When pigeons sit on eggs, they have large but completely normal droppings, not at all like the one you posted picture of.
Never give your pigeons ACV. I read such advices and tried twice. Both times my pigeons started to get sick. And i was giving them very little: 5 ml in 4 l of water. I didn't know why untill i read this advice: don't give millet to pigeons because it is base and it makes their droppings base. In base droppings all kinds of pathogens grow easily. And then i understood what happened with ACV. It also creates base reaction, moves acid-base balance of organisms into base side. If you look natural pigeon food, all kond of seeds except millet are acid. Buckwheat is neutral. For humans it is good to have more base foods, but for pigeons is opposite. And i remembered one winter i was giving my pigeons a lot of millet, about 20% and their droppings were extremely wet and many of them got sick. So i never give millet to pigeons.
If you give pigeons calcium tablets, you can easily overdose them. The pigeons must have grit - very small stones in a bowl just for that, to be healthy. It helps them to digest seeds and also provide them with all necessary calcium. And they can take it in the right amount they need. Sometimes they eat it too much, so it is good to remove it about 2 days a week.
I never give my pigeons vitamins because they are not natural products. There are little known research that man made vitamin a and another one (e if i remember correctly) that cause rapid cancer growth in cancer patients, so rapid that the research had to be stopped. Spirulina is natural source of vitamins, minerals, etc but it is base food. Just like other algae, leaves (moringa), and other natural supplements.
Always check dosages of every medicine in multiple websites before giving it to pigeons. And also other substances in medicine. For example ivermectin injection solution can be given orally to dogs (because it is extremely small dose and has small amount of propilen glicole which is predominant ingredient of that injection solution), but not to pigeons.
