Stable Isotope Ecology

The use of stable isotopes in ecology has grown steadily over the past few decades and is now considered a standard tool for assessing trophic interactions, movement patterns, and historical trends in many species. Stable isotopes are a powerful tool that allow a glimpse into an animal's diet and behavior on temporal and spatial scales that would otherwise be difficult (if not impossible) to observe. 

I work primarily with stable isotope analysis of carbon (δ13C ), nitrogen (δ15N), and sulfur (δ34S) to understand trophic interactions, foraging area, and movement in seabirds both contemporarily with field sampling and historically with museum skins. I am also interested in incorporating hydrogen (δ2H) and oxygen (δ18O) into existing projects on molt migrations and natal origins. 

Check out some of my recent stable isotope projects below. 

Tawaki / Fiordland crested penguin breeding 

Tawaki occupy one of the most diverse ranges of marine habitats of any penguin species during their breeding season. Studies have shown that penguins from coastal colonies in south Westland and on Whenua Hou/Codfish Island travel long distances away from shore during foraging trips while those breeding in Harrison Cove at the head of Milford Sound almost never leave the fjord itself. In fact the Harrison Cove penguins exhibit δ15N values that are significantly higher than those breeding outside the fjord potentially indicating reliance on prey of a higher trophic level or evidence of a higher baseline nitrogen in the fjord system. But what about other fjords or colonies with closer access to the Tasman coast?

This project builds on my MS research to ask if colony location within the fjord system matters in terms of isotopic niche. To answer this, I have collected blood, prey, and environmental samples from multiple colonies in multiple fjords in Fiordland National Park to see if fjord and distance from the open ocean impact the trophic niche of tawaki. These are being analyzed for carbon (δ13C ), nitrogen (δ15N), and sulfur (δ34S) to build a picture of their foraging dynamics within the fjord system and beyond. 

Currently, fieldwork for this project is complete and samples are being processed for analysis. Once analyzed, I will construct mixing models to estimate the proportion of various resource pools in their diet throughout the breeding season at each of these sites. 

Tawaki / Fiordland crested penguin pre-molt 

Unlike blood and other dynamic tissues, feathers are metabolically inert once they have fully emerged. This means that the stable isotopes measured in feathers reflect the time period in which the feather was grown. All penguins undergo a single, catastrophic molt per year to exchange all of their feathers. During the molt they are bound to land and forced to fast until the molt is complete. To sustain them through this fasting period, tawaki undertake a massive (up to 7,ooo km round trip!) pre-molt dispersal where they must restore fat lost during the breeding season and build up enough reserves to sustain them through the molt. This hyperphagia followed by extended fasting means that the diet fueling feather growth (and the stable isotopes measured) reflect the pre-molt period at sea. Check out the marathon penguins paper by the Tawaki Project here to learn more about the pre-molt journey!

For this project, I have collected feathers from breeding tawaki across their range from 2017-2022 to analyze for carbon (δ13C ) and nitrogen (δ15N) stable isotopes. These data will be combined with historical samples from the Tawaki Project and museum specimens from the American Museum of Natural History, the Tūhura Otago Museum, and the Museum of New Zealand Te Papa Tongarewa (dating to 1887) to assess historical trends and the stability of resources during the pre-molt.

Collection and analysis of wild samples for this project is complete and I am analyzing feather samples from museums to provide a historical context for foraging behavior. 

Erect-crested & Eastern rockhopper penguin breeding 

Erect-crested penguins are by far the least studied species of penguin. Their remote breeding range consists solely of the Bounty and Antipodes Island groups in the New Zealand's sub-Antarctic region, making routine monitoring difficult. While Erect-crested penguins are the only penguins breeding on the Bounty Islands, they share the Antipodes Island group with Eastern rockhopper penguins. Unfortunately, both species also share a declining population trend on Antipodes Island. 

This study is part of a multi-modal investigation by the Tawaki Project into the foraging ecology of these two species on both island groups. By analyzing the stable isotopes of carbon (δ13C ) and nitrogen (δ15N) derived from blood and prey samples from both island groups, I will construct stable isotope mixing models and estimate diet and foraging areas for each species. Finally, I will be able to compare this between island groups and between these two sympatric species across their breeding season. 

Currently the first season of fieldwork for this project is complete with a second season planned for December 2023. 

Erect-crested & Eastern rockhopper penguin pre-molt 

Like all crested penguins, erect-crested penguins and Eastern rockhopper penguins undergo pre-molt dispersals away from the breeding grounds to replenish fat stores lost during chick rearing and to prepare for their molting fast. However the pre-molt journey of erect-crested penguins and Eastern rockhoppers (in New Zealand) has not been extensively studied. 

For this project, I am assessing the pre-molt foraging period by analyzing the stable isotopes of carbon (δ13C ) and nitrogen (δ15N) derived from feathers collected during the breeding season. As feathers are inert once fully emerged, the stable isotopes they contain reflect the diet consumed during the pre-molt period. By comparing stable isotopes between these two species, we will determine foraging areas, the trophic level of foraging, and any overlap between these two sympatric species. 

Currently the first season of fieldwork for this project is complete with a second season planned for December 2023 with hopes of adding museum specimens in the future. 

Double-crested cormorant & anhinga foraging niche 

The Florida Everglades are home to an astounding diversity of fish eating birds many of which occupy unique niches. Two closely related diving species are ubiquitous in the region: the double-crested cormorant and the anhinga. Although they are of similar size and are often found together, their different diving profiles and hunting strategies suggest reduced overlap in prey encounters and preferences.

For this project, I ask whether the diving behavior of these species leads to differences in their isotopic niche. I have collected blood, feathers, livers, stomach contents, and environmental samples from double-crested cormorants and anhingas foraging and roosting at a series of flooded sand quarries in south-west Florida. I am focusing on the stable isotopes of carbon (δ13C ), nitrogen (δ15N), and sulfur (δ34S) to assess foraging area (quarry lake vs. roadside canal), trophic position, and marine influence, respectively. Ultimately, I will combine these data with prey and environmental data to construct stable isotope mixing models to estimate the contribution of various resource pools to their diet and identify niche width of both species. 

Currently, field work is complete for this project and sample analysis is currently underway. 

Resource partitioning in Alaskan diving ducks

The Arctic tundra of Alaska is a globally important breeding area for waterfowl. Many of North America's diving duck species breed on tundra ponds and lakes in northern Alaska during the brief summer months. Migrating waterfowl are concentrated into higher densities on ice-free ponds as they chase the spring melt northward. This places pressure on local resources which in turn should require partitioning of resources between or within species.

For this project,  I collected blood, feathers, livers, stomach contents, and environmental samples from 8 species of diving duck (Barrow's and common goldeneye, bufflehead, greater and lesser scaup, long-tailed duck, common and red-breasted merganser) from a transect from the Alaska Range to Prudhoe Bay. I measured the stable isotopes of carbon (δ13C ), nitrogen (δ15N), and sulfur (δ34S) alongside prey and environmental samples to construct stable isotope mixing models and estimate niche width and overlap between species and within clades. 

This project is currently in the final stages of manuscript preparation. Check back here for results and any resulting publications. 

Additionally, I am working on a side project with collected flight feathers from these ducks to assess whether δ13C and δ2H can be used to determine molting grounds and infer breeding status of harvested waterfowl.