Nancy E. Mathews

LANDSCAPE‐SCALE PATTERNS OF BLACK‐THROATED SPARROW (AMPHISPIZA BILINEATA) ABUNDANCE AND NEST SUCCESS

Analyses of avian demographic patterns across entire, contiguous landscapes are rare, but such analyses are important for understanding population dynamics. We se- lected the Black-throated Sparrow in the northern Chihuahuan Desert as a model to test patterns of abundance and nest success across a landscape. We integrated abundance, nest density, and nesting success measured on sampling plots with a classified satellite map of the distribution of seven habitat types to analyze spatial and temporal patterns contributing to the population dynamics of this species. Adult relative abundance ranged from ,1 bird/100 ha in pinyon-juniper habitat to 24- 39 birds/100 ha in shrubland habitats. Nest density was consistently high in mesquite, moderate to high in creosotebush, and low in black grama grassland; this value exhibited more temporal variability than relative abundance of adults. Nest success rates exhibited a strong habitat effect and ranged from 8% in mesquite to 47% in black grama grassland; overall population nest success was 0.266. In all three years, nest success in mesquite was significantly lower than in all other habitat types (P , 0.01). There was no correlation between nest success and adult relative abundance. While mesquite habitat contained about one-third of all adults in the three years of the study, it contributed as little as 10% of successful nests. In creosotebush, the relative contribution to both adult abundance and successful nests was relatively high. Mesa grass- land contained relatively few adults, but up to 44% of successful nests. We discuss how habitat selection theory suggests mechanisms for the observed patterns. Mesquite appears to be a population sink for Black-throated Sparrows and may be an ecological trap. While we do not propose that there is cause for conservation concern for this widespread species, our results underscore the pitfalls associated with using adult abundance as an indicator of habitat quality. The method presented here is applicable for many species and ecosystems and, thus, may be an important tool for conservation and management, as well as a new avenue for scientific investigation of landscape-level population dynamics.

Long-distance Commuting by Brown-headed Cowbirds in New Mexico

The Brown-headed Cowbird (Molothrus ater) is a widespread brood parasite that has attracted considerable recent attention as a possible threat to forestdwelling songbirds (e.g. Finch 1991, Robinson et al. 1995). Cowbirds are unusual among passerines in that their parasitic nature allows some populations to have spatially and temporally separate breeding and feeding activities (Rothstein et al. 1984). Cowbirds are ground foragers that feed in open habitats such as grazed grasslands, agricultural fields, livestock corrals, and mowed lawns (Friedmann 1929, Mayfield 1965, Ortega 1998). For breeding, however, cowbirds occupy a wider variety of habitats where their passerine hosts occur in higher densities (Robinson et al. 1995). This reliance on distinct habitat types for essential activities gives rise to daily commuting patterns in landscapes where breeding and feeding habitats are spatially separated (Rothstein et al. 1984, Thompson 1994, Gates and Evans 1998). Cowbirds are restricted to parasitizing nests that are within commuting distances of their feeding habitats. In recent years, recognition of the relationship between feeding and breeding habitats of cowbirds has led to management strategies involving the manipulation of feeding habitats to alter cowbird breeding distributions for the benefit of potential hosts. For example, in the Coconino National Forest, Arizona, managers rotate livestock (which enhance feeding opportunities for cowbirds) away from nesting sites of endangered Southwestern Willow Flycatchers (Empidonax traillii extimus; Goguen and Mathews 1999). For such strategies to effectively protect hosts, the scale at which they are implemented must be sufficient to discourage cowbirds from commuting. The maximum commuting distance between breeding and feeding sites of cowbirds is one of several factors needed to determine the correct scale of management efforts for cowbirds. In previous radio-telemetry studies, the longest observed commute between breeding and feeding areas of female cowbirds has been around 7 km (Rothstein et al. 1984, Thompson 1994, Gates and Evans 1998). This distance has been used as an estimate of the maximum commuting distance for female cowbirds (e.g. Gustafson and Crow 1994, Coker and Capen 1995) and is the distance used for livestock removal in some cowbird management efforts (e.g. Goguen and Mathews 1999). Given the conservation implications, it is necessary to

Nest desertion by blue-gray gnatcatchers in association with brown-headed cowbird parasitism

The blue-gray gnatcatcher, Polioptila caerulea, is one of the smallest regular hosts of the brown-headed cowbird, Molothrus ater. Gnatcatchers typically raise none of their own young when parasitized, yet anti-parasite behaviours have not been reported. A blue-gray gnatcatcher population in which pairs often desert nests when parasitized by cowbirds was studied to examine the success of a desertion response and to determine factors that stimulate desertion. Of 83 gnatcatcher nests, 76% were parasitized; of nests parasitized during the egg-laying stage, 45% were deserted. Desertion occurred independently of the timing of the parasitism event during the breeding season and the number of cowbird eggs laid per nest. All cowbird eggs laid after the completion of the gnatcatchers egg-laying stage were accepted. Parasitism rates were lower late in the breeding season, and nesting success of unparasitized nests was higher. Pairs that deserted often obtained cowbird-free clutches, allowing them to raise gnatcatcher young. Nest desertion appears to be a means of avoiding parasitism and increasing fitness. The nest desertion response was inconsistent, but it cannot be explained by the adaptiveness of acceptance or by the possibility that blue-gray gnatcatchers are in transition between being acceptors or rejectors of cowbird parasitism. Nest desertion by blue-gray gnatcatchers appears to be a generalized response to a disturbance at the nest induced by cowbirds. The cue that elicits the desertion response remains unclear. ? 1996 The Association for the Study of Animal Behaviour

BROWN-HEADED COWBIRD BEHAVIOR AND MOVEMENTS IN RELATION TO LIVESTOCK GRAZING

The Brown-headed Cowbird (Molothrus ater) is a widespread brood parasite which often engages in a commensalistic feeding relationship with domestic livestock. We studied the behavior of female cowbirds breeding in pinyon–juniper woodlands in New Mexico, USA, on two adjacent sites, one an active cattle ranch, and the other a site that was not grazed by domestic livestock throughout the songbird breeding season. In 1994, we conducted morning and afternoon surveys of cowbird abundance in pinyon–juniper and prairie habitats; from 1995 to 1997 we used radio telemetry to monitor daily and seasonal movement and behavioral patterns of female cowbirds. Our objectives were to measure how closely cowbird feeding behavior was linked to livestock grazing, and how the presence or absence of active livestock grazing within a females breeding range influenced diurnal patterns of behavior. During morning surveys, we detected cowbirds primarily in pinyon–juniper habitat, but in similar numbers in the ungrazed and actively...

White-Tailed Deer Movements in a Chronic Wasting Disease Area in South-Central Wisconsin

Abstract To focus white-tailed deer (Odocoileus virginianus) management within a chronic wasting disease–infected area in south-central Wisconsin, USA, we assessed deer movements and related dispersal to variation in landscape pattern, deer density, and harvest intensity. We radiocollared and monitored 165 deer between 2003 and 2005. Yearling males that dispersed (45%) had greater forest edge (i.e., fragmentation) within natal home ranges. Exploratory movements were rare for adult females. Transient and migratory movements were rare among all deer (<5%). Although yearling males have low chronic wasting disease prevalence rates, they may be infected before dispersal due to variable incubation times. Managers should increase yearling male harvest and consider removing young males in areas of higher forest edge.

Long-Distance Movement of a White-Tailed Deer Away From a Chronic Wasting Disease Area

Abstract Chronic wasting disease is a fatal, transmissible spongiform encephalopathy found among cervids. Spread of the disease across the landscape is believed to result from movements (dispersal, exploratory, transient, or migratory) of infected deer, serving as the vectors for the disease. We document an unusual long-distance movement of a young female, out of the chronic wasting disease eradication zone in south-central Wisconsin. This type of movement could function as a rapid, long-distance dispersing mechanism for the disease only if the following conditions are met: the deer is infected and shedding prions, the deer directly contacts other deer and transmits secretions carrying an infectious dose of prions, or an infectious dose of prions is transmitted to the environment and taken up by other deer. Despite lower prevalence rates of chronic wasting disease among young deer, we believe managers should not dismiss deer making long-distance movements such as we report, as they could serve as potential long-distance vectors of the disease.

Songbird Community Composition and Nesting Success in Grazed and Ungrazed Pinyon-Juniper Woodlands

Livestock grazing is a dominant land use of pinyon-juniper habitats in the western United States, yet the effects of grazing on breeding bird communities in this habitat have been poorly studied. We compared habitat structure, songbird abundance, and nesting productivity within pinyon-juniper woodlands on an actively grazed site and a site experiencing long-term relief from livestock grazing in northeastem New Mexico. From 1992 to 1995, we performed vegetation sampling, conducted songbird point counts, and located and monitored nests on 8 35-ha study plots. Four of these plots experienced moderate cattle grazing and 4 were ungrazed since 1973. We found no differences in habitat or vegetation features between grazed and ungrazed plots. Bird communities were similar, with only 1 of the 11 species we tested more abundant on the ungrazed treatment (western scrub-jay; Aphelocoma californicus). We detected no differences in nesting success or cause-specific rates of nest failure for 7 common bird species (P < 0.05), and we detected no differences in brown-headed cowbird (Molothrus ater) parasitism rates for the major hosts between grazed and ungrazed areas. Greater than 75% of the nests of the solitary vireo (Vireo solitarius), western tanager (Piranga ludoviciana), and blue-gray gnatcatcher (Polioptila caerulea) were parasitized on both treatments. These high parasitism rates may be the result of high densities of local cowbirds because of abundant feeding sites (i.e., livestock), the high mobility of cowbirds, and the close proximity of ungrazed plots to grazed areas (all <4 km). Our results suggest that 20 years of relief from grazing had little influence on the habitat structure or bird species composition of the pinyon-juniper woodlands on our study site. However, livestock grazing has indirectly affected the nesting success of some songbird species via the influence of grazing on cowbird abundance. Our findings highlight the need for studies that incorporate nest monitoring and landscape-scale approaches to better understand the relation between cowbirds, livestock, and songbirds, and the time required for recovery from grazing effects.

Effects of supplemental prey, vegetation, and time on success of artificial nests

Despite intensive management on many grassland areas, nest loss to predators continues to result in low nest-survival rates. Management efforts are complicated by complex relationships among habitat, predators, and prey resources. We monitored the fates of artificial nests (908 in 1993, 827 in 1994) on Conservation Reserve Program (CRP) plots from April to July to test effects of prey supplementation, vegetation density, and time (month) on nest survival in agricultural and range landscapes in northwest Texas. Supplemental prey had the greatest effect on artificial nest survival and increased nest survival in both sparse and dense vegetation. Prey supplementation may be useful when used in conjunction with habitat management for dense nesting cover or in areas that already have dense vegetation. Nest survival was highest early in the nesting season, emphasizing the importance of available nesting cover during this period. Although least important, dense vegetation increased artificial nest survival. When evaluating management options, managers should consider logistical and economic costs of using supplemental prey, as well as potential effects on predator population dynamics.

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

Abstract 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.

Reproductive costs of commuting flights in brown-headed cowbirds

Brood parasitism by brown-headed cowbirds (Molothrus ater, hereafter cowbird) has attracted much attention from conservation biologists as an edge effect over small spatial scales, reducing nesting success of songbirds near habitat boundaries. The mechanisms underlying this landscape-scale edge effect have not been studied, but this phenomenon has been attributed to the energetic costs of commuting by cowbirds-i.e., daily flights between breeding and feeding areas. We studied cowbird egg-laying rates by counting ovarian follicles at a site in New Mexico, USA, where previous research had established the presence of short-distance commuting (SDC; approx 2 km) and long-distance commuting (LDC; approx 12 km) cowbirds. We hypothesized that LDG cowbirds would lay fewer eggs than SDC cowbirds and have more atretic yolky follicles (follicles degenerating prior to ovulation) due to the greater energetic demands of their commuting flights. In 1999-2000, we found that LDC cowbirds had half the number of post-ovulatory follicles corresponding to a 5-day period (x = 1.8; n = 16) as SDC cowbirds (x = 3.5, n = 14; P = 0.008). Thus, the egg-laying rate of LDC cowbirds (0.36/day) was only half that of SDC cowbirds (0.7/day). Long-distance commuting cowbirds also had more atretic follicles (x = 1.3, n = 16; P = 0.006) than SDC (x = 0.2, n = 14), suggesting that the difference in egg-laying rates was due largely to energetic stress in cowbirds commuting longer distances. Wing length and body mass were smaller in LDC cowbirds than SDC cowbirds, suggesting that LDC cowbirds may have been subordinate individuals excluded from breeding habitat closer to feeding areas by intraspecific competition. These results suggest that management strategies, such as livestock removals, designed to reduce cowbird numbers at host nest sites may provide additional benefits to hosts by reducing the fecundity of individual cowbirds.