I spoke with a friend who is a birding expert, about seeing Pine Siskins (Carduelis pinus) at our feeder. In that context, he introduced me to term “irruption”. In general terms, an irruption is a sudden, violent invasion. In ecological terms, an irruption is “a sudden sharp increase in the relative numbers of a natural population usually associated with favorable alteration of the environment”. I.e.: there isn’t enough food at home, so they go somewhere else to find some.
Pine Siskins spend most of their time in Canada and in the mountains of the Western US. But every other year, they irrupt, erratically, in places throughout the rest of the US., apparently driven by the availability of conifer seeds in their normal range. So, I feel lucky that our feeder has been chosen as one of their alternate food sources this year.
Red-breasted Nuthatches, Red Crossbills, Evening Grosbeak, and Bohemian Waxwings also exhibit irruptive behavior.
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