September, with a modal period in the first third of May. Conspicuous breeding activity occurs from mid-April to the first third of June.