# A Collection of Best Practices for the Collection and Analysis of Bioacoustic Data
## 6 Signal Structure and Evolution: Ecological Effects  
Another productive line of research in acoustic communication focuses on how signals are influenced in structure and function by variation in ecological parameters such as resource availability, habitat structure, and conditions for rearing offspring [119; 131; 132; 133]. Ecological effects on signal structure can be direct, such as when they favor signals that transmit effectively through particular habitats [111], or indirect, such as when patterns in resource availability impact social, sexual, and life history parameters involved in communication [134; 135]. When considering acoustic signals such as echolocation clicks that are produced for navigation, object detection and localization, the environment will typically impose particularly strong effects on the characteristics of produced sounds [136; 137]. For example, in bats, calls produced by different species while in flight resemble each other, as do calls while tree-climbing or marauding over a water surface [137]. In these types of studies, as above for cultural effects, it can be tempting to ascribe observed variations in acoustic signals to external ecological causes without truly isolating the hypothesized driver. A firm demonstration of systematic variation in signal structure driven by particular ecological traits requires attention to within-treatment or within-location variation, with sample sizes sufficient to document this variation with confidence. In other words, as discussed previously, the data collected should capture the variability in all other potential environmental or ecological parameters in order to truly isolate the target parameter. True variation is thus revealed by relatively limited within-treatment variation in the acoustic signal across all other ecological parameters, as compared to between-treatment variation. Variation in signals can emerge as non-functional, indirect by-products of other processes such as cultural evolution or random drift in signal traits [55], which can be confused with the direct effects of environment-specific selective regimes. It is typically not feasible to control for all confounding factors within the scope of a single study, and in such cases, it is important to address those caveats and consider potential future work that would clarify the factors that drive signal evolution. In addition to these issues, field studies on acoustic behavior are uniquely susceptible to oversights about the context and intentions of the signaler. Understanding these things requires carefully planned observational studies with large sample sizes or controlled experimental conditions, and even these can be open to interpretation. Capturing the full variability of signals being produced often requires multiple recorders to be placed at various systematic locations throughout the study area. It is also important to record extensive metadata about the acoustic context, including parameters describing the foragingspace and annotation of the prey types being hunted [1]. This, however, can be difficult to achieve in cases where the signallers cannot be directly observed. Some recent papers on ecological effects on signal structure have focused on signal differences related to highly distinct habitats and foraging spaces. For example, in a study of mountain chickadees, [PERSON] and [PERSON] [70] tested whether song structure diverged with location and the preferences of females for local males, a pattern which would help parents to produce offspring with locally adapted genes. The authors found that these birds' songs indeed differed in structure among locally adapted populations, both by elevation as well as regionally across two mountain slopes [70]. A second, conceptually similar study in bats examined changes in echolocation characteristics at different flight elevations [71], and showed that echolocation clicks became longer, lower-frequency, and had narrower frequency bandwidths at higher elevations.