Grassland bird populations have experienced declines in recent decades that coincided with fragmentation and loss of prairie habitat. Evaluations of factors impacting reproductive success, such as food availability, are important for assessing habitat quality for these birds. Grasslands are highly variable environments and such variation affects the diversity and abundance of arthropods, which constitute the nestling diet for most grassland birds. Higher arthropod abundance might allow greater parental provisioning of nestlings and, consequently, improved nestling condition and survival. However, few studies have tested the effects of variable food availability on nestling condition in grassland birds. We examined Dickcissel (Spiza americana) nestling condition in relation to abundance of arthropod prey across 36 restored grassland sites that varied in management history (e.g., seed mixes, grazing regime) in the central Great Plains. We measured morphometrics and sampled plasma metabolites (triglycerides) from nestlings as indices of condition and sampled arthropods using sweep nets to obtain biomass estimates of prey, of which orthopterans constitute the primary food provided to Dickcissel nestlings.
Results/Conclusions
Despite much variation among fields in orthopteran (0.01 – 3.05 g) and total arthropod (0.09 – 3.24 g) biomass samples, this variation was unrelated to management history and no measure of nestling condition (mass-age residuals, mass-tarsus residuals, standard deviation of tarsus length, and plasma triglycerides) showed clear relationships with field-level variation in either biomass measure. Instead, brood size explained some variation in nestling condition with nestlings in larger broods generally having lower mass than those in smaller broods (β= -0.60, 95% CI: -0.85, -0.35 for mass-tarsus residuals). Arthropod biomass was not a strong predictor of nestling condition, perhaps because parents can compensate for food limitation in their nesting territories. Our results suggest that metrics of Dickcissel nestling condition known to affect post-fledging survival may not be affected by spatial variation in food availability, at least in some years. Our work is consistent with classic ideas regarding lack of food limitation to birds breeding in grasslands, contrasting with the more saturated avian communities in forest ecosystems.