CONCLUSION

This study established the first reproductive performance-based reference intervals for plasma Ca, P, and Ca:P ratio in captive female C. undulata and demonstrated significant associations between mineral profiles and reproductive outcomes. Outside the breeding season, females exhibited relatively stable plasma mineral concentrations, whereas breeding season birds showed marked increases in circulating Ca and P. Reproductive performance-based reference intervals were established for good layers at 10.8–27.8 mg/dL for Ca, 2.6–6.58 mg/dL for P, and 2.48–5.91 for the Ca:P ratio. Females producing soft-shelled eggs and those with retained eggs exhibited significantly higher plasma Ca concentrations than good layers, while plasma P concentrations were significantly elevated in soft-shelled egg producers. A strong positive correlation was observed between plasma Ca and P concentrations (r = 0.690, p < 0.001), further supporting the coordinated regulation of mineral homeostasis during reproduction.

Importantly, the findings indicate that eggshell abnormalities and egg retention are more likely associated with impaired mineral utilization and uterine deposition efficiency than with absolute mineral deficiency. The absence of significant differences in the Ca:P ratio among reproductive groups suggests that systemic endocrine regulation of mineral balance remains largely intact despite alterations in circulating mineral concentrations. These results provide new mechanistic insights into reproductive physiology and mineral metabolism in a conservation-dependent avian species.

From a practical perspective, routine monitoring of plasma Ca and P concentrations may serve as a valuable tool for reproductive assessment and early identification of females at risk of eggshell defects or egg retention. Integration of biochemical monitoring into captive breeding programs could facilitate evidence-based nutritional and management interventions to improve reproductive success and reduce egg losses.

A major strength of this study is the relatively large sample size, including 239 females and 136 breeding season birds, which enabled the establishment of reproductive performance-based reference intervals and the evaluation of mineral profiles across distinct reproductive categories. However, the study was limited by its cross-sectional design and the lack of measurements of ionized Ca, vitamin D metabolites, reproductive hormones, and uterine transporter activity, which may further clarify the mechanisms underlying eggshell abnormalities.

Future studies should incorporate longitudinal monitoring throughout the laying cycle, evaluate endocrine and molecular regulators of mineral transport, and investigate the relationships among nutrition, mineral metabolism, and reproductive performance under both captive and wild conditions. Such investigations would provide a more comprehensive understanding of mineral regulation and reproductive physiology in Houbara bustards.

Overall, this study demonstrates that plasma Ca and P concentrations are useful biomarkers of reproductive status in captive female Houbara bustards and provides a foundation for improving reproductive monitoring, nutritional management, and conservation breeding strategies for this vulnerable species.