Ducklings have two sets of visual memory, able to recognise images in one eye that they don't recognise in the other.
Scientists from the University of Oxford have shown that newly hatched ducklings that are shown a substitute mother object with only one eye do not recognise it when they have only the other eye available.
Ducklings and other fowl normally learn to follow their mother within a few minutes of seeing her, through a process called imprinting. This process can allow a duckling to learn to identify any moving object presented to it during its 'sensitive' period - typically within the first week or so after hatching.
In this new study, published in the journal Animal Behaviour, ducklings were initially presented with either a red or a blue duck decoy, which moved in a circular path, while wearing an eyepatch over one eye.
The ducklings 'imprinted' on this maternal surrogate, learning with this first eye to follow the coloured decoy they had been presented with. In subsequent choice tests over the next three hours, each duckling was presented with both the red and blue decoys simultaneously while wearing either no eyepatch, a patch over the same eye as in training, or a patch over the other eye - the one that had seen the decoy during training.
The ducklings that made their choice with both eyes, or with the same eye with which they had been trained, accurately preferred to follow the original decoy. But the ducklings wearing a patch over the original eye, so that only the naive eye was available during testing, showed no reliable preference between the two decoys.
The researchers say the results tell us something about how birds' brains work. In most mammals there is a large bridge connecting the two halves of the brain called the corpus callosum. This structure allows rapid transfer of information between the two brain hemispheres, including information arriving from the visual system. In birds and other vertebrates, however, there is no equivalent structure, and the direct inputs from each eye go to the opposite side of the brain. That means information obtained with one eye has to follow more complex anatomical routes to become available to the other.
This new experiment shows that this anatomical difference means that when a duck gathers visual information with one eye - even something as important as learning to identify its mother - that information, in the short term, is not available when behaviour is controlled by the other eye. For some time at least, a bird may be visually informed with one eye and naive with the other.
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