In trying to become acquainted with the birds of a locality it must be borne in mind that the bird-fauna of any region varies with the season. Some birds live in a certain region all the year through; these are called _residents_. Some spend only the summer or breeding season in the locality, coming up from the South in spring and flying back in autumn; these are _summer residents_. Some spend only the winter in the locality, coming down from the severer North at the beginning of winter and going back with the coming of spring; these are _winter residents_. Some are to be found in the locality only in spring and autumn as they are migrating north and south between their tropical winter quarters and their northern summer or breeding home; these are _migrants_. And finally an occasional representative of certain bird species whose normal habitat does not include the given locality at all will appear now and then blown aside from its regular path of migration or otherwise astray; these are _visitants_. As to the relative importance, numerically, of these various categories among the birds which may be found in a certain region and thus form its bird-fauna we may illustrate by reference to a definite region. Of the 351 species of birds which have been found in the State of Kansas (a region without distinct natural boundaries and fairly representative of any Mississippi valley region of similar extent), 51 are all-year residents; 125 are summer residents, 36 are winter residents, 104 are migrants, and 35 are rare visitants. It must also be kept in mind in using bird-keys and descriptions to determine species that the descriptions and keys refer to adult birds and in ordinary plumage. Among numerous birds the young of the year, old enough to fly and as large as the adults, still differ considerably in plumage from the latter; males differ from females, and finally both males and females may change their plumage (hence color and markings) with the season. The seasonal changes of plumage accomplished by molting may be marked or hardly noticeable. "All birds get new suits at least once a year, changing in the fall. Some change in the spring also, either partially or wholly, while others have as many as three changes--perhaps, to a slight extent, a few more.... It is claimed by some that now all new colors are acquired by molt, and by others that in some instances (young hawks) an infusion or loss, as the case may be, of pigment takes place as the feather forms, and continues so long as it grows." There is much lack and uncertainty of knowledge concerning the molting and change of plumage by birds, and careful observations by bird-students should be made on the subject.