Goose Mass and Vegetation Data, Colville River Delta, Alaska, 2012-2017

In each year, we established eight sites that each consisted of a pair of 2.6 m-2 plots in C. subspathacea. One randomly selected plot at each site was exclosed by lightweight nylon fencing attached to a PVC frame that excluded geese from the plot. The other plot was a control that remained open to grazing by geese. We measured standing crop in control and exclosed plots at 7–9 day intervals from mid-June until early August, resulting in 9-10 biomass measurements at each plot within a year. In 2012 and 2013, biomass was estimated based on clipping of aboveground live leaves of C. subspathacea at randomly located 20 x 20 cm subplots in each plot. In 2014, a portable spectrometer that was used to compute the Normalized Difference Vegetation Index (NDVI) for vegetation in 10-12 randomly selected plots. Biomass was them estimated based on a previously established relationship between NDVI and C. subspathacea biomass. At the remaining 4-6 plots sampled on each date, biomass was estimated by clipping C. subspathacea at randomly located 20 x 20 cm subplots, as in previous years. Standing crop of C. subspathacea in grazed plots was subtracted from biomass in exclosed plots to estimate offtake at a site. Each line of the data set represents the estimate of offtake at a site on a given sample date.

For hatched goslings that had not yet departed the nest bowl and for those that had pipped through eggshells but were not fully emerged, we attached a uniquely numbered tag through the web on one foot. All goslings in the nest were marked unless some had not sufficiently emerged from the egg to allow access to their foot. Pipping precedes emergence from the egg by up to 24 hours. Thus, we knew hatch date to within 1 day for all marked birds. We searched through the main nesting islands every 1–2 days from the onset of hatch until its conclusion so that birds that hatched at different dates were marked. At a small number of nests, marked goslings were abandoned as parents departed with more mobile siblings. In those cases we moved abandoned young to a nest where goslings of similar mobility were present, and where parents remained. In each year, we recaptured web-tagged goslings over a 4–5 day period between 28 July and 2 August. Because they typically hatched earlier, snow goose goslings were 27–39 days of age when they were recaptured whereas brant goslings were 22–32 days old. We used a helicopter to search the CRD and adjacent areas for flocks of snow geese and brant. Upon locating a flock with adults and goslings, we used the helicopter and a ground crew to herd flightless birds into a capture pen that we had erected nearby. We typically captured from 100–400 birds at a time. Although we observed flocks of snow geese and brant in the same habitats and often near each other, they usually did not intermingle. Thus, we typically only captured one species during each capture event. Both species were captured <25 km from their main nesting colonies. Upon capturing a flock we examined each bird to determine its sex, based on cloacal inversion, and used an electronic balance to weigh (+5 g) each gosling. Balances were tested for accuracy with a 1-kg reference weight at the onset of capture events. We noted goslings marked with web tags. We attached metal U.S. Geological Survey (USGS) tarsus bands to all goslings and any unmarked adults. Brant and female snow goose goslings were also marked with plastic tarsus bands. We released flocks en masse. Contemporary capture and marking of geese were approved by the Alaska Science Center Animal Care and Use Committee (review codes 2010-12 and 2013-05) and conducted under federal migratory bird banding permit 20022 held by the Alaska Science Center of the USGS.