Adjustments to the timing of reproduction in response to snow cover and spring temperature varied by species (Fig. 2; Table 3). The four species arrived at the Colville River Delta earlier with advancing snow melt (Fig. 2a), but this relationship was only significant for semipalmated sandpipers (0.55 days earlier arrival for each day of advancing snowmelt; Table 3). Earlier snowmelt resulted in a reduced pre-lay period for brant (Fig. 2c; Table 3) and earlier nest initiation for all four species (Fig. 2e), although the magnitude of the effect varied among species and was greatest in brant and snow geese (Table 3). For assessments of reproductive investment, earlier snowmelt resulted in larger clutches for brant and snow geese (Fig. 3a; Table 3) and increased nesting efforts by semipalmated sandpipers (Fig. 3c; Table 3). In general, the effect of spring temperatures on reproductive phenology and investment were less pronounced than snow cover. Warmer spring temperatures resulted in earlier nest initiation for brant and snow geese (Fig. 2f; Table 3), and larger clutch sizes for brant (Fig. 3b; Table 3) and increased nesting efforts for semipalmated sandpipers (Fig. 3d; Table 3). Spring temperature did not influence any of the reproductive metrics for longspurs (Fig. 2 and 3, Table 3).
TABLE 3 Response of black brant (BLBR), lesser snow geese (LSGO), semipalmated sandpipers (SESA), and Lapland longspurs (LALO) to snow cover (a; date of 50% snow cover each year) and temperature (b; accumulated thaw-degree days from 1 January–10 June each year), 2011–2018, Colville River, Alaska. Values represent statistically significant slope parameters (±95% confidence intervals) from linear least-squares regression models with date of site arrival, duration of pre-lay period, date of nest initiation, clutch size, and number of nests as response variables. There was no support for species-specific differences in site arrival or pre-lay duration as a function of temperature, but all other assessments had at least one significant species-specific response. Significance levels represented by * (P ≤ 0.05), ** (P ≤ 0.01), and *** (P ≤ 0.001); n.s. indicates P > 0.05