3. RESULTS

Over the course of our study, we monitored 1,447 brant nests, 1,374 snow goose nests, 821 semipalmated sandpiper nests, and 142 longspur nests. We experienced highly variable environmental conditions during the study period. In spring, the date of 50% snow cover ranged over a nearly three-week period from 20 May (2016)–8 June (2013) and values for accumulated thaw-degree days 1 January–10 June ranged from 7.2 (2018)–56.1 (2015). Warmer springs had earlier dates of snowmelt (adjusted R 2 = 0.53, P = 0.02), but our study period also encompassed years with mixed patterns between the two variables (e.g., years with relatively early [2015] or late [2013] snowmelt as functions of spring temperature). We also recorded prolonged periods of high (e.g., average temperature 17.3°C from 13–16 July 2016) and near-freezing temperatures (e.g., average temperature 3.4°C from 7–13 July 2015) during periods of chick growth.

3.1 Variation in reproductive phenology and investment

Mean dates of arrival at the breeding site (F3,28 = 11.2, P < 0.001; Fig. 1a), duration of the pre-lay period (F3,24 = 13.7, P < 0.001; Fig. 1b), nest initiation (F3,24 = 16.9, P < 0.001; Fig. 1c), and clutch size (F3,24 = 17.0, P< 0.001; Fig. 1d) varied considerably among the four species (see Fig. 2 for statistically significant interspecific comparisons). Regardless of spring conditions, snow geese (15 May ± 4.0 days) and longspurs (14 May ± 2.4 days) arrived earliest each spring (Fig. 1a), followed approximately one week later by brant and semipalmated sandpipers (both 22 May ± 3.6 days and ± 4.2 days, respectively). Early arrival did not necessarily confer rapid nesting, however: the pre-lay period (Fig. 1b) was longest for longspurs (mean 27.8 ± 2.9 days), and shortest for brant (12.6 ± 4.7 days), followed by snow geese (13.8 ± 5.1 days) and semipalmated sandpipers (19.3 ± 3.2 days). Due to their early arrival and short pre-lay period, snow geese consistently initiated nests before the other three species (Fig. 1c). Mean nest initiation was 30 May (± 4.6 days) for snow geese, 4 June (± 5.2 days) for brant, 8 June for longspurs (± 9.3 days), and 10 June (± 5.4 days) for semipalmated sandpipers. Longspurs had the largest clutches (4.8 ± 1.1 eggs; Fig. 1d), followed by snow geese (4.0 ± 1.2 eggs), semipalmated sandpipers (3.8 ± 0.5 eggs), and brant (3.7 ± 1.0 eggs).
FIGURE 1 Variation in arrival date (a ), pre-lay interval (b ), date of nest initiation (c ), and clutch size (d ) and of black brant (BLBR), snow geese (LSGO), semipalmated sandpipers (SESA), and Lapland longspurs (LALO) at a site on the Colville River, Alaska, 2011–2018. Horizontal lines represent the median, diamonds the mean, black circles the actual annual values, boxes the 25th and 75th percentiles, and whiskers the range of values. All interspecific comparisons of these values were statistically significant (P < 0.05) based on post-hoc Tukey HSD comparisons, except for comparisons linked by horizontal bars at bottom of each plot (e.g., mean arrival dates of black brant and semipalmated sandpipers do not statistically differ; Fig. 2a). For (a ) and (c ), ordinal date 140 is 20 May