My balcony pigeon chicks died. Was it sickness or heartbreak?

It could be infection, congenital defects, environmental issues (too cold) simply parental inexperience. Post mortem exams and diagnostics on the chicks may provide an answer. It is difficult to determine cause from an image. Consider giving the hen a rest from breeding. Clean and sanitize the cage, bowls, toys and perches. Discard anything that cannot be sanitized. Clean and sanitize the nesting boxes or replace them entirely. Speak with local breeders to determine if this is not an isolated issue. They may also be able to provide helpful tips or insight.

It would be hard to tell what caused the babies to die without a necropsy/examination. It's possible there was a genetic issue, an illness, or the babies were just weak/not fed as well and so passed on. Your parents are a bit young still and may be having trouble caring for all the babies, so helping out by supplementing with baby bird food may be best to help the remaining babies thrive.

I'm sorry to hear about Puff's passing. Unfortunately, without an necropsy (animal autopsy) it would be hard to know why he passed away. Birds are very good at hiding illness until they're VERY sick, so it may be that there wasn't anything else you could have done. If you're having trouble coping with Puff's passing, checking out some resources such as the Rainbow Bridge where you can talk with other pet owners and even create an online memorial may help in your time of loss.

Nature can be very cruel. And unfortunately, especially in the bird world, survival of the fittest is the rule. Sometimes chicks who are not thriving are sick or otherwise underdeveloped. The parents often instinctively know this. Also, the parents are less likely to care for their chicks when they have been touched by human hands. While I am certain you have the best of intentions, feeding the chicks and taking them in could possibly make matters worse.

18 days old is still a bit too young for a pigeon to be on its own, so the parents should be still caring for it. It's possible that the other baby is ill, or the parents ARE still caring for it and you just aren't catching them in the action. If you feel the baby is truly sick, injured, or not being cared for by its parents, then seeking help from a local wildlife rehab center is best. Do not attempt to move the baby or care for it yourself as it could still very well be cared for by the parents.

You can clean out the nesting box as needed. Gently scoop the chicks out and place them in a clean temporary box. Expect mom to complain and peck. Scrape the box and fill with clean nesting material. Return the chicks to the box afterwards. If the hen is not feeding them adequately, you will need to supplement​ them or pull them for handraising. Use a commercial handfeeding formula. Follow label directions. If you don't have experience hand feeding, ask your avian veterinarian or an experienced breeder. If this is Boo Boo's first clutch, the loss of some of the chicks may be due to inexperience.

Thanks for reaching out about your birds. First thoughts are of a tracheal obstruction, possible infection or aspirated seed. Either of these can lead to acute death. Ticks are extremely rare in birds and can usually be removed manually. Mites or lice are more common. Treating them with organophosphates might be risky with chicks around. If possible, wait until chicks are weaned and out of the nest. Then the cage can be thoroughly cleaned and the birds treated. A veterinary visit will be needed to prescribe a treatment. Good luck.