I finally wrote a post that fits all of my blog categories! 🙂 Years ago, Dr. T. Mitchell Aide visited my former lab and I had an opportunity to meet with him. Hearing about his work with automated classification of bird calls first got my mind churning about how we can use ARU’s for gathering field data (Aide et al. 2013). His work focused on the tropics, and thus the complexity of the animal soundscape (Acevedo et al. 2009). I further became interested as I came across the technique for monitoring my nemesis bird, yellow rail! Also, ARU’s have been tested with respect to the Breeding Bird Survey (BBS) which has been the focus of most of my research to date (Rempel et al. 2013). Soundscape ecology is a relatively new area of research (Pijanowski et al. 2011a) and is considered a branch of my current field, landscape ecology (Pijanowski et al. 2011b).
It is important to evaluate ARU’s for avian study, because vocalization accounts for most detection (Acevedo and Villanueva-Rivera 2006). The utility of automated recording units (ARU) for ecology has been investigated for well over a decade, so as anything in ecology methods and technology are always evolving (Haselmayer and Quinn 2000). Thus, there is something of a literary trail as technology has improved, both with respect to recorders and analysis (Haselmayer and Quinn 2000). For example, automated classification was out of reach by the standard of reliability not long ago (Swiston and Mennill 2009). Manual classification seemed to be the only reliable way to identify songs in recordings (Waddle et al. 2009).
ARU’s have their pros and cons, as well as applicability (Brandes 2008). Recording sounds can provide a less invasive alternative to direct observation, detect hard-to-observe species and sample a large area. For example, an early (and ongoing) application of recording was to monitor nocturnal migration (Farnsworth and Gauthreaux 2004). Additionally, recordings are reviewable, and do not have human listening bias (Digby et al. 2013). This paves the way for standardizing observer effort and capability (Hobson et al. 2002). In comparison to a point count, the visual component is lost, but detection of species by the recording units appears to be relatively high (Alquezar and Machado 2015). Yet, if it is easier to detect a target species visually, ARU’s may not sample them as well as a point count (Celis-Murillo et al. 2012). For at least some species, the best method appears to be to combine point counts and ARU’s for detection (Holmes et al. 2014). While there is now an ever-growing body of literature on the applicability of ARU’s, they are often suited better to some sound qualities over others, species, or certain components of bioacoustics such as temporal patterns (Rognan et al. 2012). ARU’s have now been tested over many different ecosystem types, and results are generally favorable (Venier et al. 2012). However, they may not be able to sample species well that vocalize infrequently and/or are sparsely distributed (Sidie-Slettedahl et al. 2015). There are different configurations of ARU’s to answer different ecological questions (Mennill et al. 2006).
Various indices aid in interpreting recordings (Towsey et al. 2014a). There is an R package “soundecology” that now calculates a number of indices from recordings!
- canonical discriminant analysis (CDA): identifying individuals (Rognan et al. 2009)
- Acoustic Complexity Index: proxy for species richness (Pieretti et al. 2011)
- Acoustic Richness index (AR)
- Acoustic dissimilarity index (D) (Depraetere et al. 2012)
- Within-group (α) indices (Sueur et al. 2014)
- Between-group (β) indices
- acoustic diversity = Shannon index of intensity per frequency (Pekin et al. 2012)
ARU’s can answer ecological questions scaling from individual monitoring to community assemblage (Blumstein et al. 2011). With bird species that are well-monitored by ARU’s, life history detail gleaned can even surpass more traditional recapture methods (Mennill 2011)! With “song fingerprints” taking the place of color bands, it is possible to map individual movement patterns (Kirschel et al. 2011). Most often, this means mapping territorial males (Frommolt and Tauchert 2014). If individuals detected at the same place are acoustically distinguishable, it may be possible to estimate abundance, and thus a given species’ population density from recording surveys (Dawson and Efford 2009). There are several species that have been shown to be distinguishable to individual with recording analysis (Ehnes and Foote 2015). This allows for broad scale population monitoring, which may be especially important for threatened species (Bardeli et al. 2010). Further, community descriptors such as species composition may be approximated by characteristics of the soundscape (Celis-Murillo et al. 2009). Community metrics have been found to correlate back to landscape metrics, which may make them useful for conservation (Tucker et al. 2014).
Where we are now
There are still logistical analytical hurdles to overcome, and the development and comparison of methods for sound analysis has paralleled many trends in ecology (Kirschel et al. 2009). For one, ARU’s can present a big data problem, so automating sound analysis is a priority (Towsey et al. 2014b). Because of the promise of ARU’s, though, long-term recording projects are being designed (Turgeon et al. 2017). Right now, we are on the journey from manual to automated classification of songs, falling somewhere in the realm of “semi-automation” (Goyette et al. 2011). Recent efforts in enhancing automated analysis focus on sampling techniques for days-worth of recordings (Wimmer et al. 2013). Now, we can automate at least some species identification in recordings (Potamitis et al. 2014). However, it appears that automation partly depends on the template-matching algorithms used (Joshi et al. 2017).
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