In the late Summer and early Fall,  birds are on their way back South, to the neotropics for the winter.  However, the birds will not be singing and it will take an astute bird watcher to pick them out among the vegetation.   Unlike in the Spring, Birds are not in the mating mode and many

can find enough food to make it through the winter.  Other short-distance migrants, such as robins, yellow-rumped warblers and woodcocks leave the northern U.S. for the deep South where they feed on fruits and invertebrates in areas without much snow cover.  Blackbirds congregate in huge mixed flocks throughout the Northeast and Mid-Atlantic on their way to  wintering grounds in the southern U.S.  In October and November, look for the first juncos (often called snowbirds) and white-throated sparrows that arrive from Canada to spend the winter at our bird feeders.  More than half of all North American bird species, however, winter in the neotropics.  The blackpoll warbler flies non-stop across the Atlantic ocean from the eastern U.S. to its wintering grounds in South America.  Likewise, our tiny ruby-throated hummingbirds that have hovered about our feeders all Summer will head South toward the Caribbean and wing over the Gulf of Mexico non-stop on a 500-mile flight, their wings fueled by a supply of brown fat.
  Migration may occur sporadically.  Some northern birds only migrate to more southerly locations when their food becomes scarce.  Northern conifer tree seed crops typically have boom and bust production cycles, called mast cycles.  When seed crops fail, redpolls, crossbills and other seed-eating birds appear in southern locations where they are not normally seen.  These irregular migrations are called irruptions.  An absence of rodents in boreal forests can lead to irruptions of snowy and great gray owls in the middle Appalachians.  Migration patterns