The birds most commonly associated with winter irruptions are the winter finches — Pine Grosbeak, Red Crossbill, White-winged Crossbill, Purple Finch, Pine Siskin, Common Redpoll, and Evening Grosbeak. Their food supply, or lack thereof, in the Canadian boreal forests where they normally overwinter, determines whether or not they will be seen as far south as the U. S. Key trees affecting finch movements in the boreal forest are spruces, birches and mountain-ashes.
Common Redpolls feed primarily on the catkins (seed-containing fruit) produced by birch and alder trees. When catkin production is low further north, as it is this winter, Common Redpolls leave these areas and irrupt into areas where food is more plentiful. (Photo: male Common Redpoll)
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