David Luther

A framework to assess evolutionary responses to anthropogenic light and sound

Human activities have caused a near-ubiquitous and evolutionarily-unprecedented increase in environmental sound levels and artificial night lighting. These stimuli reorganize communities by interfering with species-specific perception of time-cues, habitat features, and auditory and visual signals. Rapid evolutionary changes could occur in response to light and noise, given their magnitude, geographical extent, and degree to which they represent unprecedented environmental conditions. We present a framework for investigating anthropogenic light and noise as agents of selection, and as drivers of other evolutionary processes, to influence a range of behavioral and physiological traits such as phenological characters and sensory and signaling systems. In this context, opportunities abound for understanding contemporary and rapid evolution in response to human-caused environmental change.

Birdsongs keep pace with city life: changes in song over time in an urban songbird affects communication

Animals in urban environments often must adjust their vocalizations to be heard over the din of anthropogenic ambient noise. Most studies of this process use current comparisons of environments that vary in ambient noise; here, we evaluate over time the effects of the urban environment on signal structure and salience. We show that white-crowned sparrow, Zonotrichia leucophrys, song increased in minimum frequency between 1969 and 2005 in San Francisco, California, U.S.A., coincident with rising levels of traffic noise. A higher minimum frequency is less likely to be masked by low-frequency ambient noise and thus is more likely to be effective as a communication signal. To test whether this change in song affects receiver response, we presented territorial males current (2005) and historical (1969) local songs. Males responded more strongly to current than to historical songs, suggesting that current songs communicate more effectively in the current local environment. The results suggest that behavioural adjustment to anthropogenic ambient noise over time can affect cultural evolution and communication.

Sources of background noise and their influence on vertebrate acoustic communication

Many animals rely on long-range communication for species recognition, mate selection and territorial defense, but background noise from the environment can constrain their communication. Background noise from both biotic and abiotic sources is ubiquitous. In general, acoustic noise from abiotic sources, including anthropogenic noise, has energy mostly below 1 kHz. Arthropods tend to produce sounds in the 4–10 kHz range, while birds, amphibians and mammals generally have vocalizations with frequencies between 1 and 5 kHz. There are several ways that signalers could improve the efficiency of their acoustic signals to counteract the constraints of background noise. Signalers could make long-term and short-term signal adjustments to increase the detectability and discriminability of their signals. As predicted by signal detection theory adjustments can include increases in contrast between signals and noise, such as the intensity of the signal, the structure of the signal and an increase in signal redundancy. Our study reviews the sources of acoustic background noise, adjustments made by signalers to increase signal efficacy, and the influence of acoustic background noise on the evolution of acoustic communication in terrestrial vertebrate species.

Assessing the impact of local habitat variables and landscape context on riparian birds in agricultural, urbanized, and native landscapes

Large tracts of natural habitat are being replaced by agriculture and urban sprawl in Mediterranean regions worldwide. We have limited knowledge about the effects of human activities on native species in these landscapes and which, if any, management practices might enhance the conservation of native biodiversity within them. Through a citizen volunteer bird-monitoring project, we compared bird abundance and species richness in northern Californian riparian zones surrounded by vineyards, urban areas, and natural areas. We assessed both local and landscape-level variables that may enhance native bird diversity in each land use type. We also demonstrate a new statistical approach, generalized estimating equations, to analyze highly variable data, such as that collected by volunteers. Avian abundance was highly correlated with both landscape context and local habitat variables, while avian richness was correlated with local habitat variables, specifically shrub richness, and percent of tree cover. In particular, shrub species richness has a strong positive correlation with riparian-preferring bird species. This suggests that active local management of riparian zones in human-dominated landscapes can increase our ability to retain native bird species in these areas.

Patterns of Song across Natural and Anthropogenic Soundscapes Suggest That White-Crowned Sparrows Minimize Acoustic Masking and Maximize Signal Content.

Soundscapes pose both evolutionarily recent and long-standing sources of selection on acoustic communication. We currently know more about the impact of evolutionarily recent human-generated noise on communication than we do about how natural sounds such as pounding surf have shaped communication signals over evolutionary time. Based on signal detection theory, we hypothesized that acoustic phenotypes will vary with both anthropogenic and natural background noise levels and that similar mechanisms of cultural evolution and/or behavioral flexibility may underlie this variation. We studied song characteristics of white-crowned sparrows (Zonotrichia leucophrys nuttalli) across a noise gradient that includes both anthropogenic and natural sources of noise in San Francisco and Marin counties, California, USA. Both anthropogenic and natural soundscapes contain high amplitude low frequency noise (traffic or surf, respectively), so we predicted that birds would produce songs with higher minimum frequencies in areas with higher amplitude background noise to avoid auditory masking. We also anticipated that song minimum frequencies would be higher than the projected lower frequency limit of hearing based on site-specific masking profiles. Background noise was a strong predictor of song minimum frequency, both within a local noise gradient of three urban sites with the same song dialect and cultural evolutionary history, and across the regional noise gradient, which encompasses 11 urban and rural sites, several dialects, and several anthropogenic and natural sources of noise. Among rural sites alone, background noise tended to predict song minimum frequency, indicating that urban sites were not solely responsible for driving the regional pattern. These findings support the hypothesis that songs vary with local and regional soundscapes regardless of the source of noise. Song minimum frequency from five core study sites was also higher than the lower frequency limit of hearing at each site, further supporting the hypothesis that songs vary to transmit through noise in local soundscapes. Minimum frequencies leveled off at noisier sites, suggesting that minimum frequencies are constrained to an upper limit, possibly to retain the information content of wider bandwidths. We found evidence that site noise was a better predictor of song minimum frequency than territory noise in both anthropogenic and natural soundscapes, suggesting that cultural evolution rather than immediate behavioral flexibility is responsible for local song variation. Taken together, these results indicate that soundscapes shape song phenotype across both evolutionarily recent and long-standing soundscapes.

The Island Syndrome in Coastal Wetland Ecosystems: Convergent Evolution of Large Bills in Mangrove Passerines

ABSTRACT. Passerine birds on islands tend to have larger bills than their mainland relatives. The morphological shift may be related to reduced interspecific and increased intraspecific competition. Emberizid sparrows in North American salt marshes also show consistently greater bill size. We tested the hypothesis that passerines restricted to mangrove forests, another continental system with low species diversity and high population densities, also have larger bills than their closest nontidal relatives. We found a consistent pantropical pattern of longer and deeper bills in passerine birds restricted to mangroves. These results indicate that disproportionately longer bills in relation to body size in passerines restricted to coastal saline habitats, just like those found on islands, seems to be a general ecological rule. The similar pattern in bill morphology suggests that ecological and evolutionary processes thought to occur only in island systems might also occur in some continental systems.

White-crowned sparrow males show immediate flexibility in song amplitude but not in song minimum frequency in response to changes in noise levels in the field

Abstract The soundscape acts as a selective agent on organisms that use acoustic signals to communicate. A number of studies document variation in structure, amplitude, or timing of signal production in correspondence with environmental noise levels thus supporting the hypothesis that organisms are changing their signaling behaviors to avoid masking. The time scale at which organisms respond is of particular interest. Signal structure may evolve across generations through processes such as cultural or genetic transmission. Individuals may also change their behavior during development (ontogenetic change) or in real time (i.e., immediate flexibility). These are not mutually exclusive mechanisms, and all must be investigated to understand how organisms respond to selection pressures from the soundscape. Previous work on white‐crowned sparrows (Zonotrichia leucophrys) found that males holding territories in louder areas tend to sing higher frequency songs and that both noise levels and song frequency have increased over time (30 years) in urban areas. These previous findings suggest that songs are changing across generations; however, it is not known if this species also exhibits immediate flexibility. Here, we conducted an exploratory, observational study to ask whether males change the minimum frequency of their song in response to immediate changes in noise levels. We also ask whether males sing louder, as increased minimum frequency may be physiologically linked to producing sound at higher amplitudes, in response to immediate changes in environmental noise. We found that territorial males adjust song amplitude but not minimum frequency in response to changes in environmental noise levels. Our results suggest that males do not show immediate flexibility in song minimum frequency, although experimental manipulations are needed to test this hypothesis further. Our work highlights the need to investigate multiple mechanisms of adaptive response to soundscapes.

The relative response of songbirds to shifts in song amplitude and song minimum frequency

Lay Summary Male birds adjust their strength of response to songs modified with increased amplitude and minimum frequency in the presence of loud human noise. In addition, territorial rival males respond more strongly to louder songs than songs with relatively lower minimum frequencies. Thus, increasing song amplitude can compensate for any loss in signal salience due to shifting songs up in minimum frequency. Increasing only song minimum frequency in the context of high noise levels could be maladaptive.

Evidence of suboscine song plasticity in response to traffic noise fluctuations and temporary road closures

Abstract This study investigates how noise reduction (road closure) mitigates the effect of traffic noise on the acoustic communication of the Eastern wood pewee (Contopus virens) (EAWP), a suboscine passerine. Songs were passively recorded at sites where the traffic pattern of the nearest road was either relatively constant or reduced on a weekly basis during a 36 h road closure. Five song attributes, low frequency traffic noise amplitude (LAeq) measured within 20 s of each song, and full-spectrum background noise levels (LAeq) characteristic of each territory were measured and analysed in order to better understand how EAWP respond to variation in traffic noise levels. EAWP adjusted its spectral song attributes by increasing song tonality to improve transmission in immediate response to fluctuations in traffic noise. The results suggest that song adjustments are responses to traffic noise levels at the time of their song, instead of the average background noise level measured per territory. This study provides a better understanding of how suboscine communication is affected by traffic noise, as well as the potential mitigating effect of noise reduction for animal acoustic communication.

Males with larger bills sing at higher rates in a hot and dry environment

ABSTRACT Temperature and water availability affect the survival, reproductive success, and behavior of all animals; thus, traits that influence thermoregulation and water balance should be under strong selection. Avian bills can dissipate excess body heat through convection and radiation. We hypothesized that in hot, dry locations, bill size would be selected for its value in thermoregulation: males with larger bills would shed excess heat more efficiently, allowing greater activity levels during thermally challenging periods of the breeding season. We predicted that thermoregulatory challenges would require Melospiza melodia atlantica, which breeds in hot, dry sand dunes, to trade off between behaviors associated with breeding (song output) and behaviors related to thermoregulation, and that in the hot environment occupied by our study population, males with larger bills would sing at higher rates. We captured, measured, and marked individual birds and then observed singing behavior, microclimate, and microhabitat use to assess thermoregulatory challenges. Ambient weather indicated thermally challenging conditions, and birds displayed thermoregulatory behaviors, including reducing their song rates at higher temperatures. Birds with larger bills sang at higher rates than birds with smaller bills, and males with the largest bills in the population sang almost twice as many songs per minute than birds with the smallest bills. These results are consistent with the hypothesis that climate influences selection on the bill as a thermoregulatory organ.