Comparative bioacoustics: a roadmap for quantifying and comparing animal sounds across diverse taxa.

Animals produce a wide array of sounds with highly variable acoustic structures. It is possible to understand the causes and consequences of this variation across taxa with phylogenetic comparative analyses. Acoustic and evolutionary analyses are rapidly increasing in sophistication such that choosing appropriate acoustic and evolutionary approaches is increasingly difficult. However, the correct choice of analysis can have profound effects on output and evolutionary inferences. Here, we identify and address some of the challenges for this growing field by providing a roadmap for quantifying and comparing sound in a phylogenetic context for researchers with a broad range of scientific backgrounds. Sound, as a continuous, multidimensional trait can be particularly challenging to measure because it can be hard to identify variables that can be compared across taxa and it is also no small feat to process and analyse the resulting high-dimensional acoustic data using approaches that are appropriate for subsequent evolutionary analysis. Additionally, terminological inconsistencies and the role of learning in the development of acoustic traits need to be considered. Phylogenetic comparative analyses also have their own sets of caveats to consider. We provide a set of recommendations for delimiting acoustic signals into discrete, comparable acoustic units. We also present a three-stage workflow for extracting relevant acoustic data, including options for multivariate analyses and dimensionality reduction that is compatible with phylogenetic comparative analysis. We then summarize available phylogenetic comparative approaches and how they have been used in comparative bioacoustics, and address the limitations of comparative analyses with behavioural data. Lastly, we recommend how to apply these methods to acoustic data across a range of study systems. In this way, we provide an integrated framework to aid in quantitative analysis of cross-taxa variation in animal sounds for comparative phylogenetic analysis. In addition, we advocate the standardization of acoustic terminology across disciplines and taxa, adoption of automated methods for acoustic feature extraction, and establishment of strong data archival practices for acoustic recordings and data analyses. Combining such practices with our proposed workflow will greatly advance the reproducibility, biological interpretation, and longevity of comparative bioacoustic studies.

Phenological changes in North Atlantic right whale habitat use in Massachusetts Bay.

The North Atlantic right whale (Eubalaena glacialis) is one of the world's most highly endangered baleen whales, with approximately 400-450 individuals remaining. Massachusetts Bay (MB) and Cape Cod Bay (CCB) together comprise one of seven areas in the Gulf of Maine where right whales seasonally congregate. Here, we report on acoustically detected presence of right whales in MB over a nearly 6 year period, July 2007-April 2013, a time of both rapid ocean warming throughout the Gulf of Maine and apparent changes in right whale migratory dynamics. We applied an automated detection algorithm to assess hourly presence of right whale "up-calls" in recordings from a 19-channel acoustic array covering approximately 4,000 km2 in MB. Over the survey, up-calls were detected in 95% of 8 day periods. In each year, as expected, we observed a "peak season" of elevated up-call detections in late winter and early spring corresponding to the season when right whales congregate to feed in CCB. However, we also saw an increase in right whale occurrence during time periods thought to be part of the "off-season." With the exception of 2009-2010, when acoustic presence was unusually low, the mean percent of hours in which up-calls were detected increased every year, both during the peak season (from 38% in 2008 to 70% in 2012), and during the summer-fall season (from 2% in 2007 to 13% in 2012). Over the entire study, the peak season start date varied between 17 January and 26 February. Changes in right whale phenology in MB likely reflect broadscale changes in habitat use in other areas within the species range. This study demonstrates the value of continuous long-term survey datasets to detect and quantify shifts in cetacean habitat use as environmental conditions change and the long-term continued survival of right whales remains uncertain.

Long-term passive acoustic recordings track the changing distribution of North Atlantic right whales (Eubalaena glacialis) from 2004 to 2014.

Given new distribution patterns of the endangered North Atlantic right whale (NARW; Eubalaena glacialis) population in recent years, an improved understanding of spatio-temporal movements are imperative for the conservation of this species. While so far visual data have provided most information on NARW movements, passive acoustic monitoring (PAM) was used in this study in order to better capture year-round NARW presence. This project used PAM data from 2004 to 2014 collected by 19 organizations throughout the western North Atlantic Ocean. Overall, data from 324 recorders (35,600 days) were processed and analyzed using a classification and detection system. Results highlight almost year-round habitat use of the western North Atlantic Ocean, with a decrease in detections in waters off Cape Hatteras, North Carolina in summer and fall. Data collected post 2010 showed an increased NARW presence in the mid-Atlantic region and a simultaneous decrease in the northern Gulf of Maine. In addition, NARWs were widely distributed across most regions throughout winter months. This study demonstrates that a large-scale analysis of PAM data provides significant value to understanding and tracking shifts in large whale movements over long time scales.

Bowhead whale acoustic activity in the southeast Beaufort Sea during late 2008-2010.

Autonomous passive acoustic recorders were deployed to record sounds of bowhead whales (Balaena mysticetus) in the southeast Beaufort Sea for periods of 30-55 days during the late summer, open-water seasons of 2008-2010. Recordings were made in three areas licensed for hydrocarbon exploration, spanning the continental slope and adjacent outer shelf, and in a shallow inner-shelf area where bowheads have been observed congregating to feed in recent decades. Bowhead sounds were counted in samples comprising 10% of each recorded hour. In mid-August and September in all 3 years, the rate of bowhead calling at outer shelf sites exceeded that at adjacent continental slope sites by one to two orders of magnitude. Higher rates of calling occurred on the slope in late July and early August than at later dates. Calling rates varied by an order of magnitude between years in the one area that was monitored in different years. The highest rates of calling occurred on the inner shelf, offshore of the northern Tuktoyaktuk Peninsula. These trends are consistent with patterns of habitat use previously reported from aerial surveys in this and nearby areas of the Beaufort Sea and with the results of satellite tagging studies.

Ivory-billed woodpecker (Campephilus principalis) persists in continental North America.

The ivory-billed woodpecker (Campephilus principalis), long suspected to be extinct, has been rediscovered in the Big Woods region of eastern Arkansas. Visual encounters during 2004 and 2005, and analysis of a video clip from April 2004, confirm the existence of at least one male. Acoustic signatures consistent with Campephilus display drums also have been heard from the region. Extensive efforts to find birds away from the primary encounter site remain unsuccessful, but potential habitat for a thinly distributed source population is vast (over 220,000 hectares).