Bird irruption

Natural history words are so often beautiful–like irruption, which here means an unusual influx of northern birds into southern places. We’re in an irruption year now, caused by scarce food supplies up north. From this post by the CUH, local naturalists in my area:

“Irruption is closely tied to a cyclical fluctuation in the seed crops of trees the birds depend on as winter food sources. Birches, alders and conifers in the boreal forests— all important food sources for irruptive songbirds — do not bear equal seed crops every year. ‘Mast’ years when lots of seeds are produced are followed by several years of poor seed production. This appears to be an evolutionary strategy to limit the populations of creatures that eat the trees’ seeds to ensure the maximum possible number of seeds survive. In good seed crop years, seed eaters do well, and their population expands. When the next year’s crop is poor, there’s even less food for each animal than there would have been if the population hadn’t grown the year before, and the population falls. Then, when the next good year comes, there are not enough animals to fully capitalize on the seed crop, and some seeds escape. Oaks, hickories, and beeches in the deciduous forests of the East have similar cyclical fluctuations in seed production.”

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