Community-level patterns of population recruitment in a generalist avian brood parasite, the brown-headed cowbird.

The brown-headed cowbird (Molothrus ater) is a generalist brood parasite that typically parasitizes many host species in a single bird community. Population recruitment in a generalist parasite should be diverse with respect to host species; however, host-specific rates of cowbird recruitment have not been reported in any host community, and the determinants of host quality are poorly known. We studied the combined influence of parasitism level, nest abundance, and host quality on community-level patterns of cowbird recruitment in New Mexico, USA. Our objectives were to: (1) evaluate patterns of host use and quality; (2) compare cowbird egg investment and recruitment among host species; (3) identify host species of most importance to cowbird recruitment. Cowbirds parasitized 11 host species, with five "major" hosts experiencing high parasitism levels (>or=1 cowbird egg/nest) and six minor hosts experiencing low parasitism levels (<0.3 cowbird eggs/nest). Parasitism level was not correlated with host species abundance, host mass, host nestling period length, or host success at fledging cowbirds. However, tree-nesting hosts were parasitized more than ground-nesters, and foliage-gleaners more than sally-foragers and ground-foragers. Average estimated survival to fledging of cowbird eggs laid in active host nests was 0.19. Cowbird recruitment was diverse with respect to hosts but was less evenly distributed across the host community than was cowbird egg investment because western tanagers (Piranga ludovicianus) fledged cowbirds more successfully than other hosts. This success in western tanagers was due to high cowbird survivorship in tanager nests and may be associated with the larger body size of tanagers relative to other hosts.

Surgical technique for tubal ligation in white-tailed deer (Odocoileus virginianus).

Surgical tubal ligation was used to sterilize urban free-ranging white-tailed deer (Odocoileus virginianus) as a methodology of a larger study investigating the influences of intact, sterile females on population dynamics and behavior. Deer were either trapped in clover traps (n = 55) and induced with an i.m. injection of xylazine and tiletamine/zolazepam or induced by a similar protocol by dart (n = 12), then intubated and maintained on isoflurane in oxygen. Over 3 yr, individual female deer (n = 103) were captured in Highland Park, Illinois, with a subset of females sterilized using tubal ligation by ventral laparotomy (n = 63). Other sterilization procedures included tubal transection by ventral (n = 1) or right lateral (n = 2) laparoscopy and ovariohysterectomy by ventral laparotomy (n = 1). One mortality (1/ 67, 1.5%) of a doe with an advanced pregnancy was attributed to a lengthy right lateral laparoscopic surgery that was converted to a right lateral laparotomy. The initial surgical modality of laparoscopy was altered in favor of a ventral laparotomy for simplification of the project and improved surgical access in late-term gravid does. Laparotomy techniques included oviductal ligation and transection (n = 14), application of an oviductal mechanical clip (n = 9), ligation and partial salpingectomy (n = 40), and ovariohysterectomy (n = 1). As of 2 yr poststerilization, no surgical does were observed with fawns, indicating that these procedures provide sterilization with low mortality in urban white-tailed deer.

Local Gradients of Cowbird Abundance and Parasitism Relative to Livestock Grazing in a Western Landscape.

We studied local patterns of Brown-headed Cowbird (Molothrus ater) abundance, parasitism rates, and nest success of a common host, the Plumbeous Vireo (Vireo plumbeus), in relation to the distribution of livestock grazing in an undeveloped region of northeastern New Mexico, 1992-1997. We predicted that both cowbird abundance and parasitism rates of vireo nests would decrease with increasing distance from active livestock grazing, and that the nesting success of vireos would increase. We measured cowbird abundance and host density and located and monitored vireo nests in pinyon-juniper and mixed-conifer habitats that ranged from actively grazed to isolated from livestock grazing by up to 12 km. Cowbird abundance declined with distance from active livestock grazing and was not related to host density or habitat type. Brood parasitism levels of vireo nests (n = 182) decreased from> 80% in actively grazed habitats to 33% in habitats that were 8-12 km from active grazing but did not vary by habitat type or distance to forest edge. Vireo nesting success was higher in mixed-conifer habitat than in pinyon-juniper but was unrelated to distance from active livestock grazing. Nest losses due to parasitism declined with distance from active livestock grazing. Our results suggest that cowbird abundance and parasitism rates of hosts may be distributed as a declining gradient based on distance from cowbird feeding sites and that isolation from feeding sites can reduce the effects of parasitism on host populations. These findings provide support for management techniques that propose to reduce local cowbird numbers and parasitism levels by manipulating the distribution of cowbird feeding sites. The presence of parasitized nests> 8 km from active livestock grazing suggests that, in some regions, management efforts may need to occur at larger scales than previously realized.

RESUMEN: Estudiamos patrones locales de abundancia del tordo cabeza café (Molothrus ater), las tasas de parasitismo y el éxito de nidada de un hospedero común, el vireo (Vireo plumbeus), en relación con la distribución del pastoreo en una región poco desarrollada del noreste de Nuevo México, entre 1992 y 1997. Pronosticamos que tanto la abundancia del tordo, como las tasas de parasitismo de nidos de vireo disminuirían con un incremento en la distancia a las zonas de pastoreo activo de ganado y el éxito de nidada de vireos incrementaría. Medimos la abundancia de tordos y la densidad de hospederos y localizamos y monitoreamos los nidos de vireos en hábitats de pino-cedro y de coníferas mixtas que variaron desde activamente pastoreadas hasta sitios distanciados del pastoreo hasta por 12 km. La abundancia de los tordos disminuyó con la distancia de las zonas de pastoreo activo de ganado y no estuvo relacionada con la densidad de hospederos o el tipo de hábitat. Los niveles de parasitismo de las nidadas del vireo (n = 182) disminuyeron de> 80% en hábitats activamente pastoreados a 33% en hábitats que estuvieron de 8 a 12 km de distancia de los sitios de pastoreo activo, pero no variaron con el tipo de hábitat ni la distancia al borde del bosque. El éxito de nidada de vireos fue mayor en el hábitat mixto de coníferas que en el hábitat de pino-cedro, pero no estuvo relacionado con la distancia al sitio de pastoreo. Las pérdidas debidas al parasitismo disminuyeron con la distancia al sitio activo de pastoreo. Nuestros resultados sugieren que la abundancia de tordos y las tasas de parasitismo de hospederos podría estar distribuida en forma de un gradiente en descenso basado en la distancia a los sitios de alimentación de los tordos y a que el aislamiento de los sitios de alimentación puede reducir los efectos del parasitismo de las poblaciones de hospederos. Estos resultados apoyan las técnicas de manejo que proponen la reducción local de números de tordos y los niveles de parasitismo al manipular la distribución de sitios de alimentación de tordos. La presencia de nidos parasitados> 8 km de sitios con pastoreo activo sugiere que, en algunas regiones, los esfuerzos de manejo deben ocurrir a escalas mayores a lo que anteriormente se pensaba.

Evaluating spatial overlap and relatedness of white-tailed deer in a chronic wasting disease management zone.

Wildlife disease transmission, at a local scale, can occur from interactions between infected and susceptible conspecifics or from a contaminated environment. Thus, the degree of spatial overlap and rate of contact among deer is likely to impact both direct and indirect transmission of infectious diseases such chronic wasting disease (CWD) or bovine tuberculosis. We identified a strong relationship between degree of spatial overlap (volume of intersection) and genetic relatedness for female white-tailed deer in Wisconsin's area of highest CWD prevalence. We used volume of intersection as a surrogate for contact rates between deer and concluded that related deer are more likely to have contact, which may drive disease transmission dynamics. In addition, we found that age of deer influences overlap, with fawns exhibiting the highest degree of overlap with other deer. Our results further support the finding that female social groups have higher contact among related deer which can result in transmission of infectious diseases. We suggest that control of large social groups comprised of closely related deer may be an effective strategy in slowing the transmission of infectious pathogens, and CWD in particular.