Causes of infertile chicken eggs

Even though it's not much fun, I autopsy all of the eggs that don't hatch after 23 or 24 days.  At first, I found lots of partially or fully formed chicks during my post-hatch autopsies, but as I got better at incubating, more and more of the eggs that didn't hatch were simply full of liquid and no chicks.  Of the 24 eggs we stuck in the incubator at the end of July, 14 hatched, 2 were nearly fully formed but died in the shell, and 8 showed no sign of chicks at all.

Chickless eggs can have several causes.  First, it's quite possible that the eggs were never fertilized, especially since my later hatches occurred long after the peak egg fertility period.  The chart below, which I mocked up using data from The Dollar Hen, shows that hatch rate (and, presumably, egg fertility) follows the seasons, with lows in July and August and again throughout the winter.  To get the most  bang for your buck, you might choose to order eggs during the peak fertility period in middle to late spring.

Another cause for chickless eggs is embryos that died within the first few days of incubation.  If you're very astute, you might see spots of blood in these eggs, signaling that the egg was fertilized and that a miniscule chick started to grow.  You have to expect a certain number of eggs like this when ordering eggs through the mail since longer storage, rough handling, and temperature extremes before the egg goes into the incubator can all kill your chick before it really begins to grow.  If you see lots of chickless eggs from homegrown eggs, you should instead consider the viability of the parents --- are they healthy, on pasture, and young; do you have the right number of hens per rooster; is the flock suitably outbred, etc.?

I ran across one final tidbit while researching infertile (or seemingly infertile) eggs --- your hatch rate will probably correspond to the apparent fertility rate of the eggs.  For example, since 67% of our eggs from the last hatch had some signs of chick development, we should have hatched only 67% of the fertile eggs (11 chicks.)  Many of the factors that cause embryos to die very young also make the surviving chicks weaker and unable to survive all the way to hatching.  So I guess we got lucky hatching out 88% of our fertile eggs into perky chicks.