Complexity in <i>Chick‐a‐Dee</i> Calls of Mountain Chickadees (<i>Poecile gambeli</i>): Call Variation Associated With Flock Size and Flight

ABSTRACT The chick‐a‐dee call of chickadees, tits, and titmice is a vocal system used in a wide range of social contexts by both sexes throughout the year and is one of the more structurally complicated vocal systems outside of human language. Relatively little is known about the chick‐a‐dee calls of mountain chickadees, Poecile gambeli , however. This is an important species for increasing our comparative understanding of variation in chick‐a‐dee calls as they are one of the chickadee species with the largest naturally occurring flock sizes. Flock size relates to the social complexity of flocks, and the social complexity hypothesis for communication predicts that individuals in more complex social groups should communicate with greater complexity than individuals in simpler social groups. Correlational and experimental evidence in support of the hypothesis has been found in the calls of a wide range of species, including Carolina chickadees, P. carolinensis . Here, we provide the first description of the variation in note composition and note‐ordering rules in calls from mountain chickadee flocks in California and Colorado. California flocks were found to be significantly larger than Colorado flocks. Analysis of note‐type usage and transition probabilities between note types found that calls of California birds were more complex than calls of Colorado birds, supporting a key prediction of the social complexity hypothesis for communication. We also found relatively high rates of reversals of note‐ordering rules in mountain chickadee calls, which might help explain the complexity of the chick‐a‐dee calls of this species. Additionally, birds in flight produced calls with different note compositions when compared to perched birds. Generally, the note‐type ordering and transition probabilities of calls of mountain chickadees seem comparable to other better‐studied chickadee species, although their frequent note‐type order rule reversals suggest potential syntax‐like properties in this call system.