**Abstract**

The aim of this study was to establish incubation values of eggs (egg fertilization, absolute and relative embryo mortality, and hatchability of male and female chicks), morphometric indicators (preincubation egg mass; length, width, and egg shape index; hatched female and male chicken mass and their relative share in the egg mass before incubation), and the phenotype correlation between some traits in the younger parent flock (YF33—33 weeks) and the older flock (OF49—49 weeks) of the light Institut de Sélection Animale (ISA) Brown hybrid. With regard to incubation values, the younger flock (YF33) demonstrated better incubation results than the older flock (OF49). The egg fertilization rate was 95.24 and 94.22%, respectively, chick hatchability as the percentage of the total of incubated eggs was 86.51 and 84.89%, respectively, and chick hatchability as the percentage of the total of fertilized eggs was 90.83 and 90.09%, respectively. Embryo mortality rate was 8.73 and 9.17% (YF33), and 9.33 and 9.91% (OF49). Regardless of the parent flock age, eggs that hatched female chicks had lower values of observed morphometric traits than those that hatched male chicks, except for the egg shape index (77.49–77.47%, respectively) which was higher by 0.02% in eggs which hatched female chicks, but this difference was not statistically significant (P > 0.05). Contrary to effects of the chick sex, the parent flock age had considerably larger effect on observed morphometric traits, as all morphometric indicators of eggs and hatched chicks of both sexes in the older flock (OF59) had statistically significantly higher values (P < 0.001) than in the younger flock (YF33). The only exception is the relative share of the chicken in the egg mass where the measured difference (−0.03%) was not statistically significant (P > 0.05). The phenotype correlation coefficients determined (*r*p) between the egg mass before the incubation period and the egg shape index were statistically significant (P < 0.01; P < 0001), except between the egg mass of eggs which hatched female chicks and the egg shape index in the young flock (YF33), whereby the calculated coefficient (*r*xy = 0.107) was not statistically confirmed (P > 0.05). Furthermore, the egg mass and hatched chicken mass of both sexes increased with the age of the parent flock, and statistically significant absolute phenotype correlation (P < 0.001) was determined between these two indicators.

**Keywords:** morphometric measurements, sex, eggs, chicks
