候鸟导航系统，神奇出乎想象

有些候鸟在迁徙时，是借助星象来导航的；然而星象导航面临一个问题：地球的自转轴不是恒定的，而是按固定周期摇摆，这意味着，北极星（或其他确定南北极的星象）并不总是与正北方向成固定角度，而会随时间而变化，这种自转轴摇摆导致的变化在天文学上叫作岁差（precession of the equinoxes），其周期长度约27000年。当然，27000年中的微小摆幅，对于设计导航系统的人类工程师来说，可以忽略不计，但是27000年对于进化史来说，却很短，因而，对于候鸟导航系统的设计者——它们的基因组——，这个问题必须处理，它们的解决方案很漂亮：用于确定正北（或正南）方向的星象，没有被作为常量，预先编码在候鸟的神经系统中，而是作为变量处理，这个变量什么时候被赋值呢？是在小鸟被孵化后不久，当它第一次有机会看到星空时，它会连续数小时凝视星空，通过观察天穹转动，来找出北极所指方向附近的星象，并印刻（impress）到它的神经系统中。太棒了！以下摘自Steven Pinker: How The Mind Works第3章Revenge of the Nerds： Many migratory birds fly thousands of miles at night, maintaining their compass direction by looking at the constellations. As a Cub Scout I was taught how to find the North Star: locate the tip of the handle of the Little Dipper, or extrapolate from the front lip of the Big Dipper a distance seven times its depth. Birds are not born with this knowledge, not because it is unthinkable that it could be innate, but because if it were innate it would soon be obsolete. The earth's axis of rotation, and hence the celestial pole (the point in the sky corresponding to north), wobbles in a 27,000-year cycle called the precession of the equinoxes. The cycle is rapid in an evolutionary timetable, and the birds have responded by evolving a special algorithm for learning where the celestial pole is in the night sky. It all happens while they are still in the nest and cannot fly. The nestlings gaze up at the night sky for hours, watching the slow rotation of the constellations. They find the point around which the stars appear to move, and record its position with respect to several nearby constellations, acquiring the information imparted to me by the Cub Scout manual. Months later they can use any of these constellations to maintain a constant heading—say, keeping north behind them while flying south, or flying into the celestial pole the next spring to return north.