Increased genetic diversity and immigration after West Nile virus emergence in American crows: No evidence for a genetic bottleneck

Infectious diseases can cause steep declines in wildlife populations, leading to changes in genetic diversity that may affect the susceptibility of individuals to infection and the overall resilience of populations to pathogen outbreaks. Here, we examine evidence for a genetic bottleneck in a population of American crows (Corvus brachyrhynchos) before and after the emergence of West Nile virus (WNV). More than 50% of marked birds in this population were lost over the 2-year period of the epizootic, representing a 10-fold increase in adult mortality. Using analyses of single-nucleotide polymorphisms (SNPs) and microsatellite markers, we tested for evidence of a genetic bottleneck and compared levels of inbreeding and immigration in the pre- and post-WNV populations. Counter to expectations, genetic diversity (allelic diversity and the number of new alleles) increased after WNV emergence. This was likely due to increases in immigration, as the estimated membership coefficients were lower in the post-WNV population. Simultaneously, however, the frequency of inbreeding appeared to increase: Mean inbreeding coefficients were higher among SNP markers, and heterozygosity-heterozygosity correlations were stronger among microsatellite markers, in the post-WNV population. These results indicate that loss of genetic diversity at the population level is not an inevitable consequence of a population decline, particularly in the presence of gene flow. The changes observed in post-WNV crows could have very different implications for their response to future pathogen risks, potentially making the population as a whole more resilient to a changing pathogen community, while increasing the frequency of inbred individuals with elevated susceptibility to disease.

Development of mouse models for the study of chloropicrin and hydrogen fluoride ocular injury

The possibility of chemical terrorism within the United States is a rising concern, with the eye being one of the most sensitive tissues to toxicant exposure. We sought to develop mouse models of toxicant-induced ocular injury for the purpose of evaluating potential therapeutics. Chloropicrin (CP) and hydrogen fluoride (HF) were selected for the study owing to their reportedly high potential to induce ocular injury. Eyes of female BALB/c mice were exposed to CP or HF vapor in order to produce a moderate injury, as defined by acute corneal epithelial loss followed by progressive corneal pathology with the absence of injury to deeper eye structures. Clinical injury progression was evaluated up to 12 weeks postexposure, where a significant dose-dependent induction of corneal neovascularization was measured. Histopathology noted epithelial necrosis and stromal edema as early as 24 h after exposure but was resolved by 12 weeks. A significant increase in inflammatory cytokine concentrations was measured in the cornea 24 h after exposure and returned to baseline by day 14. The ocular injury models we developed here for CP and HF exposure should serve as a valuable tool for the future evaluation of novel therapeutics and the molecular mechanisms of injury.

Acoustic profiling in a complexly social species, the American crow: caws encode information on caller sex, identity, and behavioural context

Previous research on inter-individual variation in the calls of corvids has largely been restricted to single call types, such as alarm or contact calls, and has rarely considered the effects of age on call structure. This study explores structural variation in a contextually diverse set of "caw" calls of the American crow (Corvus brachyrhynchos), including alarm, foraging recruitment and territorial calls, and searches for structural features that may be associated with behavioural context and caller sex, age, and identity. Automated pitch detection algorithms are used to generate 23 pitch-related and spectral parameters for a collection of caws from 18 wild, marked crows. Using principal component analysis and mixed models, we identify independent axes of acoustic variation associated with behavioural context and with caller sex, respectively. We also have moderate success predicting caller sex and identity from call structure. However, we do not find significant acoustic variation with respect to caller age.

Factors associated with the risk of West Nile virus among crows in New York State

West Nile virus (WNV) is transmitted between avian hosts in enzootic cycles by a mosquito vector. The virus has significant disease effects on humans and equines when it bridges into an epizootic cycle. As the initial epidemic of WNV in 1999, perennial outbreaks in New York State suggest the local establishment of natural foci with perpetuation of the virus among susceptible hosts rather than reintroduction of the virus. The factors that play a role in the perpetuation of the virus are not fully understood. American crows (Corvus brachyrhynchos) are known to be highly susceptible to infection with the virus. We investigate the factors that put crows at risk of infection in Tompkins County, New York during the period of 2000-2008 in a case-control study. Cases were crow carcasses that were found dead and tested positive for WNV using real time reverse transcription or VecTest. Data on putative risk factors were collected and assessed for significance of association with the presence of WNV using logistic regression analysis to evaluate the significance of each factor while simultaneously controlling for the effect of others. The risk of a crow carcass testing WNV positive varied with age, season of the year and ecological area where the carcass was found. Crows that were more than 1-year-old were four times more likely to be WNV positive in comparison to birds that were less than 1 year of age. It was three times more likely to find WNV positive carcasses in residential areas in comparison to rural areas. The risk of testing WNV positive did not vary by sex of the crow carcasses.

Condition, innate immunity and disease mortality of inbred crows

Cooperatively breeding American crows (Corvus brachyrhynchos) suffer a severe disease-mediated survival cost from inbreeding, but the proximate mechanisms linking inbreeding to disease are unknown. Here, we examine indices of nestling body condition and innate immunocompetence in relationship to inbreeding and disease mortality. Using an estimate of microsatellite heterozygosity that predicts inbreeding in this population, we show that inbred crows were in relatively poor condition as nestlings, and that body condition index measured in the first 2-33 days after hatching, in addition to inbreeding index, predicted disease probability in the first 34 months of life. Inbred nestlings also mounted a weaker response along one axis of innate immunity: the proportion of bacteria killed in a microbiocidal assay increased as heterozygosity index increased. Relatively poor body condition and low innate immunocompetence are two mechanisms that might predispose inbred crows to ultimate disease mortality. A better understanding of condition-mediated inbreeding depression can guide efforts to minimize disease costs of inbreeding in small populations.

Non-West Nile virus-associated mortality in a population of American crows (Corvus brachyrhynchos): a gross and histopathologic study

The American crow (Corvus brachyrhynchos) is a common urban and rural inhabitant of the Northeast and Midwest United States that is commonly infected with West Nile virus (WNV). The current study was initiated to determine non-WNV-associated causes of mortality in the American crow. All animals (40/40) tested negative for WNV infection via polymerase chain reaction and had no evidence of infection based on immunohistochemistry. Common gross necropsy findings included external trauma (6/40), hepatosplenomegaly (6/40), poxviral dermatitis (5/40), and pneumonia (3/40). Common histologic findings included endoparasitism (32/40), multifocal hepatic and splenic necrosis (7/40), pigment accumulation in the spleen (5/40), and disseminated bacterial infection (3/40). The most significant and debilitating diseases included fungal pneumonia and poxvirus-associated lesions. The present report increases the knowledge of diseases present in the American crow population.

Growth and nutritional state of American Crow nestlings vary between urban and rural habitats

In urbanized areas, many adult birds find sufficient foods to survive, but the anthropogenic foods that are abundant there may be detrimental to nestling growth. In fact, American Crow (Corvus brachyrhynchos) nestlings are smaller in suburban than rural areas, possibly because of nutrient limitation. Here, we seek to identify possible causes of size differences by comparing both size and blood chemistry measures in rural and suburban crow nestlings. We quantified land use in known crow territories and distinguished three distinct environments: suburban-residential, suburban-managed (e.g., golf courses), and rural. We measured nestlings near fledging age in each environment and bled them for determination of unbound plasma calcium, total protein, and corticosterone. We supplemented a subset of broods in suburban-residential and rural areas with a food high in protein and calcium. Rural nestlings were significantly larger than suburban-residential crows and had higher total serum protein. Nestlings in suburban-managed areas were intermediate in size and serum protein but had the lowest plasma calcium levels. Nestling corticosterone levels did not differ significantly among habitats, indicating that, although suburban nestlings may be food-limited, they were not starving. Supplemented nestlings in suburban-residential areas were significantly larger in some growth measures than their unsupplemented counterparts. Unexpectedly, supplemented rural nestlings were significantly smaller than unsupplemented rural ones, suggesting that parents use easily accessible food even when it is nutritionally suboptimal. Our results indicate that nestlings in suburban areas are nutrient restricted, rather than calorie restricted.

Disease-mediated inbreeding depression in a large, open population of cooperative crows

Disease-mediated inbreeding depression is a potential cost of living in groups with kin, but its general magnitude in wild populations is unclear. We examined the relationships between inbreeding, survival and disease for 312 offspring, produced by 35 parental pairs, in a large, open population of cooperatively breeding American crows (Corvus brachyrhynchos). Genetic analyses of parentage, parental relatedness coefficients and pedigree information suggested that 23 per cent of parental dyads were first- or second-order kin. Heterozygosity-heterozygosity correlations suggested that a microsatellite-based index of individual heterozygosity predicted individual genome-wide heterozygosity in this population. After excluding birds that died traumatically, survival probability was lower for relatively inbred birds during the 2-50 months after banding: the hazard rate for the most inbred birds was 170 per cent higher than that for the least inbred birds across the range of inbreeding index values. Birds that died with disease symptoms had higher inbreeding indices than birds with other fates. Our results suggest that avoidance of close inbreeding and the absence of inbreeding depression in large, open populations should not be assumed in taxa with kin-based social systems, and that microsatellite-based indices of individual heterozygosity can be an appropriate tool for examining the inbreeding depression in populations where incest and close inbreeding occur.

Reproductive partitioning and the assumptions of reproductive skew models in the cooperatively breeding American crow

Understanding the benefits of cooperative breeding for group members of different social and demographic classes requires knowledge of their reproductive partitioning and genetic relatedness. From 2004-2007, we examined parentage as a function of relatedness and social interactions among members of 21 American crow (Corvus brachyrhynchos) family groups. Paired female breeders monopolized maternity of all offspring in their broods, whereas paired male breeders sired 82.7% of offspring, within-group auxiliary males sired 6.9% of offspring, and extragroup males sired 10.4% of offspring. Although adult females had fewer opportunities for direct reproduction as auxiliaries than males, they appeared to have earlier opportunities for independent breeding. These different opportunities for direct reproduction probably contributed to the male biased adult auxiliary sex ratio. Patterns of reproductive partitioning and conflict among males were most consistent with a synthetic reproductive skew model, in which auxiliaries struggled with breeders for a limited reproductive share, beyond which breeders could evict them. Counter to a frequent assumption of reproductive skew models, female breeders appeared to influence paternity, although their interests might have agreed with the interests of their paired males. Unusual among cooperative breeders, close inbreeding and incest occurred in this population. Incest avoidance between potential breeders did not significantly affect reproductive skew.

Field Identification of the Mice

Field identification of the White-footed Mouse (Peromyscus leucopus noveboracensis) and Long-tailed Deer Mouse (Peromyscus maniculatus gracilis) is difficult because of their similar external morphology. Peromyscus were sampled by live-trapping during a five-year period (1992-1996) at the Arnot Teaching and Research Forest, Van Etten, New York and identified to species by electrophoresis of their salivary amylase. No electromorphs were shared between P. leucopus and P. maniculatus, thus permitting unambiguous species identification of individuals. Means and ranges of four external measurements (ear, head-body, hind-foot, and tail) and tail to head-body ratio were determined for amylase-genotyped live mice. Although some body measurements did differ on average between the two species (ear, head-body, and tail for adults; hind-foot and tail for juveniles), the ranges of these overlap considerably. When the four external measurements (excluding the tail to head-body ratio) were used to construct two discriminant-function equations, they yielded correct identification of 80% of the adult P. l. noveboracensis and P. m. gracilis assessed excluding juveniles, and 71% of adult and juvenile mice combined. The function reported here allows partial field identification, but genetic analysis remains the only reliable field method for differentiation between live P. l. noveboracensis and P. m. gracilis. Includes erratum for a figure in this article.