When do baby budgies typically start leaving their nest box?

It's possible that is the baby bird poop. Most often the parents will take care of cleaning and can do a sort of technique where they press the stool to the nest wall to keep it "cleaner". I wouldn't disturb the babies until they are actively coming out of the nest and exploring on their own, which you can then use to remove the nest and clean it safely without injury a baby.

26 days is old enough that the babies should be starting to explore and eat some on their own. If only dad is feeding them he may be trying to encourage them to leave the nest and find food on their own. Making sure they have access to bird feed near where they can get to may help get them used to eating on their own and exploring some.

Thanks for reaching out about Blue. The average incubation length for budgies is 14-16 days, once she starts sitting on the eggs. A thermostat regulated heat lamp can be used to regulate the temperature. If she is going into and out of the nest box, and has laid egg in the box, she is probably comfortable with the size, as long as it is large enough to rear the young. Regarding nutrition, a wide variety of vegetables and a plant protein source (legumes) is important to replace nutrients lost in laying eggs. Good luck

If your budgie's parents are still around they will most likely care for them on their own. However if the parents aren't around or aren't caring for the babies properly, you will need to start feeding them yourself. Most pet stores will carry a basic baby bird food which has feeding instructions depending on your bird's age. You'll also need a syringe with which to mix and feed the food. As your bird grows you can start introducing an adult balanced diet to encourage them to begin eating more solid foods.

Budgies will typically eat a portion of their own food, and then regurgitate it to the babies so it can be more easily digested by them. If the parents aren't feeding the babies, there are commercial baby bird foods available that when added to water, mix up into a slurry that is easy for the baby bird to digest (since we can't regurgitate to them!). As the babies grow and start to explore on their own, they will begin to eat the regular adult food as well.

Incubation in budgies is about 18 days from the day the hen begins sitting on The eggs. You are probably correct in saying the eggs are infertile. If there is no male present, they are definitely infertile. Wait until Coco loses interest, then remove and discard the eggs. This will prevent her from laying "replacement" eggs. Remove the nest box afterwards.

Baby birds are not usually born with feathers so that would be unlikely that you would see feathers within the egg, however viable eggs tend to start off clearish and then become more opaque as they age. You can also hold them up to a candle or light source (carefully as they are fragile) to see if there is anything inside. If they are not sitting on the eggs at all, the eggs may not be viable, the parents may be too young or inexperienced. You can attempt to incubate the eggs yourself if they are viable, however if they are not, simply removing them and ensuring your birds have adequate diets and a calcium supplement can help encourage them to lay more eggs.