Comparative support for the social complexity hypothesis in the calls of two closely related species – Tufted Titmice Baeolophus bicolor and Black‐crested Titmice B. atricristatus

Tufted Titmice Baeolophus bicolor are a well‐studied species known for their gregarious nature and complex communication system. Black‐crested Titmice B. atricristatus are a closely related species, but much less is known about their communication system. In this study, we compared the chick‐a‐dee calls of Tufted Titmice and Black‐crested Titmice. We examined note use, main call sequence types, note transition probabilities, and first‐order and second‐order uncertainty (complexity) of calls. Black‐crested Titmice used more D notes and more overall notes than Tufted Titmice. Tufted Titmice used more Z notes and more hybrid D notes (D h ) notes. Tufted Titmice used more calls with the call sequence type [Z][I][D] and [Z], whereas Black‐crested Titmice used more [D] call sequence types. Tufted Titmouse calls had greater first‐ and second‐order uncertainty than Black‐crested Titmouse calls. More research needs to be done on Black‐crested Titmouse ecology and social behaviours, but early evidence suggests that Black‐crested Titmice have fewer individuals in their flocks than Tufted Titmice. The results of our study appear to lend support to the Social Complexity Hypothesis for Communication, which argues that more complex social systems (e.g. larger groups) need to have greater communicative complexity. Future research should consider whether the differences found in this study are related to genetic differences or to other differences like habitat preferences. Work should also be done to examine whether the difference in call complexity affects each species' role within their communities.