Biology subjects

Verrett, T. B.

Publications and source records attributed to Verrett, T. B..

3 recordsLinked to original sources

bioRxiv2023

Determinants of spring migration departure dates in a New World sparrow: weather variables reign supreme

Numerous factors influence the timing of spring migration in birds, yet the relative importance of intrinsic and extrinsic variables on migration initiation remains unclear. To test for interactions among weather, migration distance, parasitism, and physiology in determining spring departure date, we used Dark-eyed Juncos (Junco hyemalis hyemalis) as a model migratory species known to harbor diverse and common haemosporidian parasites. Prior to spring migration departure from their wintering grounds in Indiana, USA, we quantified the intrinsic variables of fat, body condition (i.e., mass[~]tarsus residuals), physiological stress (i.e., ratio of heterophils to lymphocytes), cellular immunity (i.e., leukocyte composition and total count), migration distance (i.e., distance to the breeding grounds) using stable isotopes of hydrogen from feathers, and haemosporidian parasite intensity. We then attached nanotags to determine the timing of spring migration departure date using the Motus Wildlife Tracking System. We used additive Cox proportional hazard mixed models to test how risk of spring migratory departure was predicted by the combined intrinsic measures, along with meteorological predictors on the evening of departure (i.e., average wind speed and direction, relative humidity, and temperature). Model comparisons found that the best predictor of spring departure date was average nightly wind direction and a principal component combining relative humidity and temperature. Juncos were more likely to depart for spring migration on nights with largely southwestern winds and on warmer and drier evenings (relative to cooler and more humid evenings). Our results indicate that weather conditions at take-off are more critical to departure decisions than the measured physiological and parasitism variables.

ecology↗

bioRxiv2023

Movement-related drivers of exposure to West Nile virus by American robins (Turdus migratorius)

The ecological processes that determine how individual animals become hosts to zoonotic pathogens is a topic of rapidly growing interest. However, how such exposure is mediated by context (e.g., season, location), host behavior (e.g., migration distance) and host demographics is generally poorly understood. We evaluated seasonal exposure to West Nile Virus of American robins sampled in Indiana and compared our results to those of previous studies. Because robins that breed in Indiana are partial migrants (i.e., only a portion of the population migrates), we evaluated their probability of exposure to WNV as a function of whether they migrated or not and of their movement distance. We also tagged a subset of breeding robins with tracking devices to evaluate their potential to disperse the virus between Indiana and other regions of the continent. We found that robins that breed in Indiana are exposed to WNV at a higher rate than that detected in previous studies, but found no correlation between robin exposure and whether a robin migrated or not, nor with migration distance, season, sex, and breeding latitude (for robins overwintering in Indiana). Our tracking data indicate that robins that breed in Indiana migrate several hundred miles to overwinter in the southeastern US. The mean duration of their return to Indiana in spring is 10.3 days, which is less than the maximum infectious period found for robins in previous studies, suggesting that they have the capacity to move WNV long distances in spring. However, we still know little about the physiological capacity of robins to migrate while being infectious, which could inhibit the dispersal of the virus through robin migration. Future research on the physiological, ecological and behavioral factors mediating the exposure of birds to WNV will lend insight into the role that robins and other birds play in the transmission ecology of the virus.

ecology↗

bioRxiv2023

Urban living can rescue Darwin's finches from the lethal effects of invasive vampire flies

Human activity changes multiple factors in the environment, which can have additive or neutralizing effects on organisms. However, few studies have explored the causal effects of multiple anthropogenic factors, such as urbanization and invasive species, on animals, and the mechanisms that mediate these interactions. This study examines the influence of urbanization on the detrimental effect of invasive avian vampire flies (Philornis downsi) on endemic Darwins finches in the Galapagos Islands. We experimentally manipulated nest fly abundance in an urban and non-urban area and then characterized nestling health, survival, diet, and gene expression patterns related to host defense. Survival of fumigated nestlings from urban and non-urban nests did not differ significantly. However, sham-fumigated, non-urban nestlings lost more blood and few nestlings survived compared to urban nestlings. Stable isotopic values ({delta}15N) from urban nestling feces were higher than non-urban nestlings, suggesting that urban nestlings are consuming more protein. {delta}15N values correlated negatively with parasite abundance, which suggests that diet might influence host defenses (e.g., tolerance and resistance). Parasitized urban nestlings differentially expressed genes within pathways associated with red blood cell production (tolerance) and pro-inflammatory response (innate immunological resistance), compared to sham-fumigated non-urban nestlings. In contrast, sham-fumigated non-urban nestlings differentially expressed genes within pathways associated with immunoglobulin production (adaptive immunological resistance). Our results suggest that urban nestlings are investing more in pro-inflammatory responses to resist parasites, but also recovering more blood cells to tolerate blood loss. Although non-urban nestlings are mounting an adaptive immune response, it is likely a last effort by the immune system rather than an effective defense against avian vampire flies since few nestlings survived.

ecology↗