Herting, Brian L. – Seasonal effects of testosterone on aggression and vocalizations in Western Screech-owls. 2001.
Chapter 1: Seasonal Variation in Testosterone-Induced Aggression in Male Western Screech-Owls.
Although testosterone (T) has been shown to influence territorial aggression during the breeding season in many species of birds, relationships among aggression, territoriality, and T outside of the breeding season have received far less attention. I used a hormone/drug implant protocol to create treatment groups in which circulating levels of T were increased, decreased, or maintained at normal levels (controls) to examine these relationships during both the breeding and non-breeding seasons in male western screech-owls (Otus kennicottii). Mated pairs inhabit and defend multipurpose territories throughout the year in my study area. I exposed focal male owls to tape-recorded vocalizations of a conspecific (playback stimulus) and assessed their approach and vocal responses. Season affected vocal response but not approach irrespective of treatment group. Focal owls approached the stimulus in both seasons but vocalized more during the breeding season. Irrespective of season, exogenous T increased aggression in relation to owls that received drugs to reduce T, which indicates that western screech-owls were sensitive to exogenous T during both the breeding and non-breeding seasons. Finally, because owls displayed territorial aggression during the non-breeding seasons when T levels are naturally low, (1) testosterone appears to enhance rather than activate aggression at this time, or (2) low levels of circulating T made by the brain may play a role in mediating aggression.
Chapter 2: Testosterone-Induced Variation in Bounce Songs of Male Western Screech-Owls
In most avian species, song production coincides with the breeding season, when circulating testosterone (T) is typically high. Additionally, many types of information can be communicated through song production and for many purposes. This study sought to determine if the steroid hormone testosterone, known to influence many aspects of song and its production, influences song characteristics, or information within songs of male western screech-owls (Otus kennicottii) throughout the year. To address this question, three treatment groups were established whereby circulating levels of T were increased, decreased, or unchanged (controls). In a playback experiment, 277 vocalizations form 20 males were examines using sonographic analysis. Exogenous T influenced characteristics of bounce songs in male western screech-owls during both the breeding and non-breeding seasons through consistent increases in the duration of notes and songs. Such longer notes and songs could communicate to conspecifics a heightened state of aggressiveness, which would be expected under exposure to T.
Chapter 3: The Bounce and Double Trill Songs of Male and Female Western Screech-Owls: Characterization and usefulness for Classification of Sex
In this study, I describe and compare bounce and double trill songs of male and female western screech-owls (Otus kennicottii) from a population in southwestern Idaho. I also developed a discriminant function to help distinguish males and females bases upon vocalizations. Using sonographic analysis, I described seven vocal characteristics of the bouncing ball or bounce songs (from 15 males and 10 females) and 5 vocal characteristics of double trill songs (from 6 males and 6 females). Females consistently sang bounce and double trill songs with higher pitch than males, even though body size was larger in the former. This is in contrast to the general pattern among birds, even where reversed size dimorphism occurs. Females also had significantly longer durations between notes in the leading portion of double trill songs. For bounce songs, I developed a discriminant function to accurately classify males and females based on frequency of notes at maximum amplitude (FMA) within songs. This discriminant model may help improve accuracy of playback surveys for western screech-owls and provide a noninvasive method for determining sex.