2. MATERIALS AND METHODS

We conducted our study on the Arctic Coastal Plain of Alaska at the Colville River Delta (70.44° N, 150.67° W). This site is ~5 km from the Beaufort Sea, and is a lowland ecosystem of lakes, polygonal ponds, graminoid-dominated wetlands, dune ridges, and upland tundra communities (Kessel & Cade, 1958; Walker, 1983). Our study period was 2011–2018, but not all data were collected in all years because we integrated new aspects to the study over time. We typically arrived at the site in early spring (~20 May) prior to the initiation of nests, and remained at the site through late July to monitor the fate of nests and measure chick growth. We conducted systematic searches for semipalmated sandpiper (2011–2018) and Lapland longspur (2015–2018) nests across a 2.6-km2 plot adjacent to our camp, and we traveled by boat within 15 km of our camp to monitor goose nests at nearby nesting areas. To standardize search efforts across known-area plots for geese, from 2015–2018 we counted the number of brant and snow goose nests at 61 randomly selected circular plots (15-m or 25-m radius, depending on nest density) within these nesting areas.

2.1 Reproductive and environmental phenology

We maintained daily checklists to determine the first arrival date for each species at our study site. If a species was present at our site upon our arrival, we instead used the first-arrival information collected from a site 10 km from our camp (see Ward et al., 2016). Once nesting commenced, we determined the initiation date of each nest using standard techniques (see Supporting Information S1.1). Nests of longspurs were only monitored from 2015–2018, but we monitored nests of the other three species from 2011–2018.
We collected a suite of environmental variables at or near our study site. We recorded the percent cover of snow at 10 (2018) or 20 (2011–2017) 25-m radius plots that we monitored each year. We assessed snow cover upon arrival at the field site and every 2–6 days (typically 2 d) thereafter until snow cover averaged <5%. We averaged the daily values of snow cover across the plots and used the annual date when snow cover averaged 50% as an indicator of annual snowmelt. To characterize spring temperatures that preceded our arrival at the field site, we accessed weather observations at a site 10 km away (Colville Village, Alaska; National Oceanic and Atmospheric Administration, 2020), and determined values for accumulated thaw-degree days for each year. We also used an on-site weather station to record hourly temperature and wind speed, values which we summarized in running 3-day averages for use in chick-growth analyses.

2.2 Resource abundance

We monitored the seasonal abundance of the primary food resources available to juvenile birds at our site. We began monitoring both herbivorous and insectivorous food resources as early in the season as possible based on snow cover and ground thaw, and monitored these resources throughout the period of chick growth. For brant and snow geese, this involved estimating the biomass of the halophytic sedgeCarex subspathacea (hereafter subspathacea ; Gadallah & Jefferies, 1995; Hupp et al., 2017) by measuring the Normalized Difference Vegetation Index (NDVI). In brief, we estimatedsubspathacea biomass at a series of vegetation plots based on NDVI, and averaged these values across plots for each sample date across the season. We fit lognormal models to each year’s sample averages to describe the non-linear seasonal trends in biomass, and used daily predictions from these year-specific estimates to determine the biomass of subspathacea that was available to goslings when they were 15 days old (see Supporting Information S1).
Chicks of semipalmated sandpipers (Holmes & Pitelka, 1968) and longspurs (Custer et al., 1986) are insectivores. To monitor the availability of these food resources we collected surface-active arthropods every three days from ten modified Malaise traps following protocols of the Arctic Shorebird Demographic Network (Brown et al., 2014; Saalfeld et al., 2019; see Supporting Information S1). We collected arthropod samples at our site from 2015–2017.

2.3 Chick growth

We measured growth rates of brant and snow goose goslings (2012–2017) and semipalmated sandpipers (2015–2017) based on juveniles that were marked at hatch, and recaptured and weighed at later dates (see Supporting Information S1.1). From 2015–2017, we visited longspur nests and weighed chicks as soon as possible after the first egg in each nest hatched, and weighed chicks at approximately 3-day intervals thereafter. We could not uniquely mark longspur chicks as they were too small to retain tags, and instead modeled the growth of longspur chicks using brood-averaged masses.

2.4 Analysis

We compared similar environmental metrics across groups in our analyses, modified as necessary due to inherent differences in relevant life-history traits. To assess factors that influenced the timing of breeding (Table 2a), we determined the length of the pre-lay period (the number of days between when a species was first detected at our study site and that species’ mean date of nest initiation) and mean nest initiation date for each species each year. We monitored nests of semipalmated sandpipers and longspurs discovered on our core study plot, and brant and snow geese at nearby colonies. We calculated the average clutch size for each species for each year of study from these samples. To evaluate nesting effort relative to environmental variables from 2015–2018, we counted nests of shorebirds and longspurs in our core study plots, and goose nests in random plots.
TABLE 2 Predictor variables used to assess variation in (a) reproductive phenology and investment and (b) chick growth of black brant (BLBR), lesser snow goose (LSGO), semipalmated sandpiper (SESA), and Lapland longspur (LALO) breeding at the Colville River, Alaska, 2011–2018