Bird eggs were used before several decades as a model for embryonic development [1], and disease infection [2], because of the importance several attempts were used to understand the yolk, and albumin characteristics in birds [3]. The qualitative characteristics of eggs are affected by several factors, which make the egg suitable for study or not. Therefore, the qualitative characteristics of the egg are divided into external and internal characteristics, which both were studied faithfully by several researchers [4, 5, 6, 7]. The external traits are two (Length, and breadth), which are important to find some related traits equation such as egg shape index (ESI), volume (EV), and egg surface area (ESA) [8, 9]. These traits affected directly on many phenomena like: egg hatchability [10], chick survivor [11], consumer desire [12]. Egg dimensions affected by several traits that play important role, such as hen age [13, 14], oviposition time [15], egg production chain [16], genetic background [17, 18, 19], and nutrition [20, 21].

Due to the development that has arisen in the poultry industry over the past years, the age of sexual maturity of hens has decreased significantly; our study is focus on the affected of the age of sexual maturity on the external egg traits in the Japanese quail.

The study was conducted in the poultry farm of the Department of Animal Production - College of Agriculture - University of Kirkuk, for the period in (2/4/2023 – 30/7/2023). Six hundred and eighty-two females were used in this experiment at the age of 30 days. When sexual maturity began, the birds were divided into four classes according to the age of first egg production, as follows: the first class from (40-49) days, the second class from (50-59), the third class from (60-69), the fourth class more than 60 days. The females were raised in vertical batteries, as each cage contained one female to facilitate the attribution of the egg to the quail hen. The quail hen was fed on a diet containing 24% protein and 2900 kcal digestible energy. As for water, it was provided freely. When the female reached the ovulation stage, the first egg was weighed with an electronic balance with a sensitivity of (0.01) g. Then the egg dimensions, represented by length and elongation, were taken using an electronic digital caliper vernier with a sensitivity of (0.01). Then the equations below were used to calculate the (ESI), (EV), and (ESA).

Table 1 presents the descriptive statistics mean values with standard errors (Mean ± S.E.) and coefficients of variation (CV%) of BW, EW, EL, and EB, categorized by four classes of age at sexual maturity. The age at which Japanese quail reached sexual maturity did not significantly affect their body weight (P > 0.05), as indicated by the consistent body weights across all classes. The overall mean body weight was 192.48 ± 0.67 g with a CV of 9.03%. Although class 1, 2, and 3 exhibited slightly varying means (192.96, 192.69, and 189.78 g, respectively), the differences were not statistically significant (P > 0.05), and class 4 showed a relatively lower mean of 182.14 g, with the highest variation (CV = 11.79%). This indicates that, in terms of body mass, early or delayed onset of sexual maturity may not substantially impact growth performance. Conversely, the age of sexual maturity had a statistically significant effect (P < 0.05) on egg weight. Quail that matured earlier (classes 1 to 3) laid heavier eggs compared to those maturing later (class 4). The first egg weight was highest in class 3 (8.62 g), followed by class 2 (8.39 g) and class 1 (8.22 g), while class 4 had significantly lighter eggs (7.20 g). The overall mean egg weight was 8.34 ± 0.05 g with a CV of 15.41%, reflecting notable variability particularly in class 3 (CV = 17.79%). These findings suggest that delayed maturity might be associated with physiological limitations affecting initial egg mass, possibly due to hormonal and reproductive organ development timing. Egg length was also significantly influenced (P < 0.01) by the age at sexual maturity. The longest eggs were recorded in class 3 (29.65 mm), followed by class 2 (29.30 mm) and class 1 (28.95 mm), while class 4 exhibited the shortest eggs (28.67 mm). These differences highlight a potential association between age-related ovarian maturation and structural egg formation. The overall mean egg length was 29.20 ± 0.06 mm with a low CV of 5.29%, suggesting relatively uniform length traits across groups, except for class 4, which showed slightly greater variability (CV = 6.28%). Regarding egg breadth, no statistically significant differences (P > 0.05) were observed among the four age groups. The average egg breadth was 23.24 ± 0.04 mm with a low CV of 4.08%. This suggests that while egg length and weight are sensitive to the age of maturity, egg breadth remains relatively stable across different maturity classes. Our study agreed with the finding of [24], Furthermore, [25] found that the transcriptional modifications of genes linked to fat metabolism and yolk precursor production were directly impacted by age-related hormone shifts. We verify that FSH regulates E2, an activator that causes hepatocytes to undergo functional and histological alterations via the ER-α pathway.

It can be concluded that the age of sexual maturity effected on the external egg traits (weight, length, surface area, and volume), and its correlated significantly with (egg length, breadth, surface area, and volume). These finding also will be useful for increase production of big eggs, by choosing suitable age of sexual maturity.