WHY WE CARE:
1) Increasing woody shrub cover will have impacts that cascade across trophic levels...
Different tundra vegetation communities provide contrasting quality and abundance in both food and shelter. Songbirds forage in vegetation for arthropods, berries, catkins, and seeds. Some make their nests in tundra vegetation, and most hide from predators and storms in shrubby areas.
Shifts in vegetation dominance will therefore lead to altered food availability, which will likely favor some songbird species while hindering others. This will lead to changes in the composition and structure of migratory songbirds we see today on the tundra.
2) Earlier spring snowmelt could lead to ‘trophic mismatches’ by altering the timing and abundance of food resource availability for migratory songbirds during their short breeding season...
Migratory songbirds arrive on the North Slope of Alaska between May 17 and 30 every year. As the snow melts, arthropods, and over-wintered berries and seeds become available for them to eat. As the tundra greens up, an abundance of high quality food resources become available to both adult birds and their young.
If snow melts our earlier, the timing of plant and arthropod based food availability and abundance will likely shift earlier as well. We are exploring the stress hormone response and the impact on reproductive success of migratory songbirds to these shifts in the timing of snowmelt and food availability.
Beginning early May until the beginning of August, for five consecutive years, we are making a suite of weekly measurements associated with the vegetation, climate, snowmelt, arthropods and migratory songbirds.
Songbird community measurements
Bird Capture and Banding:
Birds were captured using mist nests or seed baited potter traps. We collect morphometrics of wing chord, tarsus, beak length (nares and tip of beak), skull (back of skull to tip of beak), and finally mass . Fat deposits from the furcular and abdominal regions were scored on a ordinal scale from 0-5 with 0 indicating a bird with no fat while a 5 indicated large fat stores. Muscle profile was recorded on a scale from 0-3 with 0 indicating an emaciated bird with extremely small muscle while a 3 indicated large muscle that was bulging past the sternum. These measurements gave us an idea about the size of the bird and the energy stores.
Prior to release each bird was banded with an aluminum band from the U.S. Geological Survey and then given an unique set of color bands so that the individual could later be identified in the field with binoculars.
Over the past five years, we monitored nesting success in two species of arctic breeding songbirds. White-crowned sparrows (Zonotrichia leucophrys) nest in shrub dominated habitats while Lapland longspurs (Calcarius lapponicus) nest on the open tundra.
Nests were located by following birds back to the nest as they were in the process of building the nest, laying eggs, incubating, or feeding young. Nests were also found by flushing females off of the nest.
Once the nest had been located, we would GPS the location and mark the nest using flagging tape and a tongue depressor. Nests were visited repeatedly over the duration nestling phase to determine total number of eggs laid, number of eggs hatched, and the number of nestlings that fledged. We made notes on nestling mortality due natural causes and predation. These data were then used to determine reproductive success of each species.
Nestling Growth Rates:
On a subset of the nests that were found, we collected daily measurements of mass, tarsus length, and skull to generate growth curves for each species.
From these measurements, were able to compare growth rates between Lapland longspurs and White-crowned sparrows. In addition, we were able to make comparisons across years to understand how growth rates were affected by weather and bug abundance.
On the same set of nests used to determine nestling growth rates, parental behavior was monitored using GoPro cameras. Cameras were deployed at each during day two and six of the nestling period. We were able to quantify duration of brooding, feeding rates, nest maintenance, and number of nestlings fed during each feeding event.
Nest Site Characterization:
Nestlings typically depart the nest towards the end of June into the beginning of July. After the nests were empty, we characterized the nest site to understand which plant species dominated the nest site. White-crowned sparrows prefer to nest in shrubby areas that are dominated by willows and birch. Lapland longpurs prefer to nest in sedges such as cotton grass.
We were able to collect data from over a hundred nests of each species. We were then able to use a model that predicts how the landscape across the North Slope will change over the course of the next fifty years in response to climate change. From this model were were able to predict how the range for each species will either expand or contract in response to a changing landscape. A full description can found in our Global Change Biology article.
Post-breeding spatial use of the landscape and fall departure:
During the 2014 breeding season, telemetry tags were deployed on parents with known nests in the vicinity of Toolik Lake Research Station. At the termination of the breeding season birds are no longer tied to their nest since the young have fledged. Juveniles reach complete independence approximately two weeks after they leave the nest. Birds were tracked using both hand held receivers in conjunction with two automated listening stations. Both the male and female were monitored daily to understand spatial use of the landscape in relation to time elapsed from fledge, response to fall storms, and success or failure of the nest.
The only predictable thing about the arctic is that it is unpredictably. Birds arrive on their breeding territories in May when conditions are often less than hospitable and must endure a wide range of perturbations that include spring snow storms, predation, food shortages, social disputes and etc. In order to cope with these various perturbations, these birds use various endocrine signaling cascades to regulate changes in physiology and behavior to help them survive. Our laboratory focuses on the activity of the hypothalamic-pituitary-adrenal axis as measured through the final synthesis product - corticosterone. Corticosterone has been termed the stress hormone and has many positive influences on physiology on the acute time scale.
In order to assess the activity of this axis, we expose the birds to a standardized restraint handling protocol. Serial blood samples are collected to generate a stress profile so that we can investigate the total amount of hormone released over time and the rate at which the hormone increases in the blood.
Many years of research have shown that the stress response is highly plastic and is modulated over the course of the annual cycle. The stress response is highest as bird establish territories and then declines birds become parental.
For the purposes of the Team Bird project we have focused on how the stress response fluctuates during the arrival period across years as birds are exposed to different snow cover and temperatures.