I am conducting a study on methylmercury (MeHg) contamination of nestling Red-winged blackbirds ( Agelaius phoeniceus ) and Great-tailed grackles ( Quiscalus mexicanus ). I collected blood samples from nestlings to measure MeHg concentration in parts per billion, and I attempted to collect blood samples when the nestlings were 5-6 days old, 8-9 days old, and 11-12 days old. However, it was common to only get one sample per individual due to predation, nest failure, or finding the nest right before fledge.
For blackbirds, I collected 424 blood samples from 243 nestlings in 84 nests from 14 different ponds. For grackles, I collected 97 blood samples from 40 nestlings in 17 nests from 2 different ponds.
The structure of my data is:
- 1-3 blood samples per individual nestling
- Individual nestlings in a nest
- Nests in a pond
I am building models to predict blood MeHg concentration, and an important thing to understand is that aquatic ecosystems generally have higher concentrations of MeHg than terrestrial ecosystems. Aquatic ecosystems have bacteria that can convert nontoxic, inorganic Hg into the toxic MeHg form. These bacteria are either absent or rare in terrestrial ecosystems, so aquatic organisms or consumers of aquatic organisms generally have higher MeHg concentrations.
For blackbirds and grackles, the source of their MeHg contamination would come through the consumption of odonates (damselflies and dragonflies). Odonates have aquatic larval life stages (where they would be exposed to MeHg contamination), and terrestrial adult life stages (where they can transport MeHg to terrestrial predators). Therefore, I collected data on the number of odonates coming out of the ponds on a weekly basis, and I have estimates on the amount of MeHg odonates were transporting to the adjacent terrestrial ecosystem.
Now some studies have shown that odonates can make up 60-90% of a blackbirds diet, while other studies have shown that terrestrial prey like caterpillars can be the dominant prey item. I believe this might be explained by differences in prey availability between terrestrial and aquatic ecosystems, suggesting that blackbirds are generalists. I unfortunately did not collect data on terrestrial prey abundance during my field season. Instead, I used ArcPro from ESRI to create polygons of different habitat types (Pond, Grass, Tree, and Anthropogenic structures like roads and buildings) and then created 100m buffers around each nest. I believe that larger areas of grass and trees around a nest might provide more opportunity for the adult birds to forage for terrestrial prey for their nestlings.
So the models I’m looking to build will include variables for odonates, habitat type, and an interaction between odonates and habitat type.
The models I have built so far have been 4-level hierarchical models: blood samples nested within individual nested within nest nested within pond. I used pond as the highest level because I was assuming that nests within the same pond were experiencing similar conditions. However, now that I am explicitly including variables that describe the habitat and MeHg conditions around each nest, do I need to include a level for pond effects? Can I bump the levels down to a 3-level hierarchical model?
Thank you for your time, and I would appreciate to hear your opinion on the matter.