Saturday was International Migratory Bird Day, and shame on us for not having properly recognized it. According to the Environment for the Americas group, the day exists to educate and empower people to help combat the many threats to healthy bird populations. You all out there can of course be forgiven if the day passed without your notice. Not so for us, given that we’ve just published Engineering Animals: How Life Works, a book that explains the actual physical principles of animal structure and behavior. Animals as machines, basically.
One the book’s richest sections is on animal navigation, which really is the most fascinating aspect of bird migration. How do they know where to go? And how to do they know when they get there? As the book’s authors Mark Denny and Alan McFadzean explain, we’re just beginning to understand the complexity of these practices:
The past half century has seen a revolution in our understanding of animal navigation brought about by many, many thousands of studies: radar or global positioning system (GPS) tracking observations; experiments that altered the apparent direction of the sun, or the brightness or color of light; experiments in which migrating birds were displaced several hundred or thousand kilometers; experiments in which animals were blindfolded in one or both eyes, or had their sense of smell removed, or were placed in planetariums with altered star patterns, or had magnets attached, or were banded or radio-tagged. . . . It used to be thought that, for example, a migratory bird made use of a single skill, such as navigating via the sun, but our understanding has broadened considerably and with greater understanding comes greater respect for the capabilities of migrant animals: most birds use many different cues, which they prioritize in different ways when these cues conflict.
One of the things that these experiments have shown is that many migrating animals are born with routes hardwired into their brains. “Head southwest for 9 days, then turn due south until you reach a desert, then head east for four days and then south until you run out of gas.” That sort of thing. In one experiment that helped document this hardwiring, a group of migrating European starlings was displaced several hundred kilometers south while en route to their usual wintering groups in northern France. The older birds that had previously made the journey used cues such as the sun’s location in the sky to sense their displacement, adjusted course, and made it to their intended destination. The younger birds, on their first trip, just followed the hardwiring in their brains (“A direction for X days, then B direction for Y days, etc.”) without correcting for the displacement, and ended up in northern Spain. Denny and McFadzean: “Clearly, the novices were just reading the instructions that their genes handed out to them, whereas the more experienced birds were overriding these with learned cues, enabling a more flexible response to changed circumstances.” Most amazing part? Conditioned by this first voyage, the young starlings actually returned to northern Spain in subsequent years, not to the France that their genes intended.
Some birds do their migrating at night, and have been found to navigate by the stars. Experiments with birds in planetariums have shown that newborn birds stare up at the night sky all through their first summer to learn the position of the North Star based on the sky’s rotation. In one experiment, the stars in the planetarium were changed such that the sky’s rotation centered around a star other than the North Star, and the birds learned to navigate based on the new center.
Plenty of other mechanisms factor in animal navigation, including the use of infrasound, polarized light, odors, and geomagnetism. To Denny and McFadzean the main reason that animal navigational senses are so good is their redundancy. Many animals use more than one of these mechanisms, and they learn how to prioritize and integrate them based on experience. Of course, as the authors point out, birds and other animals obviously don’t know anything about these mechanisms. They’re not consciously calculating the information and weighing their senses. We may know that a bird’s eye can see polarized light and magnetic field intensity, but the bird sure doesn’t. It just sees patterns of light that its brain interprets as directional cues.
Animal navigation is just one area addressed in Engineering Animals, but, again, we’re here to celebrate International Migratory Bird Day. When Wolves Playing Chess With A Moose Day, Dolphin Echolocation Week, Examining Quadruped Gaits Day, and Robotic Cockroach Appreciation Week roll around, we’ll be back with more.
