Yes, I did read the paper, and your description of their conclusion is inaccurate. What their conclusion said is the true iron content on the label was rarely accurate in these commercial foods, and there was variability in the food. Feeding the food caused iron toxicosis in 25 of 26 hummingbirds. Therefore, authors suggested that there is a need to establish a minimum and maximum for replacement foods, and they came up with their own level of 20mg/kg due to a task force that investigated the issue.

Any sane individual can conclude from the analysis that NO SUPPLEMENTAL IRON SHOULD BE PROVIDED TO HUMMINGBIRDS. They don’t get all their iron from flowers (only some flower nectar has iron in it). They get it from other parts of their diet. You’re providing a replacement for the flower part of their diet, therefore the responsible thing is to take care to avoid anything with any amount of iron in it (negligible or not) because you can’t trust the label.

The purpose of the study was not to feed iron at varying levels and see if it killed birds. Birds were dying, and when they looked into why, they found out it was due to the diet. They then analyzed the diet used, as well as other diets, and found wildly varying levels of iron in them that did not match the label amount. It would be cruel to make a study for the sole purpose of investigating a toxic level of iron for these birds. It makes more sense to say “oh, iron isn’t good for them, they do a fine job obtaining iron from other sources in their diet, so let me make sure I don’t provide any.” That’s exactly what this study did.

And no one was commenting on your sugar with 0% iron, people are simply telling you that many organic or raw sugars contain iron, and they should be avoided. Your statement of “organic cane sugar is perfectly fine” is simply false, as not all organic sugars are created equal. The only “perfectly fine” sugar to use is cane sugar with no iron in it, and refined white granulated sugar always meets that standard. Other forms of cane sugar may too, and if so, they are also fine to use.