We've been breeding Welsh Harlequin ducks for a few years now, and I've found that selling ducklings has been a really fun way to meet new duck keepers. It's also been a lot of fun helping them get started on the right foot. With each passing year, I find that my reputation as a "duck lady" seems to grow - and with it, the number of duck-related questions that I find in my email, facebook, and text messages.
Lots of these come from folks I've never actually met, and that's totally fine by me! Duck lovers tend to be kindred spirits, and I'm more than happy to help if I can. One question I get asked a lot this time of year, comes from folks looking to start hatching out ducklings from their own duck's eggs. It's this: "How long do I need to separate duck breeds, to ensure purebred offspring?"
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Usually this comes up because a duck keeper has several kind of ducks that usually live together. But as spring comes, they'd like to hatch out ducklings that will be pure to each breed. They might want to simply increase their own flock, but most often, they'd also like to be able to sell ducklings and hatching eggs to customers who want purebred ducks of a particular breed.
I can't tell you HOW many people have texted me photos of ducks they've bought from backyard breeders who assured them they were purebred, when they were purchased as ducklings. I've seen "pure" pekins with black splotches. "Pure" Welsh Harlequins with bright yellow bills and eye stripes. "Pure" Black Swedish that turn out grey and white.
This is what happens when breeding flocks are kept together and not separated long enough, before collecting eggs for hatching.
SO - how long to separate duck breeds for purebred ducklings?
The short answer is this: At least 18 days.
That is based on every available study that I've been able to put my hands on. These studies measured fertility in eggs that were collected and incubated daily, after isolating female ducks from any male. Results in every study I read (see bibliography) varied from 12 days to 17 days.
It may be worth mentioning that there are quite a few anecdotal stories in the various duck-keeping forums, in which people report fertility lasting even longer - almost 4 weeks in some instances. Again - these are anecdotal accounts , so certainly worth taking with a grain of salt. However, since I always like to err on the side of caution, I do think it's worth mentioning.
Based on the actual scientific data available from the published studies that have been done, waiting 18 days probably is adequate to ensure purity in the offspring of your ducks.
And yes - I know this is a much more conservative timetable than a lot of the information floating around out there on some various websites and poultry forums. In fact, I corresponded with the owner of one of the major waterfowl hatcheries in the US, and he said that waiting 7 days after separating a flock will ensure purity of the offspring. It's quite likely that from a customer satisfaction standpoint, 7 days does provide a small enough margin of error to serve a commercial hatchery just fine.
But as someone who works with fairly rare heritage breeds, I hear this question and see it through the lens of rare livestock breed preservation. When I'm working with a rare heritage breed, I feel the responsibility of stewarding my flock in such a way that I'm doing everything possible to maintain the purity and integrity of the breed, while increasing its numbers. So from that standpoint, I feel it's worth looking at the data, and doing everything possible to ensure that any ducklings or hatching eggs sold are absolutely beyond-a-doubt pure.
For myself, I only keep one breed of ducks. It keeps things simple. If I were ever to add another breed (and there are some gorgeous ones that tempt me!!) I'd make sure to separate my flocks a full four weeks before beginning hatching. That's just me. I'm really hopeful that the information here will help you make a decision that you feel great about, as you manage your breeding flocks!
Need tips on how to hatch those duck eggs? Here's a guide that'll make it easy, and help you get excellent hatch rates.
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Fronda, F. M.. Zialcita, L. P., and Dalisay, A. M. (1940). The fertility of the duck eggs. Philippine Agriculture 29, 111-123
Ash, W. J., 1962. The duration of fertility and hatchability of white pekin duck eggs. Poultry Science, Volume 41, Issue 4, 1 July 1962, Pages 1123–1126