Candling eggs is a core diagnostic tool in hatching management. By shining a focused light through the shell, producers can identify infertile eggs, detect early embryo death, and remove failing eggs before they compromise biosecurity or suppress hatch of fertile percentage (Ernst et al., 2004; Clark, 2002).
What Is Egg Candling?
The procedure involves pressing a bright light source against the broad end of the egg in a darkened room, revealing the air cell, vascular network, embryo position, and signs of deterioration. Modern LED candlers have improved penetration in dark-shelled breeds where older tools offered limited visibility.
What to Look for When Candling Eggs
A viable egg shows a radiating blood vessel network around a central dark spot; an infertile egg appears uniformly clear. Embryo death presents as a blood ring formed by collapsed vasculature. A floating or mobile yolk signals decomposition (Ernst et al., 2004).
Candling Stages of Chicken Eggs Hatching
Practice differs by scale. In commercial hatcheries, day 18 is the standard window, coinciding with the transfer from setter to hatcher. Broiler batches are fully candled; layer batches by spot sample. In smaller flocks, day 7 and day 14 are standard.
Candling Eggs Day 7
The first practical window for candling chicken eggs is around day 7. A viable egg should show a distinct embryo at the centre of a radiating vascular network, with the eye visible as a dark spot (Ernst et al., 2004). Do not disturb eggs during the first three to four days, when organ systems are forming. Eggs showing no vascularisation, or a blood ring, can be removed at this stage to reduce contamination risk.
Candling Eggs Day 14
At 14 days, the embryo occupies most of the shell. The air cell is visibly larger and movement may be detectable in live embryos. Dead embryos appear dark and motionless without vessel definition.
Candling Eggs: Fertile or Not
True infertility shows as a uniformly clear egg with no vascular development; early embryo death may produce a faint blood ring before vessels fully regress. The two are not always distinguishable by candling alone; breakout analysis of a sample is required to confirm true fertility rate (Mauldin, 1993).
When elevated infertility recurs, automated daily live weight data may reveal contributing body weight deviations and decreased uniformity in the parent flock.
Individual bird weighing connects weight records directly to recurring candling failures in the breeder flock.
Candling Duck Eggs
Duck eggs follow a 28-day incubation cycle, shifting the recommended candling schedule forward. First candling should occur at days 7 to 10, with a second check at days 14 to 18 (Ernst et al., 2004). Duck shells are thicker than chicken shells and require a brighter light source.
Fertility, Body Weight, and Hatchability
Persistent candling failures often trace back to breeder flock condition. Research confirms that lighter hens are significantly less likely to be in full egg follicle production, with rates around 90% in the lightest weight group compared to above 95% in heavier birds (Sibanda et al., 2020). Uniform body weight within breed-standard targets is directly connected to the fertility outcomes candling is designed to measure.
The BAT2 Connect automatic poultry scale provides continuous live weight data without disrupting flock routines, flagging weight trends before they appear as elevated infertility in the incubator.
Weight history stored in BAT Cloud gives hatchery and production teams shared visibility into the body weight patterns that underpin candling outcomes.
For individual bird uniformity, the BAT1 manual poultry scale offers a precise, hands-on complement to the continuous automatic weight record.
Cited References
1.) Adegbenjo, A.O., Liu, L. and Ngadi, M.O. (2020). Non-destructive assessment of chicken egg fertility. Sensors, 20(19), 5546. DOI: https://doi.org/10.3390/s20195546
2.) Clark, F.D. (2002). Fertility and Embryonic Mortality in Breeders. Avian Advice, 4(2). University of Arkansas Cooperative Extension Service. URL: https://poultry-science.uark.edu/_resources/pdf/avian_advice_su2002.pdf
3.) Ernst, R.A., Bradley, F.A., Abbott, U.K. and Craig, R.M. (2004). Egg Candling and Breakout Analysis. ANR Publication 8134. University of California, Division of Agriculture and Natural Resources. DOI: https://doi.org/10.3733/ucanr.8134
4.) Mauldin, J.M. (1993). Quality control procedures for the hatchery. Bulletin B 1166. Athens: University of Georgia College of Agricultural Sciences Cooperative Extension. URL: https://secure.caes.uga.edu/extension/publications/files/pdf/B%201166_2.PDF
5.) Sibanda, T.Z., Kolakshyapati, M., Walkden-Brown, S.W., de Souza Vilela, J., Courtice, J.M. and Ruhnke, I. (2020). Body weight sub-populations are associated with significantly different welfare, health and egg production status in Australian commercial free-range laying hens in an aviary system. European Poultry Science, 84. DOI: https://doi.org/10.1399/eps.2020.295