[pks016]

Generally, you test birds in a series of discrimination experiments. So for this example, the discrimination task would be distance calls vs contact calls. Let's say, birds learn to discriminate between 20 distance calls vs 20 contact calls. They can learn this quickly (reward and punishment). After birds learned this rule (say 75% accuracy), we ask to generalize this by adding more calls. Now, birds have to discriminate 30 distance calls vs 30 contact calls (10 new for each). We can check now, for which new calls they are more likely to correct and incorrect response.

In this research field, we often perform this types of experiments (not this exact one). So, this is not that surprising to some of us. Birds also prioritize duration a lot when telling apart calls.