Recent work seeking to provide a formal account of meaning in animal communication took call form to be arbitrary (Schlenker et al. 2014, 2016b). But a long line of research (from Marler 1955 to Magrath et al. 2020) has shown that this is not always so, especially in birds: there is sometimes widespread convergence among the calls of different species, to the point that a species may understand the calls of another one that is geographically and phylogenetically extremely distant. This suggests that there might be a natural biological code by which the calls of unrelated and unfamiliar species can sometimes be understood without prior exposure. We clarify this possibility by distinguishing among three degrees of interspecies comprehension. In the first degree ('Understand Thy Neighbor'), a species understands some of the calls of a neighboring species because it is exposed to them. In the second degree ('Call Convergence'), a species may understand the calls of an unrelated and unfamiliar species by virtue of Marlerian convergence: a heterospecific call may globally resemble a conspecific call enough to yield understanding. In the third degree ('Featural Interpretation'), a species may use a rule that associates a meaning to a specific acoustic feature -e.g. higher call rate signals greater urgency, higher pitch signals greater arousal, greater noisiness signals greater negativity. This yields a kind of featural compositionality by which a species may understand a heterospecific call that does not globally resemble any familiar call, but still includes the crucial, interpretable feature. There might thus be an entirely new road to the emergence of compositionality in studies of meaning evolution. We lay out possible mechanisms of evolution of these degrees of interspecies comprehension, and isolate predictions that distinguish the third degree from the second.