The most reliable way to tell if a bird egg is fertile is candling: shining a bright light through the shell to see what’s developing inside. A fertile egg will show thin red veins spreading outward from a small dark spot (the embryo) within the first week of incubation. An infertile egg stays completely clear. You can’t determine fertility before incubation begins, so the egg needs at least a few days of warmth before any test will work.
What Candling Looks Like
Candling means holding a focused light source against the eggshell in a dark room. The light passes through the shell and illuminates the contents, letting you see whether an embryo is forming. You can use a dedicated egg candler, a bright LED flashlight, or even your phone’s flashlight pressed against the shell. The key is a focused, bright beam rather than a wide flood of light. Darker or thicker shells (like those from certain duck breeds or Marans chickens) need a stronger light to see through clearly, and very faint lights may not penetrate at all.
To candle, cup your hand around the large end of the egg and hold the light flush against the shell. Tilt the egg gently to get different angles. You’re looking for veins, a dark embryo spot, and a visible air cell at the wide end.
When to Candle for the First Time
Fertility signs can appear as early as 3 to 4 days into incubation, but they’re easy to miss at that stage, especially with brown or dark shells. The University of Georgia Cooperative Extension recommends waiting until days 8 to 10 for the clearest results, when the embryo and its surrounding membranes are large enough to see without guesswork.
If you’re impatient, a day 4 or 5 check can give you a preliminary answer. Look for a tiny dark dot with faint veins radiating outward, sometimes described as a “spider” shape. If the egg looks completely clear with no structures at all, it’s almost certainly infertile. When in doubt, put it back and check again in a few days rather than discarding a potentially viable egg too early.
Fertile vs. Infertile vs. Dead
These three categories look distinctly different under a candling light, and learning to tell them apart will save you from discarding good eggs or wasting incubator space on bad ones.
Fertile and developing: You’ll see well-defined blood vessels branching out like a road map. The embryo appears as a dark spot, and in later stages, a healthy embryo will actually move in response to the light. The blood vessels look crisp and organized.
Infertile: The egg appears uniformly clear or shows only the yolk shadow drifting inside. There are no veins, no dark spot, no structure at all. These eggs never began developing.
Early embryo death: This is the one people most often confuse with a developing egg. When an embryo dies in the first several days, the blood vessels begin breaking down. Instead of clean, branching veins, you’ll see streaky, disorganized lines under the shell. The most recognizable sign is a “blood ring,” a dark reddish-brown ring visible inside the egg where the deteriorating blood vessels have pooled in a circle. A blood ring means development started but stopped. Discard these eggs.
Late embryo death: Later in incubation, a dead embryo can be harder to distinguish from a live one because the egg is already quite dark and full. The FAO recommends looking for two things: absence of movement when held under the light for 30 to 40 seconds, and breakdown of the blood vessels (which appear blurry or streaky rather than sharply defined). A healthy embryo will respond to the candling light by moving, though sometimes it takes up to 30 or 40 seconds. If the embryo remains completely still and the vessels look degraded, it’s no longer viable.
Keeping Eggs Safe While Candling
Eggs cool quickly once removed from incubation, and a temperature drop below 99°F can stress or kill a developing embryo. Keep each candling session to 5 to 10 minutes per egg, and don’t pull all your eggs out of the incubator at once. Candle a few at a time, returning each batch before pulling the next. Work in a warm room when possible, and handle eggs gently to avoid jarring the developing embryo. The ideal incubation temperature for chicken eggs sits around 100.5°F, with a safe range of 99 to 102°F.
Does the Float Test Work?
You may have seen advice about placing eggs in water to check fertility. The float test is real, but it doesn’t test fertility. It estimates how old an egg is based on buoyancy. As eggs age, moisture evaporates through the shell, the air cell inside grows, and the egg becomes lighter. Fresh eggs sink; older eggs float higher in the water column.
This method was developed for wildlife researchers estimating the age of gamebird nests, and its accuracy is limited. Studies have found that float tests tend to overestimate age for younger eggs and underestimate it for older ones. More importantly, submerging an incubating egg in water can introduce bacteria through the shell pores and chill the embryo. For eggs you’re trying to hatch, candling is far safer and more informative.
Digital Egg Monitors
For a more definitive answer than visual candling, digital egg monitors detect the embryo’s heartbeat through the shell using infrared light. These devices, such as the Buddy digital egg monitor, work by shining near-infrared LEDs through the egg and measuring the light that passes through, picking up the tiny pulse of a beating heart. They’ve been used successfully on chicken, turkey, and even reptile eggs.
Heartbeat detection becomes reliable around day 6 of incubation in chicken eggs. By the second half of incubation, a healthy chicken embryo’s heart rate reaches roughly 200 to 270 beats per minute. These monitors remove the guesswork of interpreting shadows and veins, which makes them especially useful for dark-shelled eggs that are hard to candle visually. They’re more expensive than a simple flashlight, but breeders working with valuable eggs or rare species often consider them worth the investment.
Before Incubation Starts
There is no reliable way to determine fertility before incubation begins. A fertile egg and an infertile egg look identical from the outside and even when cracked open (the small white spot on the yolk, called the germinal disc, is present in all eggs, though it’s slightly larger and more defined in fertile ones). The only way to confirm fertility is to incubate the egg and check for development after several days. If you’re sourcing eggs for hatching, your best bet is to confirm that the hen had access to an active, healthy male within the prior 10 to 14 days.