Plasma calcium, phosphorus, and calcium-to-phosphorus ratio as indicators of eggshell quality and reproductive performance in captive female Houbara bustards (Chlamydotis undulata).

Abstract

Background and aim: The Houbara bustard (Chlamydotis undulata) is a vulnerable avian species whose conservation relies heavily on successful captive breeding. Eggshell abnormalities and egg retention are major reproductive constraints that can reduce breeding efficiency. Calcium (Ca) and phosphorus (P) play critical roles in eggshell formation and reproductive physiology; however, their relationship with reproductive performance in Houbara bustards has not been investigated. This study aimed to establish the first reproductive performance-based reference intervals for plasma Ca and P in captive female Houbara bustards, evaluate the calcium-to-phosphorus (Ca:P) ratio, and determine their associations with reproductive outcomes and eggshell abnormalities.

Materials and methods: A cross-sectional study was conducted on 239 adult female Houbara bustards aged 2-5 years. Blood samples were collected from 136 females during the breeding season and 103 females outside the breeding season. During the breeding season, birds were categorized as good layers (n = 70), soft-shelled egg producers (n = 26), retained egg (stuck egg) cases (n = 19), and non-laying females (n = 21). Plasma Ca and P concentrations were measured using an automated biochemistry analyzer, and the Ca:P ratio was calculated. Data were analyzed using descriptive statistics, the Kruskal-Wallis test, Dwass-Steel-Critchlow-Fligner post hoc comparisons, and Spearman correlation analysis.

Results: Outside the breeding season, mean plasma Ca and P concentrations were 10.5 ± 0.69 mg/dL and 3.98 ± 0.94 mg/dL, respectively, with a mean Ca:P ratio of 2.78 ± 0.58. In good layers, reproductive season reference intervals were established as 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. Significant differences among reproductive groups were observed for Ca (χ² = 16.27, p < 0.001) and P (χ² = 11.12, p = 0.011), whereas the Ca:P ratio did not differ significantly (p = 0.657). Soft-shelled egg producers and retained egg cases exhibited significantly higher Ca concentrations than good layers, while P concentrations were significantly elevated in soft-shelled egg producers. Plasma Ca and P concentrations showed a strong positive correlation (r = 0.690, p < 0.001). Elevated Ca was also associated with soft-shelled eggs (r = 0.320, p = 0.001) and retained eggs (r = 0.240, p = 0.024).

Conclusion: This study establishes the first reference intervals for plasma Ca and P based on reproductive performance in captive female Houbara bustards. Eggshell defects and egg retention were linked to altered mineral mobilization and utilization rather than Ca deficiency. Routine biochemical monitoring of Ca and P may serve as valuable biomarkers for reproductive assessment and improve breeding success in conservation programs for this vulnerable species.