CONSTRAINED PERMUTATION IN “CHICK-A-DEE”-LIKE CALLS OF A BLACK-LORED TIT PARUS XANTHOGENYS

ABSTRACT

Combinatorial calls of tits (Parus) resemble language in allowing the production of a huge variety of distinct call types based on combinations of a few note types. This study found a constrained type of permuted order of note types, which enlarges the potential for call variety and even more closely resembles properties of language. Vocalizations of the black-lored tit P. xanthogenys of India have never been studied in the wild and were known previously only through brief mention in two comparative reviews (Thielcke 1968, Hailman 1989). A wild-caught captive gave chick-a-dee-like calls in response to a stuffed owl. The tit used combinations of four note types (denoted A through D) to create 65 different call types. Zipf-Mandelbrot analysis showed that the repertoire of call types has the language-like property of “openness”: as more calls are sampled, the number of call types represented in the sample continues to rise without bound. First-order Markov-chain analysis revealed that B, C and D notes tend to be isolated from one another, with A notes given between them, within strings of the same note-type, and at the beginning and ending of calls. This constrained permutation is missing from the merely combinatorial chick-a-dee calls of the black-capped chickadee P. atricapillus, where note-types occur in fixed order both within and among calls. Calling under constant stimulus conditions changed unidirectionally during the course of mobbing, with D notes first replacing D notes in calls and then also replacing B notes as time progressed. The evolution of note types and call structure is discussed in comparison with the black-capped chickadee.