They often don't explicitly state it, but if you do the math based on seeds and percentage in the food there is around 60-70g calcium per cup which varies based on seeds they use.
The birdseed
@U_Stormcrow found did list calcium: minimum 0.25%, maximum 1.5% That is quite a wide range, but even the high end should be safe enough for non-laying chickens. At the low end, any chicken would need more calcium. A laying hen needs about 3 times as much calcium as a chicken that is not laying, so any laying hen would need much more calcium than that particular birdseed provides.
Unless other birdseed mixes have a much higher level of calcium, I would not worry about them raising the calcium of the entire diet to a dangerous level for any hens, laying or not.
Birdseed may have many other problems, but I'm not seeing "high calcium" as a serious concern when it is fed to chickens, unless there are some with MUCH higher calcium levels than the particular one where I could see the label when I went to the website.
I tried to look up how much calcium is in some birdseed ingredients, and I now have trouble believing that any birdseed can have 60-70 grams of calcium per cup.
Here is info on sunflower seeds from the USDA (human food source).
https://fdc.nal.usda.gov/food-details/170562/nutrients
1 cup is 140 grams
calcium is 109 mg (milligrams). Converted to grams, that is about 1/10 of a gram.
Here is nutrition info for millet (raw, presumably dry):
https://www.nutritionix.com/i/usda/millet-raw-1-cup/513fceb775b8dbbc21002dbb
1 cup is 200 grams
calcium is not listed
https://www.uhhospitals.org/health-...ry/article/nutritionfacts-v1/millet-raw-1-cup
This gives calcium of 16 mg in 1 cup of raw millet (that is between 1/100 and 2/100 of one gram.)
If a cup of sunflower seeds or millet weighs 140-200 grams, and I assume that other ingredients have similar weights, I don't see any possible way for it to have 60 or 70 grams of calcium. That would be 1/3 to 1/2 of the total weight being calcium! Oyster shell and chicken eggshells are about 1/3 calcium by weight. I think you must have been finding numbers for how many milligrams (mg) of calcium are in the birdseed ingredients, not how many grams (g).
A laying hen needs about 4-5 grams of calcium each day. In milligrams, that is four thousand to five thousand (4000 to 5000). A non-laying chicken needs a calcium rate about 1/3 to 1/4 what a layer does, so a non-layer might need a bit over 1 gram (1000 milligrams) of calcium each day, still much higher than is found in sunflower seeds or millet (or probably in other birdseed ingredients, although I am not going to look up every possible ingredient to try to check that.)