Sharon A. Gill

Tropical field endocrinology: Ecology and evolution of testosterone concentrations in male birds

Comparative research on natural populations of vertebrates, in particular avian species, has been instrumental in documenting the existence of interspecific variation in the hormonal regulation of behavior. Studies on tropical birds, which tend to experience ecological conditions that diverge from those of higher latitude birds, have been invaluable in showcasing such variation. Here we review recent advances in tropical avian field endocrinology, focusing on male circulating testosterone concentrations during the breeding season. We summarize the evidence for a decrease in male circulating testosterone concentrations from high towards low latitudes. We revisit both established and recently proposed ecological hypotheses that attempt to explain the existence of this pattern, as well as the variation in testosterone concentrations and dynamics within tropical populations of birds. We highlight additional social and life history variables that may need to be considered if we aim at gaining an integrated understanding of the ultimate factors that influence the relationship between hormonal signals and behavioral traits in natural populations. Understanding the ecological factors that influence circulating hormone concentrations will be an important first step in our understanding of the evolutionary processes that are at the basis of variations in hormone-behavior connections. Such findings should be supplemented by studies on functional aspects of the testosterone signal at the organismal and cellular level.

Uncovering Spatial Variation in Acoustic Environments Using Sound Mapping.

Animals select and use habitats based on environmental features relevant to their ecology and behavior. For animals that use acoustic communication, the sound environment itself may be a critical feature, yet acoustic characteristics are not commonly measured when describing habitats and as a result, how habitats vary acoustically over space and time is poorly known. Such considerations are timely, given worldwide increases in anthropogenic noise combined with rapidly accumulating evidence that noise hampers the ability of animals to detect and interpret natural sounds. Here, we used microphone arrays to record the sound environment in three terrestrial habitats (forest, prairie, and urban) under ambient conditions and during experimental noise introductions. We mapped sound pressure levels (SPLs) over spatial scales relevant to diverse taxa to explore spatial variation in acoustic habitats and to evaluate the number of microphones needed within arrays to capture this variation under both ambient and noisy conditions. Even at small spatial scales and over relatively short time spans, SPLs varied considerably, especially in forest and urban habitats, suggesting that quantifying and mapping acoustic features could improve habitat descriptions. Subset maps based on input from 4, 8, 12 and 16 microphones differed slightly (< 2 dBA/pixel) from those based on full arrays of 24 microphones under ambient conditions across habitats. Map differences were more pronounced with noise introductions, particularly in forests; maps made from only 4-microphones differed more (> 4 dBA/pixel) from full maps than the remaining subset maps, but maps with input from eight microphones resulted in smaller differences. Thus, acoustic environments varied over small spatial scales and variation could be mapped with input from 4–8 microphones. Mapping sound in different environments will improve understanding of acoustic environments and allow us to explore the influence of spatial variation in sound on animal ecology and behavior.

Antibiotic resistant bacteria are widespread in songbirds across rural and urban environments

The widespread use of antibiotics in human and veterinary medicine to treat pathogenic bacteria has resulted in the rapid emergence of antibiotic-resistant bacteria (ARB). Wild animals may enable the spread of pathogenic and non-pathogenic ARB when they are exposed to reservoirs (e.g., contaminated soil, water, or crops) and carry ARB in and on their bodies to other environments. We tested for the presence of ARB in four songbird species in southwest Michigan across a gradient of land use. Our specific objectives were to: 1) quantify the prevalence of ARB found in the gut microbiome of birds; 2) identify the specific bacteria exhibiting resistance; 3) assess whether ARB prevalence and identity varied among bird species; and 4) assess whether anthropogenic land use influenced the prevalence and identity of ARB found on birds. We sampled birds across a land use gradient consisting of urban, agricultural, and natural land covers using a randomized, spatially-balanced sampling design and cultured bacteria from fecal samples in the presence of three different antibiotics (amoxicillin, tetracycline, and ciprofloxacin). Overall prevalence of ARB was high, with 88% of total birds carrying ARB resistant to one of three antibiotics that we tested. Resistance to amoxicillin was more common (83% of sampled birds) than resistance to tetracycline (15%) or ciprofloxacin (1%). Identified ARB were diverse, and included 135 isolates representing 5 bacterial phyla and 22 genera. There was no effect of land use on ARB prevalence, with 90% of sampled birds captured in rural sites and 85% of sampled birds in urban sites carrying ARB. We provide the first analysis of ARB prevalence across multiple bird species and land uses utilizing a spatially-balanced, randomized study design. Our results demonstrate that nearly all sampled birds carried at least some ARB, and that they may serve as important dispersal agents of ARB across large spatial scales.

Decomposing an urban soundscape to reveal patterns and drivers of variation in anthropogenic noise

Continuous and intermittent noise may have different effects on humans and wildlife, therefore distinguishing temporal patterns of noise and their drivers is important for policy regarding both public health and wildlife management. We visualized patterns and explored land-use drivers of continuous and high-amplitude intermittent sound pressure levels (SPLs) on an urban campus in Michigan, U.S.A. To visualize patterns of SPLs, we introduce decibel duration curves (DDCs), which show the cumulative frequency distribution of SPLs and aid in the interpretation of statistical SPLs (Ln values) that reflect continuous versus intermittent sounds. DDCs and Ln values reveal that our 24 recording locations varied in the intensity of both continuous and intermittent noise, with intermittent high-amplitude sound events in particular contributing to variability in SPLs over the study site. Time of day influenced both continuous and intermittent SPLs, as locations relatively close to manmade structures (buildings, roads and parking lots) experienced higher SPLs as the day progressed. Continuous SPLs increased with decreasing distance to manmade structures, whereas intermittent SPLs increased with decreasing distance to roads and increasing distance to buildings. Thus, different land-use factors influenced patterns of continuous and intermittent noise, which suggests that different policy and strategies may be needed to ameliorate their effects on the public and wildlife.

Testing hypotheses for the evolution of long-term monogamy in Neotropical Buff-breasted Wrens (Cantorchilus leucotis)

Over 90% of birds breed in socially monogamous partnerships. Among monogamous passerines, species vary from those with extra-pair copulations and short-term partnerships to others with exclusive and long-term partnerships. Study of long-term monogamy in passerines has lagged behind other forms of monogamy, and the factors favouring this extreme mating system are unclear. I investigated five hypotheses for long-term monogamy (LTM) in Neotropical Buff-breasted Wrens (Cantorchilus leucotis). Familiarity with breeding partners did not increase reproductive success relative to naïve pairs, which is inconsistent with the mate familiarity hypothesis. Experienced females, and possibly males, had greater reproductive success than those breeding for the first time, supporting the hypothesis that LTM may be favoured by improved reproductive performance of individuals as they gain breeding experience. The low resource availability hypothesis was supported as territory availability was low, mates were rapidly replaced, and searching for alternative territories was costly, as birds did not always re-acquire territories following experimental removals. Evidence for territorial defence hypotheses was mixed, as pair members coordinated aggressive responses to intruders while simultaneously exhibiting sex-specific aggression. These results suggest fitness advantages, low resource availability and selection for resource monopolization by males and females may have favoured long-term monogamy in Buff-breasted Wrens.

Prevalence and diversity of avian Haemosporida infecting songbirds in southwest Michigan

Avian blood parasites from the genera Plasmodium, Haemoproteus, and Leucocytozoon (Haemosporida) affect hosts in numerous ways. They influence species interactions, host behavior, reproductive success, and cause pathology and mortality in birds. The Great Lakes region of North America has extensive aquatic and wetland habitat and supports a diverse vector community. Here we describe the community of bird-infecting Haemosporida in southwest Michigan and their host associations by measuring parasite prevalence, diversity, and host breadth across a diverse community of avian hosts. Over 700 songbirds of 55 species were screened for Haemosporida infection across southwest Michigan, including 11 species that were targeted for larger sample sizes. In total, 71 parasite lineages infected over 40% of birds. Of these, 42 were novel, yet richness estimates suggest that approximately half of the actual parasite diversity in the host community was observed despite intensive sampling of multiple host species. Parasite prevalence varied among parasite genera (7–24%) and target host species (0–85%), and parasite diversity was consistently high across most target species. Host breadth varied widely across the most prevalent parasite lineages, and we detected around 60% of host species richness for these parasite lineages. We report many new lineages and novel host-parasite associations, but substantial parasite diversity remains undiscovered in the Midwest.