Causes and consequences of loud singing in birds

Vocal amplitude is crucial for acoustic communication because it determines how far a sound travels and can convey valuable information about signal senders to signal receivers. Yet, until recently, amplitude had received comparatively little attention because it is difficult to measure reliably, especially in field conditions. As a result, fundamental questions about amplitude remain largely unanswered even in one of the most extensively studied group of singers — birds. In this dissertation I aimed to answer some of these questions to shed light both on what causes some birds to sing louder than others, and on how loud singing can impact other properties of bird songs. In Chapter I, I investigated what evolutionary factors may drive variation in vocal amplitudes across bird species. Using original amplitude data from 123 species, I found that maximum amplitudes vary broadly across species — from 64 dB LAeq at 1 m in a species of hummingbird to 122 dB in a species of cotinga —, and that species with larger body sizes and larger beak gape widths tend to achieve higher maximum amplitudes. In Chapter II, I zoomed in on an extremely loud singer — the bare-throated bellbird — to investigate a potential amplitude-maximizing behavior: wide beak gaping. I found a positive relationship between beak gape angle and amplitude, with bellbirds opening their beaks at >148° to emit the loudest sounds documented for terrestrial vertebrates. In Chapter III, I asked how loud singing can impact another birdsong domain crucial for communication: the songs’ frequency content. Investigating vocalizations of 53 species led to conclusion that, at high amplitudes, birds phonate at frequencies that vary widely depending on species but at the same time, and across a broad swath of species, tend to be clustered within narrow ranges of frequencies. Taken together, these chapters show that morphological (body size, beak gape width) and behavioral (beak gaping) traits can act jointly as causes of loud singing, and that, as a consequence, a trade-off may emerge between intensity (amplitude) and complexity (frequency variation).