For years, I read articles written by well-respected authors who advised everything from 60% relative humidity (RH) to adding no water at all to the incubator in order to have a successful waterfowl hatch.
Some even recommend spraying eggs regularly, others do not. So what is the answer?
Duck eggs will hatch under a broody hen so surely 50% RH is the right sort of humidity to maintain? Who is right and why are there so many different recommendations?
After spending the last 4 years hatching ducklings, I have kept accurate records of humidity (more recently using the recording software of the RCOM 20 Pro USB ) and I have kept records of the weight of each egg at different times during the incubation period, in a search to find the ‘right’ humidity for duck eggs.
The duck egg during incubation.
Egg shells are porous and moisture loss through the shell causes the egg to lose weight. The rate of this humidity loss depends on the humidity of the air around the egg. Have you ever tried drying washing on a damp day? The moisture won’t evaporate as quickly into the air as on a dry day and this is the same idea with eggs.
As the humidity of the air decreases, the rate at which the egg loses moisture increases and vice-versa. As moisture is lost from the egg, the air sack at the rounded end increases in size, causing air to be drawn into the sack. This air sack provides the first few breaths the duckling takes before it pips through the shell, starting to breathe air through the hole it has created.
As time passes over the 4 weeks or so of incubation, the moisture loss is making the egg lighter so we have a weight loss over the incubation period. For duck eggs, the ideal weight loss is around 14% so when I monitor the weight loss of my eggs, I can see my most successful hatches occur for eggs that have lost around 14% of their weight.
So what humidity did I use to get a 14% weight loss?
I found that the humidity had to be different for different batches and breeds of eggs. Some eggs lost weight far quicker than others so when graphed, weight loss against days of incubation, there were different lines, each heading for a different percentage weight loss at the end.
So what did this tell me? After a little head scratching, I realised that the porosity of the shell was coming into play. Some eggs were more porous than others and were losing weight faster. To overcome this, whilst incubating mixed batches of duck eggs, I tried running 3 incubators, one at 65%RH, one at 45%RH and one at 35% or as low as I could get it (it was difficult to maintain due to the humidity of the outside air).
Eggs that needed a higher humidity were placed into the 65%RH incubator for a few days, then re-weighed, they were brought back, closer to the ideal weight loss line on my graph. Eggs that needed a lower humidity were placed into the 35%RH incubator for a few days to bring them back to the ‘ideal’ loss. Each time I could get the weight loss of an egg to 14%, most of the time, they would hatch successfully.
So what humidity should you use?
The conclusion of what humidity to use to hatch duck eggs was that it varies although, the humidity requirement for most of the duck eggs I hatched during my tests was at the lower end, closer to 40% – most of my failures were because the humidity was too high. Ducklings would develop and die fully formed because they couldn’t pip and rotate around inside the egg in order to break the shell and free themselves.
If you can’t weigh eggs and run 3 different incubators (and lets face it, this was a little extreme!) there is another way to estimate the correct weight loss and that’s by the age-old method of candling and observing the size of the air sac.
Air Sac Development
The image to the right shows you the ideal air sac size at different stages of incubation in days, if you follow this, you shouldn’t go too far wrong.
If you print it out, it should be close to the actual size for most light and medium breed ducks.
There is another image of a chicken and duck egg at the bottom of our page on candling eggs that might also be useful to you too.
- Use a soft (e.g. HB) pencil to mark the extent of air sac development on the egg shell and compare it to the egg above.
- If the air sack in your duck eggs is too small, your incubator humidity is too high. If the air sack is too big, your incubator humidity is too low.
- The air sac changes slowly at first. Most of the change occurs from 18 days onwards.
- Weight loss is not proportional to the change in air sac: it will be fairly constant.
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