Each long-distance journey poses significant physiological challenges for the red knot. As a result, their bodies must make several adaptations in order to survive the trip (similar, I suppose, to the physical adaptations we all have to make when trying to cram ourselves into a much-too-narrow airplane seat for a cross-country flight). Immediately prior to each journey, the birds’ flight muscle mass increases while their leg muscle mass decreases. Their stomach and gizzard masses decrease, while fat mass increases by more than 50 percent. Throughout their wintering range they feed primarily on small mussels and other mollusks, shell and all. However, when traveling long distances they eschew those hard foods because of their shrunken gizzards. It is primarily the soft eggs of the horseshoe crab they are able to ingest. Since the red knot’s spring migration is synchronized with the release of horseshoe crab eggs, those eggs become the ideal food for a long-distance traveler. As a result of the super-abundance of the eggs across Delaware Bay beaches, the birds save considerable energy in hunting for necessary food sources.
When red knots arrive at the Delaware Bay they are exceedingly thin, almost to the point of emaciation. As a result, they need to eat constantly in order to increase their fat mass sufficient to continue their journey. It is not unusual for them to gain up to 10 percent of their body weight each day and double their body weight during their time along the Bay. There is at least one estimate that each red knot must eat approximately 135,000 horseshoe crabs eggs (in a period of about two weeks) in order to double its overall weight.
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