Can a newly hatched duckling learn an idea rather than an object?

They followed the relation, not the objects. Ducklings briefly exposed to a pair that was the same later preferred to follow new pairs that were also the same, and those exposed to a mismatched pair preferred new mismatched pairs. The shapes and the colors were different the second time, so only the abstract relation carried over. No reinforcement training was involved at any point.

Holding a logical relation between two stimuli, and then applying it to stimuli you have never seen, is called relational concept learning. It is abstract in the strict sense, because it survives changing everything concrete about the examples. A few species manage it, usually after extensive reinforcement training.

Imprinting sits at the opposite end of the reputational scale. A newly hatched bird meets something, follows it, and keeps following it, which looks like the most rigid and least intelligent learning in the catalogue. Putting the two together is the experiment.

Newly hatched domesticated mallards were briefly exposed to a pair of objects. For some ducklings the two objects matched in shape or in color; for others they differed. Later the ducklings met pairs of entirely new objects, and they preferred to follow the pairs that repeated the relation they had been exposed to.

Nothing concrete carried over, since the second pairs shared no shape or color with the first, so the only thing the duckling could have retained is the relation itself. The authors draw the conclusion plainly: even in a fast and seemingly rigid form of learning, the brain is working with abstract conceptual reasoning, a faculty usually reserved for animals we rate highly. Keep the scope narrow, though. One relation, one behavior, and following a pair of objects is a long way from reasoning in general.