Dead bird is a huge sign for toxic gas. This is why the term Canary in the coal mine has been used.

Canary in the coal mine; Etymology . An allusion to caged canaries (birds) that miners would carry down into the mine tunnels with them. If dangerous gases such as carbon monoxide collected in the mine, the gases would kill the canary before killing the miners, thus providing a warning to exit the tunnels immediately

Edit: carbon dioxide poison was common for minors so they used Canary's in the mines because the bird are more sensitive and would die letting the minors know there is poison and they need to leave