Brian Woodbridge

Mapping sources, sinks, and connectivity using a simulation model of northern spotted owls

Source-sink dynamics are an emergent property of complex species–landscape interactions. A better understanding of how human activities affect source-sink dynamics has the potential to inform and improve the management of species of conservation concern. Here we use a study of the northern spotted owl (Strix occidentalis caurina) to introduce new methods for quantifying source-sink dynamics that simultaneously describe the population-wide consequences of changes to landscape connectivity. Our spotted owl model is mechanistic, spatially-explicit, individual-based, and incorporates competition with barred owls (Strix varia). Our observations of spotted owl source-sink dynamics could not have been inferred solely from habitat quality, and were sensitive to landscape connectivity and the spatial sampling schemes employed by the model. We conclude that a clear understanding of source-sink dynamics can best be obtained from sampling simultaneously at multiple spatial scales. Our methodology is general, can be readily adapted to other systems, and will work with population models ranging from simple and low-parameter to complex and data-intensive.

Migration Patterns, Use of Stopover Areas, and Austral Summer Movements of Swainson's Hawks

Abstract. From 1995 to 1998, we tracked movements of adult Swainsons Hawks (Buteo swainsoni), using satellite telemetry to characterize migration, important stopover areas, and movements in the austral summer. We tagged 46 hawks from July to September on their nesting grounds in seven U.S. states and two Canadian provinces. Swainsons Hawks followed three basic routes south on a broad front, converged along the east coast of central Mexico, and followed a concentrated corridor to a communal area in central Argentina for the austral summer. North of 20° N, southward and northward tracks differed little for individuals from east of the continental divide but differed greatly (up to 1700 km) for individuals from west of the continental divide. Hawks left the breeding grounds mid-August to mid-October; departure dates did not differ by location, year, or sex. Southbound migration lasted 42 to 98 days, northbound migration 51 to 82 days. Southbound, 36% of the Swainsons Hawks departed the nesting grounds nearly 3 weeks earlier than the other radio-marked hawks and made stopovers 9.0–26.0 days long in seven separate areas, mainly in the southern Great Plains, southern Arizona and New Mexico, and north-central Mexico. The birds stayed in their nonbreeding range for 76 to 128 days. All used a core area in central Argentina within 23% of the 738 800-km2 austral summer range, where they frequently moved long distances (up to 1600 km). Conservation of Swainsons Hawks must be an international effort that considers habitats used during nesting and non-nesting seasons, including migration stopovers.

Correlates of survival in Swainson's hawks breeding in northern California†

ABSTRACT We used a 30-year study of breeding Swainsons hawks (Buteo swainsoni) in northern California to examine correlates of adult apparent survival using multistate models in Program MARK. Specifically, we examined age-related patterns in adult apparent survival and how adult survival was correlated with average annual nest productivity, annual reproductive output, western juniper (Juniperus occidentalis) density around nest sites, distance to agriculture, and amount of agriculture within a territory. Annual estimates of adult survival varied from 0.85 to 0.9 (SE = 0.02). There were no indications of senescence or other patterns of age-related changes in adult apparent survival. Adult survival was inversely correlated with average reproductive output, with individuals producing >2 offspring having decreased survival, reflecting a possible trade-off between reproduction and survival. Conversely, reproduction in any year was positively correlated with survival, providing evidence of individual quality influencing adult survival. The distance an individual had to travel to agriculture, where most individuals forage, was negatively related to survival. Primary productivity within the average Swainsons hawk territory was positively correlated with adult survival. Our results indicate that individuals may have higher survival and fitness in areas with high proportions of irrigated agriculture that provides high prey densities, particularly alfalfa.

Plumage polymorphism and fitness in Swainson's hawks

We examine the maintenance of a plumage polymorphism, variation in plumages among the same age and sex class within a population, in a population of Swainson’s Hawks. We take advantage of 32 years of data to examine two prevalent hypotheses used to explain the persistence of morphs: apostatic selection and heterozygous advantage. We investigate differences in fitness among three morph classes of a melanistic trait in Swainson’s Hawks: light (7% of the local breeding population), intermediate (57%) and dark (36%). Specifically, we examined morph differences in adult apparent survival, breeding success, annual number of fledglings produced, probability of offspring recruitment into the breeding population and lifetime reproductive success (LRS). If apostatic selection were a factor in maintaining morphs, we would expect that individuals with the least frequent morph would perform best in one or more of these fitness categories. Alternatively, if heterozygous advantage played a role in the maintenance of this polymorphism, we would expect heterozygotes (i.e. intermediate morphs) to have one or more increased rates in these categories. We found no difference in adult apparent survival between morph classes. Similarly, there were no differences in breeding success, nest productivity, LRS or probability of recruitment of offspring between parental morph. We conclude that neither apostatic selection nor heterozygous advantage appear to play a role in maintaining morphs in this population.

Temporal Morph Invariance of Swainson's Hawks

Persistent genetic polymorphisms (e.g., different plumage morphs irrespective of age and sex classes) have long fascinated biologists, are found in approximately 3.5% of avian populations (Roulin 2004), and have prompted many hypotheses about how plumage morphs arise and are maintained (Lank 2002). In birds, many hypotheses have been proposed to explain intrapopulation differences of plumages, such as the handicap principle (Zahavi 1975) and different mating strategies (Lank et al. 1995). Raptors (Falconiformes and Strigiformes) in particular have a high prevalence of persistent genetic polymorphisms occurring in approximately 30% of species (Fowlie and Krüger 2003). In raptors, this prevalence of polymorphism has been related to prey type (Paulson 1973), wing load and population size (Fowlie and Krüger 2003), and heterozygous advantage (Krüger et al. 2001). For buteonine hawks, it has often been assumed that plumage morph is invariant over time (i.e., plumage pattern does not change as an individual ages and molts), and this has been used to identify individuals across years in some species (e.g., Krüger and Lindström 2001). In fact, similar polymorphisms have been associated with variations in the melanocortin-1 receptor (MC1R) gene across a wide range of avian taxa (reviewed in Mundy 2005). These results indicate that morph in many avian species may be invariant over time. For example, in Ruffs (Philomachus pugnax) plumage polymorphisms remain unchanged over the course of an adult male’s life (Lank et al. 1995). However, no studies have examined the persistence of morph within individuals over time in raptors. We examined the assumption that individual morph is invariant over time in a population of Swainson’s Hawks (Buteo

Inheritance Patterns of Plumage Morph in Swainson's Hawks

PATRONES DE HERENCIA DEL MORFOTIPO DE PLUMAJE EN BUTEO SWAINSONI La herencia de los polimorfismos basados en melanina es poco conocida a pesar de ser relativamente comun entre las rapaces. Examinamos la hipotesis de que el plumaje oscuro se hereda mediante un sistema de un locus con dos alelos en Buteo swainsoni. Presumimos que los individuos de plumaje oscuro y de plumaje claro eran homocigotos, mientras que los heterocigotos presentaban alguna forma intermedia de plumaje. Examinamos el morfotipo de 31 adultos de B. swainsoni y los comparamos con los fenotipos parentales. Todos los individuos se ajustaron a nuestra hipotesis de herencia mediante un locus con dos alelos. No existio una diferencia significativa entre lo que se esperaria de acuerdo a ese sistema y lo que nosotros observamos, lo que representa evidencia adicional de que el morfotipo en B. swainsoni esta asociado con un patron de herencia de un locus con dos alelos.

Spatial and Temporal Patterns in Golden Eagle Diets in the Western United States, with Implications for Conservation Planning

Abstract Detailed information on diets and predatory ecology of Golden Eagles (Aquila chrysaetos) is essential to prioritize prey species management and to develop landscape-specific conservation strategies, including mitigation of the effects of energy development across the western United States. We compiled published and unpublished data on Golden Eagle diets to (1) summarize available information on Golden Eagle diets in the western U.S., (2) compare diets among biogeographic provinces, and (3) discuss implications for conservation planning and future research. We analyzed 35 studies conducted during the breeding season at 45 locations from 1940–2015. Golden Eagle diet differed among western ecosystems. Lower dietary breadth was associated with desert and shrub-steppe ecosystems and higher breadth with mountain ranges and the Columbia Plateau. Correlations suggest that percentage of leporids in the diet is the factor driving overall diversity of prey and percentage of other prey groups in the diet of Golden Eagles. Leporids were the primary prey of breeding Golden Eagles in 78% of study areas, with sciurids reported as primary prey in 18% of study areas. During the nonbreeding season, Golden Eagles were most frequently recorded feeding on leporids and carrion. Golden Eagles can be described as both generalist and opportunistic predators; they can feed on a wide range of prey species but most frequently feed on abundant medium-sized prey species in a given habitat. Spatial variations in Golden Eagle diet likely reflect regional differences in prey community, whereas temporal trends likely reflect responses to long-term change in prey populations. Evidence suggests dietary shifts from traditional (leporid) prey can have adverse effects on Golden Eagle reproductive rates. Land management practices that support or restore shrub-steppe ecosystem diversity should benefit Golden Eagles. More information is needed on nonbreeding-season diet to determine what food resources, such as carrion, are important for overwinter survival.

Golden Eagles in a Changing World

In recent years, Golden Eagles (Aquila chrysaetos) have emerged as a conservation concern in the United States, particularly in the West. Our nation’s effort to become more energy independent has involved a significant expansion in the development of domestic sources of both renewable and nonrenewable energy. The influence of energy development, particularly wind energy, taken together with other anthropogenic sources of mortality, including electrocution on power distribution lines, contaminants, collisions with vehicles, and illegal shooting, may be resulting in declining Golden Eagle populations (U.S. Fish and Wildlife Service [U.S.F.W.S.] 2016a). To achieve the objective of ‘‘stable or increasing breeding populations’’ promulgated under the Bald and Golden Eagle Protection Act (Eagle Act; 16 U.S.C. 668-668d) and appurtenant regulations (U.S.F.W.S. 2009, 2016b; hereafter Eagle Rule), the U.S.F.W.S. will require a greater understanding of Golden Eagle population dynamics, movement patterns, and genetic connectivity across populations, and conservation practices that reliably reduce or mitigate population limiting factors (U.S.F.W.S. 2016a, 2016b). In addition, state, federal, and tribal wildlife and land management agencies need this information to support conservation planning for Golden Eagles at local and regional scales. To support the dissemination of new information on the demography, nesting ecology, movement patterns, diet, and conservation of Golden Eagles, the Academy for the Environment, University of Nevada, Reno, and Western Golden Eagle Team, U.S. Fish and Wildlife Service, collaborated to host a symposium at the 2015 annual conference of the Raptor Research Foundation (RRF), in Sacramento, California. Invited speakers presented new information during three sessions that focused specifically on Golden Eagle demography and population trends, use of innovative research technologies and techniques, and conservation planning. Six of the presenters subsequently developed manuscripts published in this special issue. The California–Nevada Golden Eagle Working Group convened a meeting immediately prior to the RRF meeting in Sacramento, where they hosted a symposium that focused on Golden Eagle prey ecology and the need for using consistent terminology in raptor ecology. Two of the invited presenters subsequently developed manuscripts that also appear in this special issue; one of these discusses the use of terminology in describing reproduction, the other summarizes spatial and temporal patterns in eagle diets throughout the western U.S.A. Four additional manuscripts, not associated with these symposia, are also included in this special issue, since they address issues highly relevant to Golden Eagle conservation and management. These papers address several different topics, including

Patterns of Spatial Distribution of Golden Eagles Across North America: How Do They Fit into Existing Landscape-scale Mapping Systems?

Abstract Conserving wide-ranging animals requires knowledge about their year-round movements and resource use. Golden Eagles (Aquila chrysaetos) exhibit a wide range of movement patterns across North America. We combined tracking data from 571 Golden Eagles from multiple independent satellite-telemetry projects from North America to provide a comprehensive look at the magnitude and extent of these movements on a continental scale. We compared patterns of use relative to four alternative administrative and ecological mapping systems, namely Bird Conservation Regions (BCRs), U.S. administrative migratory bird flyways, Migratory Bird Joint Ventures, and Landscape Conservation Cooperatives. Our analyses suggested that eagles initially captured in eastern North America used space differently than those captured in western North America. Other groups of eagles that exhibited distinct patterns in space use included long-distance migrants from northern latitudes, and southwestern and Californian desert residents. There were also several groupings of eagles in the Intermountain West. Using this collaborative approach, we have identified large-scale movement patterns that may not have been possible with individual studies. These results will support landscape-scale conservation measures for Golden Eagles across North America.

Reproduction Versus Recruitment of Swainson's Hawks in the Butte Valley, California

Abstract. Measures of reproductive success have long been assumed to be reasonable surrogates of fitness. We examined reproductive success and recruitment at the individual and population levels in Swainsons Hawks breeding in northern California. At the population level, we investigated whether the number of individuals subsequently recruited could be predicted by the number of offspring produced in any given year, finding no evidence of differences in probability of recruitment by cohort or year. Instead, age of recruits was the best predictor of probability of their being recruited into the local breeding population. At the individual level, we used generalized linear models to examine the correlation between lifetime reproductive success of individual females and the number of their offspring recruited. For individuals, the number of fledglings produced was significantly correlated with both the number of those offspring and of grandchildren recruited into the breeding population. We also examined the relationship between the average annual reproduction of an individual and the number of offspring recruited, finding these significantly correlated. The relationship was not linear: individuals producing moderate numbers of fledglings, on average, yielded the highest numbers of recruits, reflecting a trade-off between adult survival and reproduction. These results provide evidence that monitoring of reproductive success of individuals or a population can give reasonable indices of future recruitment if the study is continued for a long period. But individuals with the highest average rate of reproduction may not be the most fit.