James W. Rivers

Sperm competition selects beyond relative testes size in birds.

Sperm morphology varies considerably across taxa, and postcopulatory sexual selection is thought to be one of the main forces responsible for this diversity. Several studies have investigated the effects of the variation in sperm design on sperm function, but the consequences of variation in sperm design on testis morphology have been overlooked. Testes size or architecture may determine the size of the sperm they produce, and selection for longer sperm may require concomitant adaptations in the testes. Relative testes size differs greatly between species and is often used as an index of sperm competition, but little is known about whether larger testes have more sperm-producing tissue or produce sperm at a faster rate. Using a comparative approach in New World Blackbirds (Icteridae), we found (1) a strong link between testis histology and sperm length, suggesting selection on testis architecture through selection on sperm size, and (2) that species under intense sperm competition had a greater proportion of sperm-producing tissue within their testes. These results support the prediction that sperm competition fosters adaptations in reproductive organs that extend beyond testes size, and raise questions about the trade-offs influencing reproductive investment.

Thresholds in forest bird occurrence as a function of the amount of early-seral broadleaf forest at landscape scales

Recent declines in broadleaf-dominated, early-seral forest globally as a function of intensive forest management and/or fire suppression have raised concern about the viability of populations dependent on such forest types. However, quantitative information about the strength and direction of species associations with broadleaf cover at landscape scales are rare. Uncovering such habitat relationships is essential for understanding the demography of species and in developing sound conservation strategies. It is particularly important to detect points in habitat reduction where rates of population decline may accelerate or the likelihood of species occurrence drops rapidly (i.e., thresholds). Here, we use a large avian point-count data set (N = 4375) from southwestern and northwestern Oregon along with segmented logistic regression to test for thresholds in forest bird occurrence as a function of broadleaf forest and early-seral broadleaf forest at local (150-m radius) and landscape (500-2000-m radius) scales. All 12 bird species examined showed positive responses to either broadleaf forest in general, and/or early-seral broadleaf forest. However, regional variation in species response to these conditions was high. We found considerable evidence for landscape thresholds in bird species occurrence as a function of broadleaf cover; threshold models received substantially greater support than linear models for eight of 12 species. Landscape thresholds in broadleaf forest ranged broadly from 1.35% to 24.55% mean canopy cover. Early-seral broadleaf thresholds tended to be much lower (0.22-1.87%). We found a strong negative relationship between the strength of species association with early-seral broadleaf forest and 42-year bird population trends; species most associated with this forest type have declined at the greatest rates. Taken together, these results provide the first support for the hypothesis that reductions in broadleaf-dominated early-seral forest due to succession and intensive forest management have led to population declines of constituent species in the Pacific northwestern United States. Forest management treatments that maintain or restore even small amounts of broadleaf vegetation could mitigate further declines.

The persistent problem of lead poisoning in birds from ammunition and fishing tackle

ABSTRACT Lead (Pb) is a metabolic poison that can negatively influence biological processes, leading to illness and mortality across a large spectrum of North American avifauna (>120 species) and other organisms. Pb poisoning can result from numerous sources, including ingestion of bullet fragments and shot pellets left in animal carcasses, spent ammunition left in the field, lost fishing tackle, Pb-based paints, large-scale mining, and Pb smelting activities. Although Pb shot has been banned for waterfowl hunting in the United States (since 1991) and Canada (since 1999), Pb exposure remains a problem for many avian species. Despite a large body of scientific literature on exposure to Pb and its toxicological effects on birds, controversy still exists regarding its impacts at a population level. We explore these issues and highlight areas in need of investigation: (1) variation in sensitivity to Pb exposure among bird species; (2) spatial extent and sources of Pb contamination in habitats in relation to bird exposure in those same locations; and (3) interactions between avian Pb exposure and other landscape-level stressors that synergistically affect bird demography. We explore multiple paths taken to reduce Pb exposure in birds that (1) recognize common ground among a range of affected interests; (2) have been applied at local to national scales; and (3) engage governmental agencies, interest groups, and professional societies to communicate the impacts of Pb ammunition and fishing tackle, and to describe approaches for reducing their availability to birds. As they have in previous times, users of fish and wildlife will play a key role in resolving the Pb poisoning issue.

Evaluation of a reproductive index to estimate dickcissel reproductive success

Intensive monitoring of bird nests to measure reproductive success is time-consuming and may influence the fate of nests. Reproductive indices that do not require searching for and visiting nests may be reasonable alternatives to nest monitoring if they provide results similar to nest-searching efforts. We evaluated the reproductive index of Vickery et al. (1992) for estimating reproductive success of the dickcissel (Spiza americana) in northeast Kansas, USA. We used nest searching and Vickery et al.s (1992) reproductive index to compare reproductive success on 20 plots (200 x 200 m). Daily nest survival (DNS) rates averaged 0.911 (SE = 0.011, n = 72 nests), and brown-headed cowbirds (Molothrus ater) accounted for 21% of all nest failures. Surveyors underestimated reproductive index ranks when compared to nest-searching efforts and were inaccurate in their assignment of reproductive success. In particular, surveyors reported successful nests on 3 study plots that fledged no young, probably because young dickcissels moved onto plots after fledging from their natal territories. Our results indicate that the reproductive index of Vickery et al. (1992) may be inappropriate for wary species or those heavily parasitized by brood parasites. We suggest that before relying on the index alone, investigators should use pilot trials to determine whether results from this index are concordant with results from intensive nest-searching efforts for species of interest.

Sperm competition leads to functional adaptations in avian testes to maximize sperm quantity and quality

The outcome of sperm competition (i.e. competition for fertilization between ejaculates from different males) is primarily determined by the relative number and quality of rival sperm. Therefore, the testes are under strong selection to maximize both sperm number and quality, which are likely to result in trade-offs in the process of spermatogenesis (e.g. between the rate of spermatogenesis and sperm length or sperm energetics). Comparative studies have shown positive associations between the level of sperm competition and both relative testis size and the proportion of seminiferous (sperm-producing) tissue within the testes. However, it is unknown how the seminiferous tissue itself or the process of spermatogenesis might evolve in response to sperm competition. Therefore, we quantified the different germ cell types and Sertoli cells (SC) in testes to assess the efficiency of sperm production and its associations with sperm length and mating system across 10 species of New World Blackbirds (Icteridae) that show marked variation in sperm length and sperm competition level. We found that species under strong sperm competition generate more round spermatids (RS)/spermatogonium and have SC that support a greater number of germ cells, both of which are likely to increase the maximum sperm output. However, fewer of the RS appeared to elongate to mature spermatozoa in these species, which might be the result of selection for discarding spermatids with undesirable characteristics as they develop. Our results suggest that, in addition to overall size and gross morphology, testes have also evolved functional adaptations to maximize sperm quantity and quality.

Brown-headed cowbird nestlings influence nestmate begging, but not parental feeding, in hosts of three distinct sizes

Avian brood parasites typically depress the fitness of their hosts by reducing the number of host offspring produced, yet little is known about how parasitic nestlings influence the behaviour of host parents and host offspring. In this study, we used three hosts of the brown-headed cowbird, Molothrus ater, that varied in size (i.e. smaller, similar to and larger than cowbirds of a given age) to determine whether parasitic nestlings altered patterns of food provisioning by host parents and begging by host young under field conditions. Adult provisioning did not change in the presence of a cowbird but instead was influenced by feeding treatment and host size. In parasitized broods where nestlings differed in size (i.e. the small and large hosts), the larger nestling received the majority of food brought to the nest, regardless of whether it was the cowbird or host nestling. In contrast, similar-sized host nestlings received a similar amount of food in parasitized and unparasitized host broods. Relative to unparasitized broods, the presence of a cowbird led to increased begging intensity by the small host, had no clear effect on begging behaviour of the intermediate-sized host, and reduced begging intensity of the large host. Taken together, these results suggest the presence of a cowbird did not lead to changes in provisioning behaviour in parents, and the extent to which cowbirds influenced host begging behaviour depended on the size of the host.

Community-Level Patterns of Host Use by the Brown-Headed Cowbird (Molothrus ater), a Generalist Brood Parasite

ABSTRACT. Quantifying community-level host use by generalist brood parasites is important because it provides a measure of the resources that parasites need for reproduction. During the 2002–2007 breeding seasons, we quantified host use by the Brown-headed Cowbird (Molothrus ater) at Konza Prairie Biological Station in northeast Kansas. Overall, 54.4% of nests (n = 2,621) of 22 passerine host species that typically accept parasitic eggs were parasitized, and parasitized nests contained an average of 1.9 ± 1.03 (SD) Brown-headed Cowbird eggs, nestlings, or both. Multiple parasitism was common on the study site in all years: 55.7% of parasitized nests (n = 1,425) received ≥2 Brown-headed Cowbird eggs. The Bells Vireo (Vireo bellii; 70.5%) and the Dickcissel (Spiza americana; 69.6%) were parasitized at a significantly greater rate than the Red-winged Blackbird (Agelaius phoeniceus, 21.9%); collectively, these three species represented 85.8% of potential host nests found in all years. The host preference of Brown-headed Cowbirds did not appear to shift among the three species and instead they exhibited a consistent preference for Dickcissel nests over the course of the breeding season and among years. Both the rate and the intensity of parasitism on Dickcissels were significantly lower during the present study than in one conducted several decades earlier at the same site. Our results indicate that cowbirds in northeast Kansas differentially parasitize hosts, that most cowbird eggs are laid in the nests of a small number of host species, and that the Dickcissel appears to be preferred over other hosts.

HIGH LEVELS OF RELATEDNESS BETWEEN BROWN-HEADED COWBIRD (MOLOTHRUS ATER) NESTMATES IN A HEAVILY PARASITIZED HOST COMMUNITY

ABSTRACT. Multiple parasitism of host nests by generalist brood parasites reflects the decisions of laying females and may influence the development and behavior of parasitic young. We used microsatellite and mtDNA control-region haplotype data to examine the relatedness of Brown-headed Cowbird (Molothrus ater) offspring in multiply parasitized nests sampled from a heavily parasitized host community in northeastern Kansas. We also examined how host nest availability influenced the degree of multiple parasitism, and used community-wide parasitism data to explore whether female cowbirds were constrained in their laying decisions. Relatedness estimates for all suitable pairwise comparisons (n = 94 from 41 multiply parasitized nests) found that the mean likelihood than an individual cowbird in a multiply parasitized nest shared its nest with a full sibling was 40.4% (95% confidence interval: 28.4–52.4%), indicating that many cowbird offspring were reared with full siblings. Extensive community-wide parasitism data revealed that most cowbird offspring shared the nest with ≥1 other conspecific. Additionally, we found that the proportion of available host nests increased steeply at the start of the breeding season and remained high for most of the breeding season, but that the degree of multiple parasitism was unrelated to the number of new nests. We found evidence that laying decisions of female cowbirds were constrained, which suggests that heavy parasitism levels were due to a high degree of competition for host nests. This intense competition for host nests, in turn, results in cowbird offspring often competing with conspecifics in our population, including full siblings, for host parental care.

Spatiotemporal patterns and risk factors for lead exposure in endangered California condors during 15 years of reintroduction.

Large-scale poisoning events are common to scavenging bird species that forage communally, many of which are in decline. To reduce the threat of poisoning and compensate for other persistent threats, management, including supplemental feeding, is ongoing for many reintroduced and endangered vulture populations. Through a longitudinal study of lead exposure in California condors (Gymnogyps californianus), we illustrate the conservation challenges inherent in reintroduction of an endangered species to the wild when pervasive threats have not been eliminated. We evaluated population-wide patterns in blood lead levels from 1997 to 2011 and assessed a broad range of putative demographic, behavioral, and environmental risk factors for elevated lead exposure among reintroduced California condors in California (United States). We also assessed the effectiveness of lead ammunition regulations within the condors range in California by comparing condor blood lead levels before and after implementation of the regulations. Lead exposure was a pervasive threat to California condors despite recent regulations limiting lead ammunition use. In addition, condor lead levels significantly increased as age and independence from intensive management increased, including increasing time spent away from managed release sites, and decreasing reliance on food provisions. Greater independence among an increasing number of reintroduced condors has therefore elevated the populations risk of lead exposure and limited the effectiveness of lead reduction efforts to date. Our findings highlight the challenges of restoring endangered vulture populations as they mature and become less reliant on management actions necessary to compensate for persistent threats.

Parent-absent begging in the Brown-headed Cowbird (Molothrus ater): the role of short-term need and nestmate size

Although it is well-established that nestlings of many altricial species beg when parents are away from the nest, we have a poor understanding of parent-absent begging in brood parasites, including the proximate factors that may influence begging frequency and intensity. In this study, I examined how parent-absent begging was influenced by competitive asymmetries between host and Brown-headed Cowbird (Molothrus ater) nestlings under disparate levels of short-term need. Food-deprived cowbird nestlings begged more frequently and for a greater proportion of parent-absent period than when food-supplemented, with similar patterns observed in hosts of different sizes. In contrast, three metrics of cowbird begging intensity varied relative to host size but not due to differences in short-term need. Cowbirds consistently begged more frequently and intensively than host nestlings for a given level of short-term need, providing evidence that cowbird begging displays are more frequent and intense than non-parasitic nestlings during both feeding visits and parent-absent periods. In sum, the frequency of begging by cowbirds was only influenced by short-term need, whereas begging intensity during parent-absent events was only influenced by the host against which cowbirds competed. This study demonstrates that host size and short-term need had differing influences on the frequency and intensity of parent-absent begging in cowbirds, although both factors are likely important in limiting the evolution of parent-absent begging in cowbirds. Because it appears to provide no immediate benefits yet may decrease fitness, parent-absent begging should be included in future theoretical models investigating the evolution of begging displays in nestling birds.