So sorry about your chickens.
Seems like you’re right about Coryza. Ducks are pretty resistant to disease and parasites in general. I haven’t given my birds any vaccinations or had any get ill yet.
Ducks can possibly be kept in the same yard / run with your geese. I’m not sure what your geese are like, but it’s recommended they at least have separate coops over night, as geese tend to like having their own space to sleep & nest and can be very territorial otherwise.
For housing, ducks don’t
need roosting bars or nesting boxes (maybe with the exception of Muscovy ducks - I hear they like to perch & nest in trees!!). They are perfectly content just having some bedding for their coop floor. I find I have to change out their bedding every 1-2 weeks. I’ve heard there’s some ‘deep litter method’ with chickens, but I’m not sure that works with ducks. Housing materials and sizing are the same, I believe; 4-6 sqft for the coop, 10-15 sqft for the run. With proper protection from the wind and rain, they can withstand well into the negative temperatures.
I recommend around a 1:4 or 1:5 male-to-female ratio. You may need more or less depending on the drake’s (male duck) temperament. Personally, I have a 1:5, and it’s been great for my birds.
Drakes
do have their role in the flock, but definitely not as much of a role as roosters do. A good drake will keep watch for predators, lead the group, and let the hens eat first. But, they cannot / will not protect the hens from threats. Ducks are not built for fighting whatsoever.
For feed, most duck, waterfowl, and all-flock feeds work very well. Personally, my ducks get Purina duck feed + some of my geese’s Mazuri waterfowl feed as a treat. The biggest lookout for in a feed is protein content (16-20%) and niacin content (minimum 55 ppm / 25 mg/lb, but larger ducks may need more). They need a deep water dish in order to eat properly, else they will sneeze and choke. They also need it to clean out their nares and eyes. They need grit provided if it’s unavailable in the yard, and extra calcium when laying.
For treats, corn, oats, peas, lettuce, mealworms, minnows, grapes, tomatoes, etc. are good.
There’s an old thread here about treats.
Minimum swim time 1-2 days per week, but more is better. My ducks swim every day and love it!
Ducks much prefer to mate in the water, but they will mate on land if necessary. They can be a little crazy about mating, hens on hens, triple stack, etc., but if your ratios are good, there’s little concern.
Like geese, ducks are susceptible to angel wing when young (<16 weeks), but it’s easy to treat, and with good diet & exercise, most likely it won’t even appear. And ample niacin will prevent / help any leg issues.
Yes, there are bantam, light, medium, and heavy-weight ducks, with different appearances, purposes, and general temperaments. Best to keeps ducks of the same or adjacent weight class imo.
My ducks have learned to come when called and climb stairs (took 1 week to learn, and they need railings!). They follow me around, eat from my hand, and don’t mind me being nearby, but.. they still don’t like when
I approach them, or try to pick them up. And they can be pretty skittish around new things, people, or experiences. It’s just how they are.. have patience!
Not sure what else to write. Definitely feel free to ask more questions
