Why do Duck eggs have longer expiry than Chicken Eggs?

First of all, eggs are generally much more robust than commonly thought. Food safety advices for eggs vary a lot, but healthy (chicken) eggs can stay good for many months. If the eggs are contaminated with bacteria when laid or during processing, they can of course spoil or rot before their "best before" date as well. I didn't find any publications, but in this interview, a researcher from Nofima (a Norwegian food research institute) tells that they were not able to find any harmful bacteria in 7 months old eggs and even after 12 months, eggs are usually edible.

Back to the difference between chicken and duck eggs ... Chicken eggs are good for at least three weeks even without refrigeration. The reason for this is that the eggs contain natural preservatives, which are required for the egg to stay good during the nesting period. The natural purpose of the egg is of course to provide nutrition to the contained chicken, and it would probably not do the chicken any good, if it has to stay with a rotten egg yolk for several weeks. It varies between different duck species, but ducks generally breed their eggs much longer than hens. I am now just assuming, but I suppose that duck eggs have a similar natural protection as well, to keep them good throughout the longer breeding period and that may explain the longer recommended shelf life for duck eggs.

Duck eggs have a heavier, more waxy coating on them than chicken eggs do. Duck eggs must be more resistant to bacteria in moist environments since they are much more likely to be exposed to it than a chicken egg is since, in nature, the duck spends much more time in the water. A heavier, more waxy coating means less evaporation and less chance for bacteria to enter the egg and multiply in storage.

I suspect it's an issue of volume. Chicken eggs are a huge commodity and a lot of time and effort goes into USDA grading, etc... Duck eggs have a smaller market and not usually graded which means they can get from the farm to the store much faster.

In many countries, eggs aren't power washed and bleached like they are in the US so eggs can be sold and stored unrefrigerated for long periods without any ill effects.

There shouldn't be any difference in shelf life if the duck eggs and chicken eggs are handled and stored in the same manner. However, it has been shown by investigative reporters that chicken eggs are often stored unrefrigerated by supermarkets, allowing them to stand at room temperature for hours before being put in the display case, so the apparently shorter shelf life of chicken eggs may be due to the producers erring on the side of caution when they label the cartons.

I keep my refrigerator colder than recommended, two to four degrees above freezing, and I've successfully stored chicken eggs for three months without spoilage. Even if the eggs don't spoil, however, they lose moisture, and the yolk and white may become quite thick over time. I've also encountered rotten chicken eggs that were well within the use-by date, presumably because the egg got infected even as it was developing inside the chicken.

I keep both ducks and chickens, and I find quite the opposite. My ducks' eggs begin to dry out in the space of a couple of weeks after collection (with or without washing), and once drying has begun, they can go off quite quickly, ie. inedible within 4 weeks of being laid. I came here hoping for tips to get them to last longer.