Goose Migration and Egg Nutrient Allocation, Colville River Delta, Alaska, 2011–2015

We attached light-archival geolocators to female geese in order to determine general wintering areas and measure the interval between their arrival in the Arctic and the onset of incubation. Between 2011 and 2013 we attached geolocators to Snow Geese, Brant, and White-fronted Geese, mainly by capturing females on their nests during late incubation. We used MK-18H (British Antarctic Survey, Cambridge, UK) and C65K (Migrate Technology, Cambridge, UK) geolocators that recorded light intensity at 5-minute intervals. Geolocators were secured to uniquely coded, plastic tarsus bands that we attached to geese. To recover the geolocators, we used spotting scopes to locate marked females on their nests and recaptured them with bow traps 1–3 years after initial capture (2012–2014). We also recovered some geolocators during late summer brood drives and from hunters.

Each year 2011–2013, we collected 7–10 adult females of each goose species prior to nest initiation. We removed food items from esophagai, and identified the plant species and plant parts consumed (aboveground vs belowground). Based on dried mass (+0.01 g), we calculated aggregate proportion of mass of each food item to summarize the pre-laying diet for each species. Upon collection of geese we removed 1–2 g of tissue from several areas of abdominal fat and pectoralis to serve as source material for isotopic analysis of endogenous lipid and protein reserves, respectively. Tissue samples were frozen within several hours of collection.

We collected plant species that were consumed by geese in the areas where we had collected birds, or in nearby habitats that were similar. We collected 8–10 samples (~0.5–1 g/sample) of most forage plant species in each year from 2011–2013. However, some species were not collected until 2014. All samples were collected in the last week of May or first week of June. Plant samples were kept frozen until they were analyzed in the laboratory.

We collected the apparent first laid (most stained) and last laid (least stained) egg from 15 nests of each goose species annually from 2011–2013, except in 2012 when we collected eggs from 10 White-fronted Goose nests. We mainly collected eggs from nests with completed clutches, based either on embryo development or from records of clutch size on previous visits. Most (84%) eggs were collected within the first 3 days of incubation and no eggs were collected from nests with embryos >5 days of age. After collection, we heated eggs in boiling water for 20 minutes to solidify contents. The eggs were then frozen until they were shipped to the laboratory. Prior to isotopic analysis, we removed the shell and separated the egg into the yolk and albumen.

We freeze-dried goose pectoralis, lipid, and egg tissues to constant mass. Plant samples were rinsed in deionized water and freeze dried. Dried samples were ground to a powder in a mortar and pestle. We extracted lipids from the powdered pectoralis and yolk using a solution of 2:1 chloroform:methanol and extracts from were evaporated under a fume hood to recover lipids for further isotope analysis. Egg tissues were separated into the lipid-free yolk, yolk lipids, and albumen. We analyzed samples for δ13C and δ15N using continuous flow isotope ratio mass spectrometry (CFIRMS). Only δ13C was measured for yolk lipids. For δ13C and δ15N analyses, ~1.0 mg of material was combusted online using a Eurovector 3000 (Milan, Italy–www.eurovector.it) elemental analyzer. The resulting CO2 and N2 gases were separated by gas chromatography (GC) and introduced into a Nu Horizon (Nu Instruments, Wrexham, UK–www.nu-ins.com) triple-collector isotope-ratio mass-spectrometer via an open split and compared to a pure CO2 or N2 reference gas. We expressed stable nitrogen (15N/14N) and carbon (13C/12C) isotope ratios in δ notation, as parts per thousand (‰) deviation from the primary standards, atmospheric AIR and Vienna Pee Dee Belemnite (VPDB). Using previously calibrated internal laboratory standards (powdered keratin [BWB II: δ13C = -20.0‰, δ15N = +14.1‰ and gelatin:[δ13C= -13.6‰, δ15N = +4.7‰), within-run precision for δ15N and δ13C measurements was ~ ± 0.15‰ (n = 5).