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.