By placing an eye patch on homing pigeons, researchers confirm that vision plays a major role in the birds’ remarking abilities to find home. But while these one-eyed pigeons still relied on vision to fly homewards from familiar locations, they couldn’t repeat that same route if the patch was switched to the other eye. The findings, published in Proceedings of the Royal Society B this week, show that their visual input is asymmetric.
The homing routes of pigeons (Columba livia) from familiar release points are idiosyncratic and tightly recapitulated on later flights – suggesting that learning plays a role in establishing routes. But how they find their way home has been debated for years. Previous work have looked at olfactory cues, sun and magnetic compasses, and the use of long, linear landmarks such as roads, railway lines, and rivers. However, the visual system of pigeons is different than that of mammals: The left and right halves of bird brains are thought to act more autonomously than ours since they don’t have a structure called the corpus callosum, where we pass information between brain hemispheres. Instead, their visual pathways share inputs from each eye asymmetrically to the brain hemispheres. So how does information from each eye contribute to route establishment?
Each pigeon was fitted with two eye rings. The hook side of the Velcro was attached to the feathers to make sure forward vision wouldn’t be compromised. The eye caps were constructed of a double layer of flexible etched plastic. A. Martinho et al., Proc. R. Soc. B 2015
Oxford University’s Antone Martinho and colleagues wanted to see if this left-right autonomy affected homing behaviors. They attached GPS loggers to 12 pigeons, placed an eye patch on one of their eyes, and then trained them to fly home for 18 flights before flying another 18 flights from the same release point but with the opposite eye blocked. This was then followed by five flights with the original eye blocked and five flights with normal (two-eyed, or binocular) vision.
The routes learned solely with one eye were not recapitulated when flying with the other. The two groups of birds – trained first with the left eye or first with the right – formed new routes after switching eyes. That means their brain hemispheres learn and act independently to some extent. Interestingly, pigeons that flew first with the left eye formed new routes that were closer to their original paths.