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Message: <blockquote data-quote="jurek" data-source="post: 1933624" data-attributes="member: 3357">Call differences are, quite frankly, also subjective and elusive. Only a handful of crossbill researchers claim they can distinguish call types, and only using sonagrams. I never seen a blind test when to two humans independently validated call types from random sets of recordings.I would expect to see objective definitions what differs in call types and where the borders cross into different call types. I would also expect that when some objective characteristics of the sound (eg. frequency plus change) were plotted, then calls of randomly chosen set of individuals would fall into clear clusters corresponding to call types. And yes, finches are famous for learning sounds - also to learn and change songs during life. I would expect experiments when captive crossbills were housed for several years with different call types, or in presence of recordings, and they kept their calls.</blockquote>