It has been demonstrated experimentally that Andean sparrows, resident in equatorial regions, come into breeding condition twice annually entirely independent of changing light periods (Miller 1963); evidently the breeding cycle is controlled by periodic internal stimuli. Probably northern migrants that winter in equatorial regions and beyond have their migratory urges controlled by similar rhythms or biological clocks. Also, no evidence suggests that the southward migration of birds is controlled by changing periods of light even among species such as white-crowned sparrows, for which this is a controlling factor in the spring. The fall stimulus is probably an innate cyclic occurrence brought on by a biological mechanism of unknown nature (King, Barker, and Farner 1963). It is pertinent to point out that the migratory instinct appears to be more or less transitory and not persistent over an extended period. Migratory birds may be delayed en route, either by natural conditions such as unusually abundant food supplies or forcibly by man. If detained until the end of the migratory season, migrants may not attempt to finish the journey because they apparently lose the migratory impulse. In the fall and early winter of 1929, abundant food and open water caused an unusual number of mallards to arrest their migration and remain in western Montana and northern Idaho. Later, however, when a heavy snowfall with subzero temperatures suddenly cut off the food supply, great numbers of the birds subsequently starved to death; a flight of a few hours could have carried them to a region of open water and abundant food. =WHEN BIRDS MIGRATE= One ordinarily thinks of the world of birds as sedentary during two periods each year, at nesting time, and in winter. For individuals this is obviously the case, but when the entire avifauna of North America or the world is considered, it is found that at almost all periods there are some latitudinal movements of birds. A few of these movements reoccur year after year with calendar-like regularity. Each species, or group of species, migrates at a particular time of the year and some at a particular time of the day. In this section some of the interesting differences will be discussed as to when birds migrate. Time of Year