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Past Research



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Photos: Autumn Iverson
Migration Ecology of Golden-crowned Sparrows

Project Lead: Autumn Iverson

Seventy percent of temperate-zone migrating songbirds have shown widespread decline in western North America over the last few decades. Along migration routes, songbirds face many threats such as navigational challenges, a changing predator assemblage, and shifting resources. They also face human related threats such as habitat loss and land-use changes. Due to the speed of these landscape changes, it is thought that songbirds are unlikely to evolve genetic adaptations that can keep pace.  Most research on migratory birds has occurred during the breeding season and migration is the least understood life history stage. Therefore, delineating migration paths as well as habitat quality and threats along those paths are critical research needs, especially on wide landscape scales. 


For seasonal passerine migrants, understanding how important the limiting factors experienced during migration are to the full life-cycle requires knowledge on different fronts, including how populations are connected between breeding and wintering ranges, how they utilize and choose stopovers during migration, and how climate changes in disparate home ranges may be affecting population numbers.


Golden-crowned Sparrows (Zonotrichia atricapilla; GCSP) breed in northern latitudes of North America such as Alaska, British Columbia and the Yukon, and winter along the west coast from British Columbia south to Baja California. The GCSP is considered a long-distance migrant with the entire population migrating seasonally between these locations. Although GCSP populations are not currently considered at risk, there is some indication that the species may be experiencing range shifts due to climate change.

This research will investigate migration in a temperate-zone migrating songbird, the Golden-crowned Sparrow, by estimating migratory connectivity across the wintering range using stable isotope analysis, determining drivers for stopover locations using new miniaturized GPS technology, and investigating the effects of climate on annual population trends using Christmas Bird Count data.


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Photos: Ryan Bourbour
Barn Owl Movement and Rodenticide Exposure in Industrial Agriculture

Project Lead: Ryan Bourbour

Barn Owls (Tyto alba) are incredible hunters: a single nest can consume thousands of pest rodents. For this reason, they are an effective component of an Integrative Pest Management program in agriculture, open spaces, and even urban areas. However, anticoagulant rodenticides are often applied to the landscape in conjunction with recruiting Barn Owls via artificial nest boxes to control rodent pests.


Ultimately, lethal and sub-lethal secondary poisoning from rodenticides can diminish the ecosystem services provided by Barn Owls, among other negative effects. We currently have little understanding of how often living Barn Owls in California are exposed to rodenticides and what effect this exposure has on hunting behavior and reproductive success. To gain a clearer picture of the relationship between Barn Owls and rodenticide applications, we will be using GPS-technology to track hunting and movement patterns across an industrial agricultural landscape where we will identify locations that Barn Owls may be capturing prey that have fed at rodenticide bait stations.


Our main objective is to map the hunting locations of Barn Owls across multiple crop and habitat types in relation to rodenticide bait stations and rodenticide exposure of growing chicks being provisioned at the nest. This information will allow farmers to plan rodenticide applications in the future to avoid placing baits in areas Barn Owls prefer to hunt while simultaneously targeting areas where Barn Owls are not providing effective pest control services. This will be the first study to describe hunting behavior around rodenticide bait stations and relating it to rodenticide exposure in both nestlings and adults.


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Migration Ecology of Sharp-shinned Hawks & Merlins
Project Lead:  Ryan Bourbour
Falcons and Accipiters use powered flight during migration and must hunt in order to fuel their journey. Both Sharp-shinned Hawks (Accipiter striatus) and Merlins (Falco columbarius) have a diet dominated by passerines, and are thought to track flocks of migrating songbirds during migration. Determining the diet of migrating raptors has been difficult in the past, however, by utilizing modern genetic techiniques we can uncover the specific diet of these two predators during their fall migration. We aim to develop a novel dietary study technique that not only has the potential to reveal ecological interactions within migratory flyways, but also opens up opportunities to safely and non-invasively study the diet of enigmatic raptor species around the world.
Photo: Bourbour et al. 2019
Josh Hull UC Davis
Photo: Chris Tyson
Behavioral Benefits of Mate Familiarity in Pelagic Seabirds

Project Lead: Chris Tyson

For many socially monogamous birds, reproductive success increases with mate familiarity, creating positive selection for long-term pair bonds. This pattern is especially common among pelagic seabirds, many of which display remarkable mate fidelity. Despite the prevalence of this mating system, little is known about the specific behaviors that benefit from mate familiarity. In general, however, it is largely thought that pair experience influences behaviors that rely on coordination during breeding, such as incubating and provisioning. In this project, we are investigating whether increased pair experience leads to improved coordination of breeding behaviors in several species of pelagic seabirds. To do so, we combine extensive datasets of mating histories with detailed observations of breeding behaviors. This approach allows us to compare the behaviors of newly established pairs to that of more experienced pairs. are currently focusing on three species: Leach’s storm-petrels (Oceanodroma leucorhoa), Cassin’s auklets (Ptychoramphus aleuticus), and Manx shearwaters (Puffinus puffinus).

Nesting Ecology and Foraging Behavior of the American Kestrel

Project Lead: Jessica Schlarbaum

The American Kestrel (Falco sparverius) is a widespread falcon species found all over North America and parts of South America. There is evidence that their population has been declining for some time with no definitive cause. While mass amounts of studies have been conducted on this species, few have been conducted on the west coast, and even fewer have looked at the relationship between habitat type and nesting success. This study hopes to give insight into which habitats are most suitable for nesting American Kestrels and possibly direct future nest box projects to install boxes in known successful habitats. Utilizing the availability of nesting pairs of American Kestrels in this study, individual foraging behavior will also be documented in order to assess the hypothesis that American Kestrels each employ a different hunting strategy and prey preference, as opposed to most studies which have analyzed diet and hunting in the species as a whole. 

Photo: Jessica Schlarbaum
Josh Hull UC Davis
Photo: Ryan Bourbour
Migrating Raptors and Mercury Trends

Project Lead: Ryan Bourbour

Mercury (Hg) from anthropogenic emissions can be detected in even the most remote ecosystems of the world. Once released into the environment, Hg becomes biologically available and enters food webs leaving top predators, including humans and raptors, vulnerable to toxic levels. Raptors occupy a variety of ecosystems and are increasingly being valued as sentinels for biomonitoring over large regions, especially in Europe. However, little is known about how Hg exposure differs between raptor species in western North America, how Hg exposure fluctuates in raptor populations over time, and how Hg levels vary for a single raptor species across the continent. To address these questions, we are analyzing breast feathers collected from raptor migration monitoring sites across North America. This study will highlight the role raptors play as indicators of environmental health and the utility of using raptor migration monitoring operations to assess environmental toxins for managing threats to human and wildlife health.

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Investigating the impact of rodent control on beneficial hawks and owls

Project Lead: Breanna Martinico

This project focuses on quantifying the frequency of secondary exposure to rodenticides in raptors that provide natural pest control on farms. In the agricultural landscape, the majority of a raptor’s diet consists of rodent pest species that reduce yields and damage infrastructure. Integrated pest management (IPM) strategies for rodent pests frequently incorporate natural pest control provided by raptors, however, rodenticides are often applied concurrently. Rodenticide exposure can cause lethal or sub­lethal secondary poisoning in raptors, ultimately lowering the pest control they provide and increasing farmers’ need for rodenticide use. Studies to date have focused primarily on spring and summer months for both understanding the role that raptors play in providing pest control and for monitoring rodenticide exposure—whereas both occur year­round. To gain a better understanding of how rodenticide exposure in raptors varies across seasons and between species in agriculture, I plan to investigate rodenticide exposure rates in hawks and owls across multiple seasons and varied crop types, and as a function of proximity to rodenticide application sites to answer two critical questions.

Photo: Ryan Bourbour

Reproductive and Movement Ecology of Northern Harriers

Project Lead:  Shannon Skalos

The Northern Harrier (Circus hudsonius) is a widespread raptor species that breeds and forages in marshes, associated grasslands and agricultural fields. Prey varies seasonally and geographically from rodents to passerines to waterfowl. In California, local breeding populations have experienced steady declines, and as a result, harriers are listed as a Species of Special Concern by the California Department of Fish and Wildlife. Suisun Marsh once had one of the largest breeding populations of harriers in the state, but the current breeding population is small. In collaboration with the USGS, we are studying the reproductive, foraging and movement ecology of harriers in Suisun Marsh using a variety of field and telemetry techniques to determine effects of prey and habitat selection on harrier population demographics and behavior. To better understand harrier ecology and inform their management, it is imperative that we investigate not only breeding ecology of harriers in Suisun, but also movement, habitat selection and interactions with prey species across the four major phases of their annual cycle (breeding, fall migration, wintering, spring migration), both in Suisun and across their range. As such, this research aims to address to following objectives: 1) determine how prey influences reproductive success and provisioning to nestlings, 2) examine nest site characteristics and habitat selection across different spatial scales, and 3) examine movement and site fidelity across different temporal scales and age-classes.


Additionally, because Suisun Marsh is an important breeding area for waterfowl, and harriers breed concurrently with waterfowl in upland habitats, we will study the potential impacts of harrier foraging on waterfowl reproductive success. We will also study reproductive success, foraging and movement ecology of Red-tailed Hawks (Buteo jamaicensis), Great Horned Owls (Bubo virginianus) and Common Ravens (Corvus corax), three species nesting in Suisun Marsh and identified as potential avian predators effecting waterfowl reproductive success. The results of this research will add to our understanding of avian predator behavior and population biology, as well as predator-prey interactions between avian predators and breeding waterfowl in California.

Photos: Shannon Skalos
Adaptation and Diversification of the Galápagos Hawk

Project Lead: Emily Abernathy

While recent technological advances have made studying the mechanisms behind speciation and adaptation more feasible, empirical studies in natural populations are lacking. The Galápagos hawk (Buteo galapagoensis) is thought to be undergoing active speciation and is therefore an ideal system to study adaptive evolution. The Galápagos hawk and its most recent ancestor show large variations in morphology despite diverging only 126,000 years ago. We are using next-generation sequencing to study the mechanisms behind the divergence of the Galápagos hawk by using restriction associated DNA sequencing (RADseq) to 1) resolve the phylogenetic relationship between the Galápagos hawk and its most recent ancestor and 2) locate regions of the genome associated with local adaptation and rapid morphological change. This study will advance our understanding of the roles of neutral and adaptive evolution while providing insight into the interplay of evolutionary forces that lead to the formation of new species.


Merlin Population Genetics and Phylogeography

Project Lead: Breanna Martinico

We are currently investigating the patterns of morphological and genetic differentiation across the North American range of Merlin (Falco columbarius) in relation to contemporary anthropogenic factors and historic impacts (Pleistocene glaciation). In the mid-20th century, many raptor species experienced population declines due to the widespread use of the pesticide DDT, an organochlorine compound with long-lived metabolites. Currently, little is known about the genetic impacts on many species that faced population bottlenecks in the DDT-era but received little conservation attention, such as Merlin in North America. We are investigating the genetic impacts of the DDT-era population bottleneck and the impacts on population structure associated with historic Pleistocene glaciation in North American Merlins.


Additionally, the three North American Merlin subspecies (F. c. suckleyi, columbarius, and richardsonii) are described to have different plumage characteristics and distinct ranges. F. c. suckleyi is found in the Pacific northwest and southeastern Alaska and has the darkest plumage of the three subspecies. F. c. columbarius is paler than F. c. suckleyi and found from Newfoundland to Alaska.  F. c. richardsonii is the palest of the three subspecies and breeding in central Canada and the north central portion of the United States. However, the true environmental and/or genetic mechanism(s) responsible for maintenance of plumage variation in North American Merlin are yet to be discovered.

Josh Hull UC Davis
Photos: Ryan Bourbour
Phenotypic Variation in Buteos

Project Lead: Megan Mayo

The Harlan’s hawk (Buteo jamaicensis harlani) is a raptor that exhibits exceptionally high levels of phenotypic variation in tail plumage color and pattern, even when compared to a conspecific subspecies, the western red-tailed hawk, Buteo jamaicensis calurus. Phenotypic divergence between Harlan’s hawks, which breed mostly in Alaska, and western red-tailed hawks, which breed in the western US and Canada, is not due to reproductive isolation; a recent study documented ongoing gene flow between the two subspecies, highlighting the question of what mechanism is responsible for maintenance of the polymorphism in Harlan’s hawks. How is variation maintained in one subspecies but not another? Little is known about the mechanism responsible for continued existence of such variation, though recent research has indicated that the melanocortin 1 receptor, a gene that has been shown to control polymorphism in many mammal and bird species, does not control polymorphism in Harlan’s hawks.  


Megan Mayo UC Davis
Population Genetics of Rough-legged Hawks

Project Lead: Megan Mayo

The Rough-legged Hawk (Buteo lagopus) is a widespread, panboreal raptor found in North America, Europe, and Asia. Despite its widespread distribution, we know very little about population structure or connectivity and gene flow between continents. We are using NextGen sequencing to examine DNA from a combination of wintering and breeding individuals to describe phylogenetic and population level differences among rough-legged hawk  populations. Through multiple collaborations, we have collected samples from N. America, Europe, and Asia.


 UC Davis Josh Hull Megan Mayo
Megan Mayo UC Davis Brian Smithers
Bristlecone Pines and Limber Pines

Project Lead: Megan Mayo and Brian Smithers

Great Basin bristlecone pines (Pinus longaeva) are the oldest non-clonal species on earth. Despite the fact that many of these trees have survived thousands of years, we still know very little about population structure, gene flow, and local adaptation of these ancient pines. With global climate change come threats such as blister rust, increased fire, drought, and novel interspecific competition, all of which threaten the continued survival of these living fossils. Perhaps the most interesting, unpredictable, and rapidly changing of these factors is invasion of limber pines (Pinus flexilis) into historically exclusive P. longaeva habitat. P. flexilis is a widespread species generally found lower in elevation on granitic soils in the White Mountains, but within the past fifty years, it has begun to establish upslope on dolomitic soil, with new recruits within the past 20 years establishing within P. longaeva habitat on Big Horn Sheep Peak in the White Mountains.  Limber pines may be better adapted to take advantage of a rapidly changing climate, meaning that much bristlecone pine treeline could be dominated by limber pine in the near future.  It is unclear whether this recent colonization event is the result of global climate change or how P. longaeva will be affected.


Pisaster Seastars 

Project Lead: Megan Mayo and Grace Ha

Pisaster ochraceus is a keystone predator of the rocky intertidal. Ranging in color from brilliant purple to outrageous orange, they are as charismatic as they are voracious. While the variation in coloration is obvious, the genes responsible have not yet been identified. Using NextGen sequencing and captive feeding experiments, we are investigating the underlying gene or genes responsible for phenotypic polymorphism in Pisaster ochraceus. 


Megan Mayo UC Davis Grace Ha
Brian Prochazka UC Davis
Greater Sage Grouse

Project Lead: Brian Prochazka

The greater sage grouse (Centrocercus urophasianus) is a lekking species found in the Northwestern United States. For this species, we are 1) quantifying resource selection via GPS location data and dynamic brownian bridge movement models, 2) estimating risk associated with various land cover types via shared frailty models, 3) estimating the effects of wildfire over the last 30 years on population growth rates for 800+ leks located throughout the Great Basin, 4) estimating abundance via state-space models, and 5) quantifying winter resource selection; specifically, nutrient vs anti-nutrient acquisition/avoidance strategies for a single population of sage-grouse located in northeastern Nevada.

Stable Isotopes and Ridgway's Rail

Project Lead: Angela Merritt

Little is known regarding the diet of the California Ridgway’s Rail. A 1941 stomach-content analysis completed by a San Francisco Bay naturalist (Moffit) indicated an opportunistic rail diet spanning several trophic levels. No predominant prey items, or a lack of, have been confirmed since. Based on the Moffit work and current location data, most researchers believe that Ridgway's rails forage exclusively in the brackish tidal marsh. A comparison of δ13C, δ15N, δ34S isotope values from clapper rail feathers and an updated gut content analysis is underway to confirm this. The final product will detail the foraging preference for three geographically distinct rail populations, documenting any rails with freshwater isotope signatures. Using museum rail specimens dating back to 1880 will broaden the spatial and temporal scale and ultimately capture diet adaptations in the face of a rising sea level.



Josh Hull UC Davis
Great Gray Owls in Yosemite

Project Lead: Joseph Medley 

A geographically-isolated population of Great Gray Owls (Strix nebulosa)(GGOW), consisting of approximately 150-200 individuals, is distributed in the central Sierra Nevada, California. Our recent research has documented that this population is genetically unique from other populations in North America. These population genetic results, in conjunction with management concerns regarding impacts of timber harvest, grazing, recreation and water management on GGOWs and their habitat, has generated significant management interest by the Forest Service and other agencies in the development of monitoring methods to track GGOW population status and trends. To date, we have collaborated with the Forest Service to develop and quantitatively-evaluate broadcast and meadow-search methods to document GGOW occupancy based on the presence and detection of owls at survey sites.  However, we also commonly collect molted GGOW feathers when conducting our meadow searches as part of the occupancy surveys. These molted feathers provide a potential source of DNA that can be used to identify individual GGOWs. The objective of our proposed project is to evaluate if the molted feathers can be used to identify individual GGOWs. If individuals can indeed be identified from these molted feather samples, then it becomes possible to use mark-recapture modeling methods to estimate survival and population trend (lambda), and to garner data on dispersal. Thus, it may be possible to develop passive and cost-efficient monitoring designs based on collecting molted feathers at occupied meadow sites, as compared to logistically-difficult and expensive demographic methods (i.e., does not require actual capture and annual resighting of individuals owls). Our goal is to develop scientifically-defensible, effective and cost-efficient monitoring methods and designs that can provide monitoring data on GGOW occupancy, survival, reproduction and population trends to meet the increasing information demands being placed on land managers and regulatory agencies regarding the status of this unique population.

Bay Area Raptor Nesting Survey

In collaboration with the Golden Gate Raptor Observatory, we are using data collected during a long-term citizen science project to describe the nest site preferences of Red-shouldered and Red-tailed Hawks nesting in Sonoma and Napa counties.


Josh Hull UC Davis
Raptor trend analysis

In collaboration with the Golden Gate Raptor Observatory and the US Forest Service, we are analyzing hawk count data collected in the Marin Headlands to identify population trends in 19 species of migratory raptors.

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