THE CHICK-A-DEE CALL SYSTEM OF THE MEXICAN CHICKADEE

Chick-a-dee calls of the Mexican Chickadee (Panus sclateri) are compos ed of combinations of three common note types (A, C and D) and one ver y rare type (B). Calls have the invariant sequence of notes A-B-C-D, w here any note type may be omitted, given once or repeated a variable n umber of times before transiting to the next type. The B and C notes a re phonologically similar to the B and C notes of chick-a-dee calls of the Black-capped Chickadee (P. atricapillus), but the A note is marke dly different and the D note somewhat different from equivalent notes of the congener. A total of 2,071 calls recorded yielded 60 different call types, and Zipf-Mandelbrot plots show that the call system is ''o pen''; as the sample size is increased new call types will be found wi thout demonstrable bound. In relatively undisturbed contexts (with mat e on territory, in fall flocks, alone in fall) birds gave mainly [A][D ] calls with lesser numbers of [A] and [C] calls, where brackets indic ate variable repetition of note types. In disturbed contexts (mobbing plastic Great Horned Owl, mobbing speaker playing calls of the Norther n Pygmy-Owl, observer sitting under the nest cavity) the birds gave mo re [C] calls with [A][C] as well. In the longest mobbing session to ow l calls, birds gave mainly [A] calls when approaching, switched to [C] calls while flying about the speaker, and then resumed [A] calls and moved off when the playback was stopped. Outside of human language, th is is the second truly combinatorial system of vocal communication fou nd in animals, the first being chick-a-dee calls of the Black-capped C hickadee. This study provides the first data substantiating quantitati ve differences in calls from different contexts, an important step tow ard understanding what kinds of information combinatorial chick-a-dee calls encode.