How an Eight-Sided ‘Egg’ Ended Up in a Robin’s Nest – The New York Times

The birds seem to be responding to variables that matter in nature. Cowbird eggs, for instance, are noticeably wider than their own, so robins may have evolved a canny sense of when width is off.
They seem to be quite hesitant about rejecting eggs when the variable that we changed was not natural, Dr. Hauber said, referring to the angular, pointed eggs. Robins dont know what to do with it, because theyve never evolved to respond to it.
And rather than toss out one of their own eggs by mistake, they…

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