I got PGC woodworking for wildlife booklet. It says it should be constrcted of wood that is stong and can be made weather resistant. It can be painted, stained, or treated. However, treatment is not necessary if weather resistant lumber is used on the outside.
I bought 1" X 12" X 12' Pine as it was lot less expensive than weather resistant type wood. Not having a lot of woodworking tools this made it easy with having just to make crosscuts with a skill saw for most of the box.
I had left over black paint and white paint setting around.
I mixed them together to get a light gray.
Boxes have been up approx. 6 years now and seem to be withstanding the weather just fine. Gray color blends in with the surroundings and the wood ducks have been using anywhere betwen 3 or 4 of the six that I have built and set out.
We use rough sawn Pine, 5/4 X 12. Some of the boxes have been up 10 years and still in good shape. If you can glue your edges it will help. Painting is also a good option, just try not to do the inside.
Using anything other than cedar and you're looking at a lot of upkeep. The waste from the ducklings and the hatching process will eat the bottom away regardless, but cedar gives you a lot longer lifespan than soft woods like pine. Painting wood seals moisture in and doesn't allow the wood to expand and contract freely. For example, you don't see pine decks on a house painted, they're stained. Stain only give you one, maybe two, seasons. Using cedar extends the life similar to how a cedar shingle will last for years on your roof. Our Delta chapter is using 100% of our funds to build cedar wood duck boxes and we've foun it runs aaround $38 a box to construct a single box. We've decided to build boxes that last much longer, opposed to building 3x more boxes of a lesser material.
We buy rough sawn 1X12X10' for $4.50, the same peice of cedar is $37 and only 5/8" thick. When high water takes a box it does'nt hurt so much. We will be building over 100 boxes this month and just could'nt afford Cedar.
Allot of wood porch flooring lumber is yellow pine and is painted, thats what helps protect it from moisture penetrating inward ( the exterior paint ). Grant it, up scale houses have ceder or better yet mahogany and these woods by far withstand weather better with out being treated, but for a duck box! Seems like over kill unless your building with cut offs. Moisture would only be in a board if it is green or not properly dried. Hemlock(a pine species), would be a very good choice for duck or any kind of bird boxes. Local sawmills usually have plenty of hemlock boards on site. Even a rough sawed hemlock board will withstand many years of the weather untreated. I believe the wood is more dense than other pine species so it helps repel water and will stain naturally to a gray over time. You would pay roughly the same as what dively has posted for a rough board. You make one box per 10' board. I have made wood duck boxes with hemlock and have had no problems with it. I would only suggest picking through the stack for the better boards and try to buy boards that have at the least been air dried for a year.