Birds are fascinating creatures.  They have so many different forms, a result of being adapted to neatly fill a vast number of biological niches in the environment.   This characteristic makes them sensitive indicators to changes in habitat; it's no surprise that the phrase "the canary in the coal mine" refers to a bird.  The Ruby-Crowned Kinglet (Regulus calendula) is a small bird, only about four inches long, and because of its dull green color is often overlooked in our yards.  

If you want to see them, hang a suet feeder near a window and plant some pine, fir, or spruce trees which are a favorite habitat.  The male bird, pictured here, has a scarlet crest that is hidden beneath green feathers most of the time.  But when he gets excited he will display it for a second or two.  When I took this photo, a female Kinglet was just off camera and he popped up his crest, wooing her in a bid for her favor.  

Where I live, Kinglets appear in the winter months and are a cheery sight during the gray, rainy days.  They have a clipped "jit-dit" call that signals they are near.