The long-term performance of broilers is closely tied to the quality of day-old chicks at placement. While farm management is often credited with flock success, the real foundation is laid much earlier. Chick quality is shaped by pre-incubation conditions, specific incubation parameters, and early post-hatch handling. The timing of hatching, and especially the length of the hatching window, the period during which already-hatched chicks remain exposed to the high temperature and humidity of the hatching compartment, plays a critical role in early development. This article reviews the factors that influence the hatching window. It begins with pre-incubation determinants such as breeder age, egg characteristics, and storage conditions, then turns to the core incubation variables, temperature, humidity, egg turning, and oxygen and carbon dioxide concentrations, that govern hatch timing and the width of the hatching window (Bergoug et al., 2013).
Hatching Window
The hatching window is a key indicator of incubation success and uniformity. It is defined as the time interval between the emergence of the first and the last chick within a single incubator batch. This window is primarily influenced by incubation temperature and must be carefully managed within the standard commercial incubation period of 508 to 518 hours to ensure chicks are pulled from the hatching compartment at the optimal time. A prolonged or desynchronized hatching window forces the earliest-hatched chicks to remain too long in the hot, humid hatching environment, leading to dehydration, stress, and uneven post-hatch performance. When a wide hatching window is compounded by delays in subsequent processing, vaccination, and transport, chicks can go without feed and water for up to 72 hours, a prolonged fasting period that significantly compromises early growth, development, and survival at placement (Willemsen et al., 2010).
Factors Affecting the Hatching Window
Egg Characteristics
Egg traits such as weight, size, and shell structure have a direct physical influence on incubation length and embryonic development. There is a clear positive correlation between egg weight and total hatch time: smaller eggs consistently hatch faster, and as egg weight increases, incubation time lengthens significantly. For example, research shows that 52g eggs average a hatch time of 488 hours, while larger 72g eggs require up to 494 hours (Burton and Tullett, 1985). Larger eggs also produce heavier embryos with the same relative eggshell porosity, meaning these embryos have greater difficulty dissipating metabolic heat into the surrounding micro-environment. If incubation temperature is not actively reduced (for example, from 37.5°C to 36.5°C) during the second half of incubation, this heat retention can cause thermal stress, increasing early or late embryonic mortality and potentially skewing the hatch window (Lourens et al., 2005). In addition, the physical quality of the shell, including its thickness and the number and size of its pores, directly governs how effectively the developing embryo exchanges gases, fluids, and heat with its environment, which in turn affects overall hatchability (Hulet et al., 2007).
Breeder Flock
The age of breeder hens shapes egg characteristics and introduces additional variables that affect hatch timing. Breeder age directly influences egg weight, which increases from an average of 55g at 27 weeks to 70g at 60 weeks. Because heavier eggs take longer to hatch, the aging of a breeder flock naturally shifts the baseline hatch time (Tona et al., 2001). Older breeders also show declining fertility and reduced hatchability, dropping to 73% by 59 to 61 weeks, largely driven by increased total embryonic mortality. At the other end of the spectrum, very young flocks fed restricted or low rations can experience compromised nutrient transfer to the egg, leading to late embryonic death and poorer chick uniformity (Abiola et al., 2008).
Implications for the hatching window: Because commercial incubator batches frequently mix eggs of varying weights or combine eggs from breeder flocks of different ages, these factors inherently widen the hatching window. Setting a uniform batch with highly variable egg weights or mixed breeder ages will naturally produce a desynchronized hatch, stretching the interval between the first and last chick to emerge, due to differences in embryonic heat-loss capacity and baseline incubation timelines.
Egg Storage
Prolonged egg storage has a direct, compounding effect on the overall incubation timeline, specifically delaying the onset and peak of the hatch. Extending the storage period significantly pushes back the total incubation time required: research shows that eggs stored for 3 days reach their peak hatch (50% of total chicks hatched) at 486 hours, while eggs stored for 18 days need an additional 18 hours of incubation (Tona et al., 2003). Multiple studies confirm this predictable delay, noting that each day of storage adds roughly 0.7 to 1.28 hours to the total incubation period (Tiwary and Maeda, 2005). To maximize hatchability in older eggs, hatcheries must therefore plan for a longer overall incubation time. Notably, data from both Tona et al. (2003) and Mather and Laughlin (1976) show that long storage periods, ranging from 14 to 18 days, substantially delay the mean total incubation time without affecting the width of the hatching window. In other words, the process shifts later as a cohesive block rather than spreading out or desynchronizing the interval between the first and last chick to hatch.
Temperature
Because chicken embryos are ectothermic and cannot regulate their own body temperature, ambient incubation temperature directly controls their developmental pace. High temperature shifts, in the range of 39.5°C to 40°C, accelerate embryonic development and result in an earlier hatch, but at the cost of significantly increased late embryonic mortality (Leksrisompong et al., 2007). Conversely, low temperature shifts around 36°C, particularly during the final third of incubation, suppress the embryo’s metabolic activity and result in a prolonged incubation time and a delayed hatch (Yildirim and Yetisir, 2004).
Humidity
Humidity is the primary environmental factor controlling egg weight loss during incubation, and it directly affects the physical condition of the chick at emergence. Moisture and gas exchange between the egg and its environment are driven both by incubator humidity and by the egg’s natural shell quality, including pore count and thickness. The direct effect of incubator humidity on hatching time itself is not clearly established, but humidity has a clear impact on chick quality and livability when the hatching window is prolonged. Lower humidity environments, around 45% relative humidity, cause eggs to lose significantly more weight during incubation, producing chicks that are smaller, more dehydrated, and more prone to unhealed navels, though these chicks are physically better adapted to dehydrating conditions after hatch. Higher relative humidity, around 55%, limits egg weight loss and yields heavier chicks at hatch, but because these chicks retain more moisture, they tend to lose weight more rapidly if exposed to high temperatures during the post-hatch period (Barbosa et al., 2008).
Carbon Dioxide Levels
Gas concentrations inside the incubator do more than influence embryo viability; they actively shape the timing and synchronization of the hatch. Gradually increasing CO2 levels to between 1% and 1.2% within the first 10 days of incubation speeds up hatch time, because the elevated gas concentration accelerates physical embryo development specifically between days 6 and 10 (De Smit et al., 2006). However, while early CO2 exposure accelerates development, it simultaneously widens the hatching window (De Smit et al., 2006). This earlier hatch is biologically linked to lower albumen pH and a higher partial pressure of CO2 in the egg’s air cell. CO2 levels during the hatching process itself also induce physiological changes that directly control the timing of external pipping and the final hatching events (Bruggeman et al., 2007).
Oxygen Levels
A reduction in oxygen levels, which occurs naturally with increasing altitude, slows the embryo’s biological clock and delays development. Lower oxygen levels have a direct, adverse impact on embryonic growth rates, and severe hypoxia at 10% oxygen can completely compromise embryo viability (Szdzuy et al., 2008). Mild hypoxia at 15% oxygen, sustained over 6-day blocks during incubation, significantly reduces embryonic growth compared to normal conditions; if this occurs during the critical internal pipping stage, it restricts the chick’s oxygen intake and alters its final weight at hatching (Chan and Burggren, 2005). The timing of oxygen deprivation also determines its consequences. Hypoxia is most lethal during the first eleven days of incubation (E0 to E10), the critical phase for structural development, whereas the last ten days (E11 to E18) represent a phase in which the embryo’s organs can mount a compensatory survival response to low oxygen (Zhang and Burggren, 2012).
Because oxygen availability acts as a strict bottleneck in late-stage incubation, artificially raising oxygen levels can significantly compress the developmental timeline. Introducing hyperoxia at 60% oxygen late in incubation, specifically between days 16 and 18, bypasses the shell’s diffusion limitation, allowing the embryo to grow faster and accelerating overall fetal development and hatch timing. Acute hyperoxic spikes of 60% oxygen for 48 hours at mid-to-late stages (days 10 to 11, 14 to 15, and 18 to 19) significantly increase the total mass of both the embryo and its vital organs (Van Golde et al., 1998). These results, however, should be interpreted with caution given the era in which the research was conducted; modern genotypes may respond differently.
Light
The duration of light exposure also plays a role in embryonic synchronization and mortality. Providing eggs with light during incubation, whether through a continuous regimen (23 to 24 hours of light) or an intermittent regimen (12 hours of light), shortens the hatching window and improves overall hatchability compared to standard dark incubation (Riaz et al., 2021). An intermittent lighting schedule appears particularly beneficial: it reduces embryonic mortality and raises melatonin levels on day 19 of incubation (Riaz et al., 2021; Archer and Mench, 2014). A continuous light regimen, by contrast, can produce negative secondary effects, including elevated eggshell temperatures and structural damage to avian eyes (Rozenboim et al., 2004; Archer et al., 2009). Specific wavelengths, particularly green LED light (measured at 522 nm and 520 to 525 nm), have also been shown to shorten total hatch time compared to incubation in complete darkness (Wang et al., 2020). This acceleration is associated with increased growth hormone and insulin-like growth factor during embryonic development when stimulated by green LED light at 565 nm and 15 lux (Zhang et al., 2014).
Egg Turning
The physical arc of egg rotation during incubation has a direct, molecular impact on how efficiently the embryo uses nutrients to progress toward hatching. Standard industrial practice involves turning eggs to 90° and 45° on either side of vertical; dropping below these optimal angles increases embryonic mortality and the incidence of malpositioned embryos. The complete absence of turning, or static incubation, has particularly severe effects at specific developmental milestones, arresting embryo growth and gas exchange (Elibol and Brake, 2004):
1.) Early-stage window (days 3 to 7): This is a highly critical phase. Failing to turn eggs during this window impairs the expansion of the area vasculosa (the early vascular network) and stalls the production of sub-embryonic fluid, which permanently reduces embryo growth rates and lowers final hatchability (Baggot et al., 2002).
2.) Late-stage window (days 12 to 19): Halting rotation during this final third of development restricts late embryonic growth by impairing oxygen consumption through the chorioallantoic gas exchanger (Pearson et al., 1996).
Conclusion
Optimizing hatchability and narrowing the hatching window depends on an interconnected set of pre-incubation and incubation variables. Breeder age, together with egg storage period, is a primary driver, directly dictating egg size and baseline quality. Within the incubator, environmental factors cannot be managed in isolation: changes in heating, cooling, and ventilation simultaneously alter eggshell temperature, humidity, and gas concentrations, and gas levels must be dynamically adjusted during the second half of incubation to maximize hatchability. Ultimately, the modern hatchery’s central challenge is managing the volatile, three-dimensional microclimate gradients inside large incubation machines, balancing ambient air temperature against actual eggshell temperature, a refinement made increasingly important by the elevated metabolic demands of genetically selected modern embryos.
Cited Sources
Abiola, S. S., Meshioye, O. O., Oyerinde, B. O., & Bamgbose, M. A. (2008). Effect of egg size on hatchability of broiler chicks. Archivos de zootecnia, 57(217), 83-86.
Archer, G. S., & Mench, J. A. (2014). The effects of the duration and onset of light stimulation during incubation on the behavior, plasma melatonin levels, and productivity of broiler chickens. Journal of animal science, 92(4), 1753-1758.
Archer, G. S., Shivaprasad, H. L., & Mench, J. A. (2009). Effect of providing light during incubation on the health, productivity, and behavior of broiler chickens. Poultry science, 88(1), 29-37.
Baggott, G. K., Deeming, D. C., & Latter, G. V. (2002). Electrolyte and water balance of the early avian embryo: effects of egg turning. Avian and Poultry Biology Reviews, 13(2), 105-119.
Barbosa, V. M., Cancado, S. V., Baiao, N. C., Lana, A. M. Q., Lara, L. J. C., & Souza, M. R. (2008). Effects of relative air humidity in the hatchery and breeder hen age on the incubation yield. Arquivo Brasileiro de Medicina Veterinária e Zootecnia, 60, 741-748.
Bergoug, H., Burel, C., Guinebretiere, M., Tong, Q., Roulston, N., Romanini, C. E. B., … & Eterradossi, N. (2013). Effect of pre-incubation and incubation conditions on hatchability, hatch time and hatch window, and effect of post-hatch handling on chick quality at placement. World’s Poultry Science Journal, 69(2), 313-334.
Bruggeman, V., Witters, A., De Smit, L., Debonne, M., Everaert, N., Kamers, B., … & Decuypere, E. (2007). Acid-base balance in chicken embryos (Gallus domesticus) incubated under high CO2 concentrations during the first 10 days of incubation. Respiratory Physiology & Neurobiology, 159(2), 147-154.
Burton, F. G., & Tullett, S. G. (1985). The effects of egg weight and shell porosity on the growth and water balance of the chicken embryo. Comparative Biochemistry and Physiology Part A: Physiology, 81(2), 377-385.
Chan, T., & Burggren, W. (2005). Hypoxic incubation creates differential morphological effects during specific developmental critical windows in the embryo of the chicken (Gallus gallus). Respiratory Physiology & Neurobiology, 145(2-3), 251-263.
De Smit, L., Bruggeman, V., Tona, J. K., Debonne, M., Onagbesan, O., Arckens, L., … & Decuypere, E. (2006). Embryonic developmental plasticity of the chick: Increased CO2 during early stages of incubation changes the developmental trajectories during prenatal and postnatal growth. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 145(2), 166-175.
Elibol, O. K. A. N., & Brake, J. (2004). Identification of critical periods for turning broiler hatching eggs during incubation. British poultry science, 45(5), 631-637.
Hulet, R., Gladys, G., Hill, D., Meijerhof, R., & El-Shiekh, T. (2007). Influence of egg shell embryonic incubation temperature and broiler breeder flock age on posthatch growth performance and carcass characteristics. Poultry science, 86(2), 408-412.
Leksrisompong, N., Romero-Sanchez, H., Plumstead, P. W., Brannan, K. E., & Brake, J. (2007). Broiler incubation. 1. Effect of elevated temperature during late incubation on body weight and organs of chicks. Poultry Science, 86(12), 2685-2691.
Lourens, A., Van den Brand, H., Meijerhof, R., & Kemp, B. (2005). Effect of eggshell temperature during incubation on embryo development, hatchability, and posthatch development. Poultry science, 84(6), 914-920.
Mather, C. M., & Laughlin, K. F. (1976). Storage of hatching eggs: the effect on total incubation period. British Poultry Science, 17(5), 471-479.
Pearson, J. T., Haque, M. A., Hou, P. L., & Tazawa, H. (1996). Developmental patterns of O2 consumption, heart rate and O2 pulse in unturned eggs. Respiration Physiology, 103(1), 83-87.
Riaz, M. F., Mahmud, A., Hussain, J., Rehman, A. U., Usman, M., Mehmood, S., & Ahmad, S. (2021). Impact of light stimulation during incubation on hatching traits and post-hatch performance of commercial broilers. Tropical animal health and production, 53(1), 107.
Rozenboim, I., Biran, I., Chaiseha, Y., Yahav, S., Rosenstrauch, A., Sklan, D., & Halevy, O. (2004). The effect of a green and blue monochromatic light combination on broiler growth and development. Poultry science, 83(5), 842-845.
Szdzuy, K., Fong, L. M., & Mortola, J. P. (2008). Oxygenation and establishment of thermogenesis in the avian embryo. Life Sciences, 82(1-2), 50-58.
Tiwary, A. K., & Maeda, T. (2005). Effects of egg storage position and injection of solutions in stored eggs on hatchability in chickens (Gallus domesticus). The Journal of Poultry Science, 42(4), 356-362.
Tona, K., Bamelis, F. B. V. V. M. J. O. E., De Ketelaere, B., Bruggeman, V., Moraes, V. M., Buyse, J., … & Decuypere, E. (2003). Effects of egg storage time on spread of hatch, chick quality, and chick juvenile growth. Poultry science, 82(5), 736-741.
Tona, K., Bamelis, F., Coucke, W., Bruggeman, V., & Decuypere, E. (2001). Relationship between broiler breeder’s age and egg weight loss and embryonic mortality during incubation in large-scale conditions. Journal of Applied Poultry Research, 10(3), 221-227.
van Golde, J. M. C. G., Borm, P. J., Wolfs, M., Gerver, W., & Blanco, C. E. (1998). The effect of hyperoxia on embryonic and organ mass in the developing chick embryo. Respiration physiology, 113(1), 75-82.
Wang, P., Sun, Y., Fan, J., Zong, Y., Li, Y., Shi, L., … & Chen, J. (2020). Effects of monochromatic green light stimulation during embryogenesis on hatching and posthatch performance of four strains of layer breeder. Poultry science, 99(11), 5501-5508.
Willemsen, H., Kamers, B., Dahlke, F., Han, H., Song, Z., Pirsaraei, Z. A., … & Everaert, N. (2010). High-and low-temperature manipulation during late incubation: effects on embryonic development, the hatching process, and metabolism in broilers. Poultry Science, 89(12), 2678-2690.
Yildirim, I., & Yetisir, R. (2004). Effects of different hatcher temperatures on hatching traits of broiler embryos during the last five days of incubation. South African Journal of Animal Science, 34(4), 211-216.
Zhang, H., & Burggren, W. W. (2012). Hypoxic level and duration differentially affect embryonic organ system development of the chicken (Gallus gallus). Poultry Science, 91(12), 3191-3201.
Zhang, L., Zhang, H. J., Wang, J., Wu, S. G., Qiao, X., Yue, H. Y., … & Qi, G. H. (2014). Stimulation with monochromatic green light during incubation alters satellite cell mitotic activity and gene expression in relation to embryonic and posthatch muscle growth of broiler chickens. Animal, 8(1), 86-93.