It will be clear from the foregoing that not all bird movements are restricted to spring and autumn; not all long-distance movements occur on a north-south axis, or even on a consistent axis; and not all movements occur at the same times from year to year. The irregularity of some of these movements implies that they occur as a direct response to habitat and food conditions at the time, and are in no sense 'anticipatory' as is much regular seasonal migration. For the most part, movement patterns have been inferred from local numerical changes, and in very few species have they been supported by abundant ring recoveries. This is not surprising in species which live mainly in areas where human population density is extremely low, and whose movements are so variable from year to year. More extensive use of satellite-based radio-tracking is one obvious means of gaining more understanding of the nomadic movements of individual birds. In the meantime, many questions remain, notably: how do nomadic species know when and where to go? From how far away can they detect suitable conditions? How far do individuals move? And to what extent are their lifetime movements directional? As mentioned above, many birds classed as nomadic still have a north-south component in their movements, in line with latitudinal-seasonal trends in temperature and rainfall. Once again we see the different kinds of bird movements grading into one another, whether migration and nomadism or migration and dispersal. In any one population, however, one or two kinds usually prevail.
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