INTRODUCTION

Compared to temperate and tropical ecosystems, the reproductive period of most animals in the Arctic is compressed due to the brief availability of food and short period of benign weather (MacLean & Pitelka, 1971; Wingfield & Hunt, 2002). Nevertheless, the sheer abundance of resources at these sites supports the reproduction of a diversity of animal groups that employ a variety of life-history strategies to exploit conditions in the Arctic. For migratory birds, these traits include reproductive strategies along the capital-income spectrum of resource allocation (Drent & Daan, 1980; Klaassen et al., 2001), the altricial-precocial spectrum of chick development (Starck & Ricklefs, 1998), and variable investments in clutch size (Jetz et al., 2008; Winkler & Walters, 1983). Life history theory predicts an optimization of such traits within a species (Roff, 2002; Stearns, 1992), and comparison of these traits among co-occurring species over multiple breeding occasions provides insight into factors that promote successful reproduction across varying environmental conditions.
Such comparisons are especially relevant in the Arctic considering the rapid pace of ecosystem change due to climate effects (Berteaux et al., 2004; Hoffmann & Sgrò, 2011). The effects of climate change are disproportionately expressed at high-latitude regions (Arctic Climate Impact Assessment, 2004; IPCC, 2013), where the rate of warming is rapid (Bekryaev et al., 2010; Serreze & Barry, 2011), the onset of spring is advancing (Høye et al., 2007; Parmesan & Yohe, 2003), and the growing season is lengthening (Piao et al., 2007; Tucker et al., 2001). In addition to these steady trends, the Arctic is also experiencing an increased frequency of punctuated, extreme climatic events (Landrum & Holland, 2020). Taken together, these rapidly changing conditions present new challenges to organisms already inhabiting extreme Arctic environments (Berteaux et al., 2004; Gilg et al., 2012).
We studied the breeding ecology of migratory birds at an Arctic site in Alaska to assess the response of the avian community to climate-related environmental variation. We focused our research efforts on the four most common species at the site: two geese (black brant [Branta bernicla nigricans ] and lesser snow geese [Chen caerulescens caerulescens ]), one shorebird (semipalmated sandpiper [Calidris pusilla ]), and one passeriform landbird [Lapland longspur (Calcarius lapponicus ]). These species encompass a range of life-history traits relating to reproductive effort (Table 1). Black brant (hereafter brant) and lesser snow geese (hereafter snow geese) are large-bodied, herbivorous waterfowl. Both species deposit endogenous nutrients into eggs (Schmutz et al., 2006; Sharp et al., 2013), but snow geese acquire relatively more exogenous nutrients from Arctic plants when foraging conditions prior to nesting are favorable (Hupp et al., 2018). In contrast, semipalmated sandpipers and Lapland longspurs (hereafter longspurs) are small-bodied birds that rely on exogenously derived nutrients (insects and seeds) for egg production (Hobson & Jehl, 2010; Klaassen et al., 2001; Meijer & Drent, 1999). Brant (2–6 eggs; Lewis et al., 2020), snow geese (2–6 eggs; Hamann et al., 1986), and longspurs (2–8 eggs; Custer & Pitelka, 1977) also regulate their reproductive investment by producing variable numbers of eggs, but the clutch size of semipalmated sandpipers is invariant (4 eggs; MacLean, 1972; Sandercock, 1997). Finally, brant (Lewis et al., 2020), snow geese (Mowbray et al., 2020), and semipalmated sandpipers (Holmes & Pitelka, 1968) produce precocial chicks that exit the nest shortly after hatch and are self-feeding, but longspur chicks are altricial and derive all their food resources from the provisioning efforts of adult longspurs (Custer & Pitelka, 1977).
TABLE 1 Life-history variation among reproductive traits for four species of Arctic-breeding bird, black brant (BLBR), lesser snow goose (LSGO), semipalmated sandpiper (SESA), and Lapland longspur (LALO)