a For SESA and LALO, age was
log10-transformed in all models to control for
heteroscedasticity.
b Females are the reference level; sexes unknown for
SESA and LALO.
The effect of the environmental covariates on chick growth varied by
species (Table 4). For brant and snow geese, age-specific body mass of
goslings increased with earlier nest timing (nest incubation date –
date of 50% snow cover; Table 4). For brant, lower resource abundance
(g m-2 subspathacea biomass at 15 days of age;
Table 4) was associated with increased gosling mass, and the body mass
of snow goose goslings declined as thaw-degree days increased (Table 4).
For semipalmated sandpipers, higher age-specific chick masses were
associated with warmer temperatures and nests that were initiated early
with respect to the date of 50% snow cover (Table 4). For longspurs,
all model-averaged parameters for environmental variables were small and
with confidence intervals that broadly overlapped zero (Table 4),
indicating that none of the environmental variables that we included in
our models meaningfully predicted the mass of chicks of this species.
Finally, wind speed (all species) and arthropod abundance (insectivores)
did not meaningfully influence chick mass as indicated by confidence
intervals on parameter estimates that overlapped zero (Table 4).
For environmental covariates that influenced chick growth (Table 4), we
estimated age-specific chick masses for each species using values of the
25th and 75th quartiles of these
predictors to assess the effect of optimal and sub-optimal (see Methods:
Analysis) values of these variables on chick mass. Across the ages and
sexes of chicks for which we modeled growth, the difference between
optimal and sub-optimal chick-growth conditions resulted in body mass
differences of 5.0–8.5% for brant, 9.1–14.3% for snow geese, and
24.4% for semipalmated sandpipers. For snow geese (Figs 4c, 4d) and
semipalmated sandpipers (Fig. 4e), the 95% confidence intervals on
estimates derived under optimal conditions did not overlap those derived
under sub-optimal conditions, but those for brant overlapped slightly
(Figs 4a, 4b).
FIGURE 4 Model-averaged predictions of chick growth of female
(a ) and male (b ) black brant, female (c ) and male
(d ) snow geese, semipalmated sandpipers (e ), and Lapland
longspurs (f ) at the Colville River, Alaska. Brant and snow goose
goslings measured from 2012–2017, semipalmated sandpiper and Lapland
longspur chicks from 2015–2017. Measured values are represented by
black circles, and black lines represent estimates under average
environmental conditions. Red and blue lines for brant, snow geese, and
semipalmated sandpipers represent body-mass estimates of chicks under
optimal (red lines) and sub-optimal (blue lines) environmental
conditions based on variation in biologically meaningful predictor
variables (see Table 3 for species-specific variables). For all colors,
heavy lines represent model-averaged estimates, and fine lines represent
the associated 95% confidence intervals