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 45% 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.
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.
I found that the humidity had to be different for different 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, 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 in conclusion, I believe the answer is that all of the authors are right! Different humidities work in different incubators at different times of the year but even if you are using the same incubator as the author of the article, different egg shells have slightly different porosity so you may not get the same results. To get the very best chances of a successful hatch, think about using the weight loss method of incubation. Since I can't run 3 incubators continually, I now average the measured weights in the batch of eggs (assuming the batch is large enough), make adjustments to my humidity to get the correct weight loss, and usually have a good success rate.
If you do not want to invest in accurate scales (mine shown in the picture cost me £45 or so in 2007 and are accurate to 0.1g) to weigh your eggs, another way is to look at the size of the air sack. This shows you how much weight has been lost and if the air sack is the wrong size at a given time, you need to make adjustments to your humidity.