Immunity and Growth Plasticity of Asian Short-Toed Lark Nestlings in Response to Changes in Food Conditions: Can It Buffer the Challenge of Climate Change-Induced Trophic Mismatch?

**Background:** Climate change-induced trophic mismatch occurs when passerine nestlings hatch outside the peak period of insect larva abundance, leading to malnutrition and reduced survival. While adult birds may increase feeding efforts, it is unclear whether nestlings possess physiological mechanisms to cope with sub-optimal food conditions. Phenotypic plasticity—the ability to alter gene expression in response to environmental challenges—may allow nestlings to prioritize immune function over growth during temporary food shortages. This study tested the hypothesis that poor food conditions induce higher immune responses and lower growth rates in nestlings, and that such plasticity supports survival.

**Methods:** The study was conducted from 15 April to 15 June 2019 in the Hulun Lake National Nature Reserve, Inner Mongolia. A total of 135 Asian short-toed lark nestlings were monitored daily. Grasshopper nymph abundance was quantified using 10 parallel transects (2 m × 100 m) sampled daily; nymph biomass proportion (NBP) was calculated as the proportion of daily nymph biomass relative to total biomass across all survey days. Blood samples (100 µL) were collected from the first-hatched four-day-old nestling in each nest. RT-qPCR was used to measure IFN-γ, TNF-α, and IL-1β gene expression in blood cells, normalized to GAPDH. Plasma IGF-1 concentration was measured using enzyme immunoassay kits (Enzo Life Sciences). Nest temperature was recorded daily at 6:00 am. Linear mixed models (LMMs) assessed effects of NBP and nest temperature on cytokine expression and IGF-1 levels. Quadratic polynomial regression analyzed relationships between NBP and physiological outcomes. Body mass was measured daily from day 1 to day 7 post-hatching.

**Key Results:** NBP significantly influenced expression of all three cytokine genes (IFN-γ, TNF-α, IL-1β) and plasma IGF-1 levels, while nest temperature had no significant effect. Quadratic polynomial regression showed significant negative correlations between NBP and IFN-γ (R² = 0.87, p < 0.001), TNF-α (R² = 0.87, p < 0.001), and IL-1β (R² = 0.89, p < 0.001) gene expression. Plasma IGF-1 was positively correlated with NBP (R² = 0.73, p < 0.001). Body mass of 1–7-day-old nestlings increased with NBP across all ages (R² values from 0.954 to 0.987, all p < 0.001). Growth rates differed by NBP group: 2.11 (NBP ≤ 2%), 1.72 (2% < NBP ≤ 5%), and 1.49 (NBP > 5%). Plasma IGF-1 was negatively correlated with IFN-γ (R² = 0.86, p < 0.001), TNF-α (R² = 0.92, p < 0.001), and IL-1β (R² = 0.81, p < 0.001). Fledge rate increased with NBP (R² = 0.98, p < 0.001), but the lowest fledge rate remained above 60%.

**Clinical Implications:** This study provides evidence that nestlings possess physiological plasticity—upregulating immune cytokines and downregulating growth factors—that may buffer the short-term survival challenges of trophic mismatch. The finding that over 60% of nestlings fledged even under the poorest food conditions suggests this plasticity is functionally significant. However, the authors caution that this mechanism may be insufficient under severe or prolonged food shortages, such as those caused by extreme weather events linked to climate change. Reduced body mass at fledging may have species-specific long-term fitness consequences, including impacts on foraging ability, survival, and reproductive performance. These findings are relevant for understanding avian adaptation to climate change and for conservation planning in grassland ecosystems.