My friend Fonda asked, "Where do you buy your birdseed and what kind".
Here is the "short" answer.
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My experience is feed types change with the seasons. The very best time to attract birds is in Winter and Spring. In Summer and Fall it is more of a challenge to compete with Mother Nature.
Different types of feeders are best for different birds. I have many, change them in and out as the seasons change. Basically you need tube types which mimic tree limbs to perch on and hopper types to mimic jumping around on the ground. One MUST have a suet feeder to attract those that don't care as much as to seeds. I am always on the look out for some new/clever design.
On this blog I put up a step by step easy way to make a perfect hopper feeder. https://yardcrap.typepad.com/yardcrap_birds/2019/03/build-a-bird-feeder.html
--- Sunflower seed is the standard. Not all birds care as to it, but many do. Best place to buy is Tractor supply where the seed is fresh (unlike supermarket crap). A 40# bag is most times under $20. I generally pick up a few bags.
--- Shelled sunflower is good. There is less mess, and pound for pound as to actual food and not shell, it is comparable to in the shell as to $$. At Tractor Supply it is about $30 for 20#. Some birds who can't open sunflower in the shell will also eat this.
--- White millet is a must in Winter and some eat it all year. Don't buy that "mixed seed" crap at the supermarket or Lowes. Just buy white millet. I get mine at a feed store in 50# bags. Go to a feed store, if they don't have it, they can get it in.
--- Suet, I have some real suet right now, but what they prefer is a mix I keep out year. If you want to attract the song birds, this is the way. Put a pot on the stove and mix in crunchy peanut butter and cornmeal. For the Winter mix I add real lard. Melt the peanut butter in the jar and lard in the tub in the microwave and pour it in with the cornmeal. I make a big batch in my beer brewing pot a couple times a year. Birds who live on insects love it. Pure seed eaters pass on this. Don't mix in other things like bird seed. Birds have enough problem cracking those, making them oily does not help.
---Thistle is good for finch, but expensive, I rarely buy it.
--- Safflower is one many like. Squirrels don't. A new version came out a couple years ago called Nutra-Saff . The hulls are tender and more nutritious, or so they claim. It is more expensive, best buy is Walmart online with free delivery.
--- Peanuts are fun, in and out of the shell. I mostly use shelled. I chop some as birds like the Carolina Wren need a little help. Many birds love these. I get a #50 pound bag on Amazon and it last a year.