Why are dead things in my chimney and why can't I burn them up?

Sometimes they nest, then abandon the nests and create a fire hazzard.

First I love your name.
Second. The birds and animals treat a cool chimney like a hole in a tree. It makes a great home.
You can try to burn them up but if the parts fell down into your house or got stuck in the chimney that would be a tragedy. I am sure their bodies did smell when they rot but the damper, that is a plate to close the flu above the fire box, may be shut so the smell went up.
Also you can have a chinmey cap installed on the flu to prevent critters from entering . Concerning the cap I’d ask an expert first as chimney’s differ.
Cleaning is good also because fires can leave creosote on the inside of the flu and it can get very hot when it burns and in quantity can set a chimney on fire just as critter bodies can.

I get my chimney cleaned every 3 years to make sure everything is good. I use my fireplace alot…..It’s less than a hundred bucks. It checks for safety issues as well.

Much easier for them to take take of critters and such than you….eh????

In the meantime, put a screen or pottery in front of the fireplace so you dont freak about birds,...

Our chimneys are cleaned every year. Burning out debris from a chimney is a bad idea, chimney fires can easily lead to burning down the whole house; the fire could spread through cracks in the masonry or embers land on the roof. A spark-surpressor screen at the top of the chimney will keep the critters out of the chimney.

Worse than there being dead things in there…you could have live things in there. So you spark up the fireplace only to hear the cries of the slowly incinerated. Better to have it cleaned out before you spark it up.

All chimney dampers leak. In the winter, warm air rises out of your house and out the chimney. That warm, sheltered opening is a perfect place for birds and small critters to nest. Sometimes one unlucky sap might fall down and cannot get out. That is where they stay until they decompose. You don’t smell it because the air is being drawn out of your house.
Before you light the firebplace:
1) Go up on the roof with a flashlight and look down the flue. See anything?
2) Go back inside and open the damper, with a mirror and flashlight see if you can look up the chimney to see the sky. If it looks clear, you saved yourself $100. If not, then you need brushes, weights, etc… Not worth the effort. Call a pro and give someone some work.

I have a reburning type fireplace insert. It puts out less than a gram of particulate per hour and no smoke. (Old stoves put out 40 g/hr) The top of my chimney is covered with a cap and screen that never gets dirty and is too small let animals in. The flue pipe stays clean because I run the system hot during the cold parts of winter and don’t let it idle.
Every year I check it and only felt I had to clean it one time -mostly to satisfy myself that it really works. Fireplace design has come a long way.