SOUNDSCAPE PREFERENCE AND AUDITORY SIGNALING STRATEGIES OF AMERICAN WOODCOCK (SCOLOPAX MINOR)

Abstract

American woodcock (Scolopax minor) courtship displays are used widely to monitor populations, yet these displays are also complex behavioral performances shaped by the acoustic environment and by individual male phenotype. Males perform during short crepuscular periods, producing repeated ground based vocalizations and aerial mechanical sounds within breeding habitats that vary in vegetation structure, biological activity, and natural and anthropogenic noise. Although habitat management for woodcock has traditionally emphasized early-successional vegetation, the acoustic conditions within those habitats may also influence where, when, and how males display. This dissertation evaluates American woodcock courtship as an integrated acoustic, behavioral, and morphological system and its relationship with soundscape structure; individual analyses focus on how acoustic competition, wing morphology, and courtship signaling and display rate respond to soundscape variation. Chapter 1 documented previously undescribed harmonic structure in American woodcock peents and evaluated whether these vocalizations were associated with acoustic crowding and multivariate soundscape conditions. Woodcock peents are typically described as occurring within the biological frequency range of 2 to 8 kHz, but passive acoustic recordings from breeding and wintering habitats in Minnesota, Ohio, New York, and Louisiana revealed multi-band harmonically stacked peents with harmonic energy extending into the 14 to 22 kHz range. I used BirdNET detections to identify candidate peents and full-minute waveform segmentation to quantify vocal overlap, acoustic saturation within frequency bands, and acoustic indices describing surrounding soundscape structure. Across 16 sites, the harmonic dataset included 55,205 total vocalizations, of which 10,056 contained harmonic structure. Harmonic vocalizations were not distributed randomly across acoustic conditions. Instead, they occurred disproportionately when immediate acoustic competition in the fundamental frequency band was reduced. Exact overlap within the fundamental band and surrounding fundamental band saturation were negatively associated with harmonic probability, whereas acoustic complexity was positively associated with harmonic probability. Multivariate analyses suggested that harmonic vocalizations tend to occupy distinct regions of acoustic space, and generalized linear mixed models provided stronger, but limited, support for broad soundscape gradients than for individual overlap metrics alone. These results suggest that harmonic peents are associated with biologically active soundscapes but become less likely during intense direct masking within the primary vocal frequency band. Full-minute waveform segmentation also identified 8.1% more candidate peent events than BirdNET detections alone, indicating that biologically relevant vocal activity can occur outside fixed classifier-defined detection windows. Chapter 2 evaluated whether soundscape composition disturbed woodcock courtship activity across breeding sites in Minnesota, Ohio, and New York. I used passive acoustic monitoring to aggregate BirdNET-detected woodcock vocalizations into temporally clustered courtship events and assigned each event a courtship probability. I then tested how courtship probability varied with soundscape indices, interspecific acoustic activity, and anthropogenic noise prevalence. Courtship probability was associated with both acoustic structure and co-occurring species. Negative associations occurred between woodcock courtship activity and several vocally active species, including Swamp Sparrow, Northern Cardinal, Tufted Titmouse, and Tree Swallow, as well as with acoustic indices representing high spectral diversity and temporal variability. However, woodcock did not avoid acoustically active environments. Courtship probability was positively associated with American Robin, Mallard, entropy, Bioacoustic Index, and higher Normalized Difference Sound Index values, suggesting that males frequently displayed in biologically active soundscapes and in areas with intermittent anthropogenic sounds. These results support soundscape composition as a functionally relevant component of breeding habitat, rather than a secondary feature of vegetation-defined habitat. Incorporating acoustic conditions into habitat assessment may improve early-successional habitat management by identifying soundscape features associated with increased courtship occurrence. Passive acoustic monitoring can also be a practical tool for evaluating management outcomes and for maintaining feedback cycles that support adaptive, hypothesis driven conservation. Chapter 3 tested whether individual variation in male wing morphology was associated with courtship rate and territorial behavior. Display based surveys assume that males detected during survey provide comparable information about abundance, but males may differ in display activity for reasons unrelated to abundance alone. Because woodcock courtship requires repeated vocalization, aerial display, movement, and territorial response, morphology may influence how much effort males invest in display behavior. I combined traditional morphometric measurements, geometric morphometric analyses of wing shape, and behavioral ethograms from playback-assisted capture recordings. Wing morphology was summarized using Hand-Wing Index, wing chord, body mass, and principal components of Procrustes aligned wing landmarks. Behavioral responses were quantified as rates of peent production, courtship display, flight activity, aggressive cackles, and pecking directed toward an audio lure. Wing shape varied continuously among males, with Shape PC1, PC2, and PC3 explaining 31.0%, 20.4%, and 14.8% of total shape variation, respectively. Behavioral PC1 explained 43.8% of behavioral variation and was driven primarily by peent rate and courtship rate. Hand-Wing Index was the strongest predictor of behavior, explaining 55.1% of variation in Behavioral PC1. Males with higher Hand-Wing Index values produced courtship behaviors at higher rates, indicating that males with more pointed wings displayed more intensely during playback-assisted territorial interactions. Shape PC1 was negatively related to Behavioral PC1 and peent rate, explaining 28.9% and 29.8% of variation, respectively, although these relationships were not statistically significant. Multivariate analyses identified morphology-behavior structure but provided weaker inferential support than targeted models. Two-block Partial Least Squares analysis produced a moderate-to-strong morphology-behavior correlation, and redundancy analysis showed that morphological predictors collectively explained 55.16% of behavioral variation, although neither multivariate test was statistically significant. These findings provide the strongest support for a positive relationship between wing pointedness and courtship rate, suggesting that display behavior reflects male morphology and performance capacity as well as immediate behavioral motivation. In summary, these chapters show that American woodcock courtship behavior is shaped by the interaction between external acoustic conditions and individual male traits. Males display within dynamic soundscapes, produce harmonic vocalizations under specific acoustic conditions, vary in courtship occurrence across acoustic gradients, and differ in display rate according to wing morphology. These findings expand the interpretation of woodcock courtship from a simple presence based monitoring signal to a behavioral performance influenced by soundscape context and morphology-linked display capacity. This distinction is critical for management and monitoring programs that rely on short durations to count displaying males. Acoustic conditions can affect when males display, and morphology can affect how intensely males display, meaning variation in courtship rate may reflect differences in detectability and display behavior as well as differences in abundance. By integrating passive acoustic monitoring, soundscape ecology, and morphology-behavior relationships, this dissertation provides a framework for interpreting courtship displays as both reproductive signals and management relevant indicators of population status. Incorporating acoustic conditions into habitat evaluation and recognizing individual variation in display behavior can strengthen monitoring programs and support more adaptive, mechanism based conservation of American woodcock breeding habitat.