Changing durations of southern resident killer whale ( Orcinus orca ) discrete calls between two periods spanning 28 years

Killer whales vocalize in order to navigate, forage, communicate, and engage in social behavior. The sounds made by killer whales are classified into three types of vocalizations: clicks, whistles, and discrete calls. Discrete calls are pulsed calls that have been categorized into stereotyped call types (Ford 1987). Discrete calls are easily differentiated by the human ear and also provide a unique spectrographic structure. Discrete call usage in killer whales of the eastern North Pacific represents one case in which long-term studies of underwater acoustic communication are available. There are several distinct populations of killer whales that use the inland waters of British Columbia and Washington. This study focuses on the fish-eating Southern Resident population that utilizes the waters off the southern end of Vancouver Island. The Southern Residents comprise three pods ( J-, K-, and L-Pods) that, as of July 2009, totaled 85 animals (Center for Whale Research 2009). The Southern Resident population is currently listed as endangered under the Endangered Species Act in the United States (as of 2005) and the Species at Risk Act in Canada (as of 2003). A standard catalogue of the 25 discrete call types used by Southern Residents has been developed (Ford 1987). Each call type has an alphanumeric designation made up of a letter that denotes the community of whales that produce that call and an arbitrary number that indicates the order in which the call was first identified. “S” is the prefix for the 25 call types and their associated subtypes made by Southern Residents (Ford 1987). In general, vocalizations can vary in multiple parameters, such as call rate, frequency, amplitude, or duration, and these parameters can change for many reasons. The factors influencing vocalization parameters vary between species and between vocalizations. For example, the calls of greater horseshoe bats (Rhinolophus ferrumequinum) are known to undergo maturational effects ( Jones and Ransome 1993), whereas the vocalizations of Weddell seals (Leptonychotes weddellii) vary with seasonality (Rouget et al. 2007). On a more rapid timescale, changing behavioral contexts are known to influence the parameters of vocalizations in species like bottlenose dolphins (Tursiops truncatus) ( Janik and Slater 1998). On a more prolonged timescale, factors in the social environment, such as group membership, will lead to changes in call