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Bayou-Diversity (21 October 2018) MIGRATION The ridgeline of the roof of my house in the woods is oriented north-south. In October this can be determined without a compass, without watching the sunrise or sunset, and certainly without checking to see which side of the white oaks that moss is growing on. From the back deck one has only to watch the sky through the hole in the canopy above the house. If the timing is right, avian compasses appear in the form of small groups of songbirds. The individuals are bunched tightly together; their flight is hurried and gives the impression of a communal determination. They fly right to left straight down the roof ridgeline and disappear in seconds. They are a part of one of the most mysterious acts of nature – bird migration. Even after years of scientific study the exact methods birds use to navigate across continents and oceans in biannual journeys are poorly understood. Some are known to use the sun as a reference point, but that assumes that birds also must have an internal clock in order to know where the sun should be at a given time of day. And what about those species that migrate at night? Tests have shown that some orient by the stars, a conclusion made with the caveat that these birds must also possess a knowledge of the night sky – a type of pre-programmed star map for each night at every location on their migration route spring and fall. Then there is the issue of how birds orient when the sun and stars are obscured by clouds, which they seem to do with ease. It turns out that the same iron atoms in the metal roof of my house are found deep inside the brains of some birds, and they are sensitive to the earth’s magnetic pole, thus serving as a built-in compass. Watching these birds hurry on their way, it is comforting to me that every revelation hatches a dozen new questions. (adapted from “Bayou-Diversity: Nature & People in the Louisiana Bayou Country,” LSU Press)


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